Attix FH, Roesch WC, Tochilin E, editors. Radiation dosimetry. (Pts I,II,III) 2nd ed. New York: Academic Press; 1966–1969.

Greening JR. Fundamentals of radiation dosimetry. 2nd ed. Bristol [England]: A. Hilger in collaboration with the Hospital Physicists’ Association; 1985.

Johns HE, Cunningham JR. The physics of radiology. 4th ed. Springfield, IL: Thomas.

Kase KR, Bja¨rngard B, Attix FH, editors. The Dosimetry of ionizing radiation. (Pts I, II, III) Orlando, FL: Academic; 1985.

Klevenhagen SC. Physics and Dosimetry of Therapy Electron Beams. Madison, WI: Medical Physics Publishing; 1993.

See also IONIZING RADIATION, BIOLOGICAL EFFECTS OF; RADIATION THERAPY SIMULATOR.

RADIATION DOSIMETRY,

THREE-DIMENSIONAL

GEOFFREY S. IBBOTT

Anderson Cancer Center

Houston, Texas

INTRODUCTION

The goal of radiation therapy is to obtain the greatest possible local and regional tumor control, with the fewest complications. The response of many tissues to radiation can be characterized by a sigmoid curve. Relatively little response is seen until the dose reaches some threshold value, after which the response is quite rapid (1,2). In the region of steep response, relatively small variations in dose can yield significant differences in the response of both tumors and normal tissue (3). To minimize the variability of tissue response, the ICRU has recommended that the uncertainty in dose delivery be maintained below 5% (4–6). Delivering a dose to a patient with a tolerance of 5% is not a simple matter (7). It has been estimated that the equipment used by most medical physicists to calibrate therapeutic radiation beams is itself calibrated with an overall uncertainty (expressed at the 95% confidence level) of 1.5% (8). Uncertainties associated with the characterization of radiation beams, patient anatomy, and location of the target volume, as well as reproducibility of the treatment from day to day must be considered (9,10).

A comprehensive radiation therapy quality assurance program must address all sources of variability in the treatment of patients, in an effort to minimize variations. Technical aspects of quality assurance (QA) must address a wide array of issues, including the performance of simulation, treatment, and treatment planning equipment, the stability of measurement and test equipment, the accuracy and appropriateness of treatment planning calculations, and the accuracy and completeness of documentation. Technical quality assurance procedures should also address inventory, calibration, and treatment planning with brachytherapy sources. Recommendations for QA procedures can be found in a number of publications (11–23).

As the equipment used to deliver radiation therapy has evolved, methods of radiation dosimetry have also changed. Multifield, conformal radiation therapy (CRT), intensitymodulated radiation therapy (IMRT), stereotactic radio-

surgery (SRS), and stereotactic radiation therapy (SRT) all produce dose distributions that can be highly irregular in three dimensions. Conventional two-dimensional (2D) planning and dosimetry systems are not adequate to simulate and measure such distributions. Instead, new dosimetry systems are required that can record and display these complex distributions (24). This article addresses recent developments in dosimetry systems, and their advantages and complications.

QUALITY ASSURANCE PROCEDURES REQUIRING DOSIMETRY SYSTEMS

External Beam Calibration Consistency-Basic Parameters

Detector systems are required for measurement of accelerator output, for compliance with published recommendations for quality assurance. Most published recommendations suggest that accelerator output constancy be monitored on a daily basis. Consequently, a dosimeter system that is rugged, reliable, and easy to operate is required. Most recommendations for daily output consistency suggest that deviations on the order 2–5% be detectable; therefore the dosimetry system does not need extremely high accuracy.

In addition, measurements of beam flatness and symmetry are recommended on a periodic basis, often weekly. Again, as these measurements are to demonstrate consistency of operation at the 2–5% level, high precision is not required. Several of the available array dosimeters systems are suitable for such frequent QA measurements of treatment unit performance.

External Beam Treatment Delivery, Planning Verification

Several treatment applications require the verification of delivered dose with relatively high accuracy. For example, IMRT requires the precise delivery of relatively small doses through a large number of fields. Even small errors in dose delivery can accumulate and result in a large error in the final dose. Monitoring of dose delivery during IMRT is done best using a real time measuring device, such as online portal imaging. Similarly, CRT delivery demands confirmation that the correct dose has been delivered. As CRT is generally delivered through static fields, point detectors may be used to measure the delivered dose in a suitable phantom. Several of the simpler point dosimeter systems described earlier are suitable for this purpose.

Likewise, SRS and SRT delivered with accelerators may need verification, particularly as SRS is delivered in single large fractions. Again, under most circumstances, point dosimeters are suitable here. However, the characterization of radiation beams for SRS–SRT requires a dosimeter with high spatial accuracy. Several of the detector systems described above would satisfy this requirement, although questions of electronic equilibrium must be addressed (25).

Total body treatments, such as photon TBI for systemic bone marrow ablation, or total skin electron therapy for cutaneous t-cell lymphoma, may require dosimetry to confirm the correct delivery of dose under these conditions of unusual field size and distances.

482 RADIATION DOSIMETRY, THREE-DIMENSIONAL

External Beam Treatment Delivery In Vivo

Modern external treatment delivery requires that doses be delivered with accuracy never before required. Procedures, such as IMRT, are delivered through many field segments, each delivering a small increment of dose. A systematic error in dose delivery can result in a significant error in the final dose received by the patient. Consequently, dosimetry devices for confirming correct dose delivery are necessary. These fall into three broad classes: surface dose measurements, transmission measurements, and true In vivo measurements.

Brachytherapy (LDR, HDR, IVBT)

Dosimeter systems are required for at least three purposes related to brachytherapy: source characterization, confirmation of dose distributions from arrangements of multiple sources, and In vivo dose measurements (26,27).

Imaging Procedures

Dosimetry measurements are required in cardiology, for procedures, such as cardiac ablation, in which patients can receive significant doses. A detector to be used in imaging must not be intrusive, meaning that it must be virtually transparent to the beam. It must measure dose over a large area, although accommodation needs to be made for the possibility that the beam may be moved during irradiation. Finally, a device attached to the source of radiation, such as a dose area product meter, may be used.

REQUIRED CHARACTERISTICS OF DOSIMETERS FOR QUALITY ASSURANCE

A dosimeter for modern CRT must possess a number of important characteristics. It must be tissue equivalent, as the dosimeter itself must not perturb the dose distribution. It must have a linear dose response over a clinically useful range. Ideally, its response would be independent of dose rate and of beam modality, making it useful for mapping dose distributions from isotope units, linear accelerators, or particle accelerator beams. Some dosimeters must be able to fill a volume, or conform to a surface. This will enable the dosimeter to either mimic any portion of human anatomy, or conform to a section of an anthropomorphic phantom.

The dosimeter must either provide immediate results or be stable for a sufficiently long period to enable irradiation and analysis. Under some circumstances, the delivery of the intended dose distribution may take some time, as is the case with brachytherapy. It is important that the dosimeter remain uniformly sensitive, and unaffected in response over the time required for irradiation. Further, the dosimeter must maintain the dose-deposition information throughout the time required for analysis. For some applications, it may be desirable to transport the dosimeter to another facility for analysis. The dosimeter must remain stable during shipment, unaffected by a variety of environmental conditions, throughout the analysis.

The accuracy and precision required of dosimeters for radiation therapy measurements depend on the intended

use of the detector. Devices intended for reference calibration of treatment units should enable the determination of dose with an uncertainty of no more than 0.5%, expressed at the 95% confidence level (k ¼ 2) (28,29). Dosimeters intended for verification of dose distributions should provide an uncertainty in dose measurement of no more than 2%, again expressed at the 95% confidence level (k ¼ 2).

DETECTORS FOR THREE-DIMENSIONAL DOSIMETRY

Detector Arrays

A number of manufacturers have marketed arrays of conventional detectors using either ion chambers or diodes. These devices are not true three-dimensional (3D) dosimeters, but are included here because they provide 3D information through the use of one, or at most two, manipulations, such as translation across a beam. For example, linear diode arrays are available for the Scanditronix water phantom system, and for stand-alone QA device, such as the Sun Nuclear profiler. An array of ionization chambers has been described for verifying treatment planning for IMRT (30). The ion chambers are arranged in several parallel linear arrays, each one offset from the next. Twenty-four chambers, each 0.03 cm3 in volume, are arranged in boreholes of a plastic-mounting frame. The assembly is positioned in a water phantom and maybe positioned in different orientations to allow measurements in different plains. Commercial ion chamber devices include the Thebes marketed by Nuclear Associates, an ion chamber array marketed by Wellhofer, the RBA-5 marketed by Gammex, and other devices. These devices range in number of detectors from few (four or five) to many (Fig. 1).

Plastic Scintillator

Some organic plastics fluoresce visible light when irradiated with ionizing radiation. Unlike the fluorescent screens used in imaging, organic scintillators have the

Figure 1. A diode array designed to display the intensity map of a therapy beam in real-time. (Courtesy of Sun Nuclear Corp.)

additional advantage of being approximately tissueequivalent (31). However, this tissue-equivalence at present only exists at the energies conventionally used for megavoltage treatment. Most of the plastic scintillators currently available exhibit significant differences in the mass energy absorption coefficient relative to that of water. More recently, plastic scintillators have been developed for low energy photon dosimetry that are radiologically waterequivalent, have improved sensitivity over some others scintillators, and offer the potential for high spatial resolution (32,33).

Plastic scintillators may be used as point detectors, in which their potential for manufacturing into very small sizes yields the possibility for improved spatial resolution of measurements. Efforts also have been made to use plastic scintillators as 2D and 3D detectors (34). Two techniques have been used; the first being the use of plastic scintillators as a detector system themselves, using optical coupling through a light pipe assembly to a video detector. This method has been described previously (35–37). Significant difficulties still exist with the spatial resolution of these systems. Light emitted by the scintillator can travel some distance, in any direction, before reaching the light detector. Unless the plastic scintillator is thin, the resolution of the image will be degraded considerably. Some efforts have been to quench the light by adding dyes to the scintillator, to reduce the distance traveled by the light obliquely through the scintillator. Until this problem is resolved, the quality of the imaged dose distribution will not be adequate for radiation dosimetry.

A second technique involves the use of plastic scintillators to enhance the response of another detector, such as radiographic film (38). In this technique, radiographic film is sandwiched between sheets of organic plastic scintillator. Several investigators have noted that radiographic film has a tendency to overrespond to low energy photons (39,40). The use of an organic plastic scintillator has been proposed to enhance the response of radiographic film to higher energy photons, thus making the energy response of the film detector system more uniform.

Film

Radiographic film has long been used as a radiation detector, and as a QA device. Again, film itself is not a 3D dosimeter, but stacks of film have been used to measure dose distributions in 3D. The difficulties with film are well known; energy dependence, requirements for processing, variations from one batch to the next, dose rate dependence, positional dependence, and other issues have been discussed by a number of investigators (41). More recently, use of radiochromic film has been proposed. Radiochromic film requires no processing, has very little energy dependence, no known dose rate dependence, and requires minimal special handling techniques (42,43). The linearity of response of a recently developed model of film is shown in Fig. 2.

The use of film for verification of conformal and IMRT dose distributions has been recommended. At least one manufacturer has marketed a phantom intended for use with IMRT (see Fig. 3, for an example of such a device).

2.00

(a)Improve radiochromic film

Figure 2. Energy dependence and linearity with dose of an improved radiochromic film. (Reproduced with permission from Ref. 42).

TLD Sheets and Plates

Lithium fluoride, a thermoluminescent material, has been used for many decades as a radiation detector (44). Its use has been limited principally to point measurements, because the dosimeter is provided either as extruded rods or chips, or as a powder that is encapsulated for use. Thermoluminescent dosimetry has a number of limitations, among them energy dependence, but most notably a requirement for delay between irradiation and processing. In addition, an expensive piece of equipment is required for readout of the material. The limitation of the device to point measurements has been addressed recently by the development of TLD sheets. In these, TL material is distributed in an array across a sheet of backing film. The film can be irradiated in much the same manner as conventional radiographic film, and may be immersed in a water phantom as necessary. As with film, 3D measurements can be made only by stacking multiple sheets of TL

Figure 3. A phantom marketed for evaluating IMRT dose distributions. (Courtesy of Med-Tec Corp.)

484 RADIATION DOSIMETRY, THREE-DIMENSIONAL

material. After irradiation, and following the requisite delay, the film is inserted into a readout device that selectively heats the individual dosimeter regions using a laser. Light is collected from the heated regions using a photomultiplier tube. Through an automated operation, a matrix of data can be obtained quickly and efficiently. However, due to the cost of the reader, this dosimetry system is presently available only as a service (Inovision, Inc.)

Electronic Portal Imaging Devices

An important aspect of quality assurance in radiation therapy involves not just the dose delivered to the patient, but the correct positioning of the patient. For many years, positioning has been verified through the use of conventional radiographic film, or through the use of video imaging techniques (45,46). Video imaging permits only a check of the relative position of external landmarks. Radiographic film permits verification of the patient position through the visualization of internal boning anatomy, but requires a delay while the film is processed. The introduction of electronic portal imaging has brought to the clinic the possibility of immediate verification of patient position. With the introduction to clinical radiation therapy of modern techniques, such as IMRT, immediate verification of correct beam delivery is crucial. The failure or incorrect programming of a multileaf collimator can result in a completely unacceptable dose distribution. With on-line portal imaging, such errors may be detectable promptly, even during treatment (47–52). A further improvement has been the introduction of transmission flat-panel detectors upand downstream from the patient. These allow the measurement of photon beam fluence entering and exiting the patient, and the estimation of dose within the patient. When combined with images of the patient made at multiple beam angles, as is done for multifield conformal treatment, or IMRT, it may be possible to reconstruct the dose distribution actually delivered to the patient in three dimensions.

GEL DOSIMETRY

Gel dosimetry has been examined as a clinical dosimeter since the 1950s (53,54). During the last two decades, however, the number of investigators has increased rapidly, and the body of knowledge regarding gel dosimetry has expanded considerably (55,56). Gel dosimetry is still considered by some to be a research project, and the introduction of this tool into clinical use is proceeding slowly. However, the interest in, and potential of, gel dosimetry for clinical use is demonstrated by the level of participation in three successful international workshops held to date on this subject (57–59). This section reviews the development of gel dosimetry, several of the formulations that have been investigated intensively, the characteristics of gel dosimetry that make it desirable for clinical use, the postulated and demonstrated applications of gel dosimetry, and some complications, setbacks, and failures that have contributed to the slow introduction into routine clinical use.

Fricke Gels

Nuclear magnetic resonance (NMR)-based gel dosimetry was introduced by Gore who recognized that the ferrous sulfate Fricke dosimeter (60,61) could be examined with magnetic resonance rather than spectrophotometry (55). The Fricke dosimeter is based on the radiation-induced and dose dependent transformation of ferrous (Fe2þ) ions into ferric (Fe3þ) ions. These two ions have different electron paramagnetic spin states and different ionic radii (60,61). Gore realized that the NMR spin–lattice and spin–spin relaxation rates (1/T1 and 1/T2, respectively) of the water protons in the Fricke dosimeter are dependent on the amount of ferric ion present in the solution and that, because changes in these parameters produce the contrast of MR images, radiation induced changes in the solution should be visible by MRI (55). Soon afterward, other researchers began investigating the use of Fricke solutions incorporated into gel matrices (Fricke gels) to provide spatial stability of the dosimeter (62–65). The most common matrices investigated were gelatin, agarose, and sephadex. Each of these systems had its advantages and limitations, but agarose was probably used more than any other detector system. While agarose dosimeters are more sensitive to dose than gelatin-based systems, they are more difficult to produce because they must be bubbled with oxygen to ensure a uniform dose response.

Fricke gel dosimeters have a number of advantages; principle among them is the well-described understanding of the radiation chemistry of this system. In addition, the basic and NMR processes leading to the dosimetry response are well understood (66,67). Fricke gel dosimeters are tissue equivalent over a large range of photon energies. Like other gel dosimeters, they are prepared in a liquid form so that phantoms containing heterogeneities or conforming to anthropomorphic geometries can be constructed.

However, there are a number of significant problems associated with the use of Fricke gels for radiation dosimetry. The dosimeters require high doses, on the order of 10–40 Gy, for the radiation-induced changes to be observed by magnetic resonance imaging (MRI). The ferric ions produced by absorption of radiation diffuse readily through the gel or agarose matrix, leading to a decrease in signal intensity, and a loss of spatial information (64,66–69). Imaging must be performed within 2 h of irradiation to avoid serious degradation of the dosimetric detail (70). The diffusion has been reduced by replacing the gelatin matrix with a poly (vinyl acohol) (PVA) matrix, which is less porous to the ferric ions (71). Other investigators have developed further methods to delay diffusion, although imaging must still be performed quite soon after irradiation (72). Some improvement in the diffusion of ions can be achieved by cooling the gel, but this is rarely practical in a clinical setting. Consequently, Fricke gel dosimetry has seen only limited clinical use.

Several improvements have been reported recently. For example, a Fricke gel dosimeter manufactured using a PVA cryogel technology has been described. The PVA is a common water-soluble polymer that can be cross-linked into its cryogel form by simply freezing and thawing. The cryogel is a rubber like material that holds its shape even at elevated temperatures. Preliminary reports of the PVA

Fricke gel dosimeter indicate that its (1/T1) response has been found to be linear from 0 to 10 Gy, and the ion diffusion constant was found to be only 0.2–0.5 that of traditional preparations in gelatin or agarose (73,74). Representative ion diffusion constants are presented in Table 1 for several gel mixtures (68).

Some preliminary work using Fricke gel dosimetry in anthropomorphic phantoms has been reported (79). Several different gel compositions were investigated, including a lung equivalent gel that was developed with a density of 0.4 g cm 3. This allows measurements of dose within the heterogeneity itself. However, diffusion of ions continues to be a problem with this dosimetry system.

Polymer Gels

Gels that replaced the Fricke solution with acrylic monomers were introduced in 1992 (80–82). Early work was conducted using a polyacrylamide gel based on the radia- tion-induced polymerization and cross-linking of bis and acrylamide. The formation of acrylic polymer chains largely resolved the problem of diffusion exhibited by Fricke gels,

as the long polymer chains were too large to diffuse rapidly. The reciprocal of T2, or R2, the relaxation rate, was found to vary proportionally with dose, and MR imaging of polymer gels was shown to yield quantitative dose distributions (81). Subsequently, alternative gel formulations have been developed in which the bis and acrylamide are replaced with acrylic acid or methacrylic acid, which has yielded increased sensitivity of the gels, and reduced toxicity (83,84). However, the polymer gels continued to show another disadvantage; their response was inhibited by the presence of oxygen. This effect was addressed though the recent introduction of a class of polymer gel dosimeters containing oxygen scavengers (85,86). Several variations of these normoxic gel dosimeters (so-called because they can be prepared under normoxic conditions) have been characterized (87).

To avoid the disadvantages of the Fricke gel systems, a polymerizing gel dosimetry system was developed (MGS Research, Inc., Guilford, CT). A variety of polymerizing gels have been developed, many of which are based on acrylamide or acrylic acid, and are referred to as polyacrylamide gels (PAG). The dosimeters are based on

486 RADIATION DOSIMETRY, THREE-DIMENSIONAL

Table 2. Composition of BANG3 Polymer Gel Dosimeter

6% Methacrylic acid

1% Sodium hydroxide

5% Gelatin

88% Water

radiation-induced chain polymerization of acrylic monomers dispersed in a tissue-equivalent gel. The BANG polymer gel system is a proprietary PAG dosimeter made of a mixture of acrylic monomers in a tissue-equivalent gel. Early BANG gels were made from acrylic acid monomers and methylene-bis(acrylamide) cross-linker. More recently, the BANG3 dosimeter was introduce, which contains methacrylic acid monomer (see Table 2, from Ref. 84). Other proprietary response modifiers were added to adjust the dose range and sensitivity. Dissolved oxygen inhibits free radical polymerization reactions and is removed from the mixture by passing nitrogen through it while the gel remains above the gelling temperature, prior to sealing the vessel. Consequently, vessels of glass or other material not permeable to oxygen must be used for irradiating and imaging the gels.

The gelling agent in the BANG dosimeter is gelatin, which is used because the transverse NMR relaxation rate of water (R2 ¼ 1/T2) in a gelatin gel is nearly an order of magnitude lower than that in agarose gels. Therefore the background R2 in the gel is substantially reduced, which improves its dynamic range.

MR Imaging of Polymer Gels

Irradiation of the polymer gels induces polymerization and cross-linking of the acrylic monomers. As polymer microparticles are formed, they reduce the NMR relaxation times of neighboring water protons. Magnetic resonance imaging can be used to measure dose distributions in the gel (81,82,88). Water proton NMR (1H NMR) transverse relaxation time T2 can be t determined from multiple spin–echo images. Images can be acquired using the Hahn spin–echo pulse sequence: 908 –t – 1808 –t– acquire for four or more different values of t. Typical pulse sequence parameters are TR ¼ 2 s, TE ¼ 11, 200, 400, and 600 ms. A field of view of 24 cm and a matrix of 128 256 can be used, with one acquisition and a 3 mm slice thickness.

More recently, it has been shown that spin–echo sequences other than the Hahn sequence described above can be used for gel imaging. Improved dose resolution can be achieved through the use of multiple spin–echo pulse sequences (89,90). Optimization of the imaging sequence is necessary, especially with regard to the number of echoes measured. The use of multiplanar imaging can reduce imaging times but can also lead to interference between image planes.

Once MR images have been obtained, they are most conveniently transferred via network to a computer for which a data analysis and display program has been written. One example of such a program has been described previously (82). The program calculates R2 maps on the basis of multiple TE images, using a monoexponential nonlinear least-squares fit based on the Levenberg–

4

gel I gel II gel III

1

Figure 4. The dose dependence of the transverse relaxation rate (R2) as a function of dose. Data from several experiments are shown indicating reproducibility over a wide range of doses (92).

Marquardt algorithm (91). The program also creates a dose-to-R2 calibration function by fitting a polynomial to a set of dose and R2 data points, obtained from gels irradiated in test tubes to known doses. This function can then be applied to any other R2 map so that a dose map can be computed and displayed.

Figure 4 shows values of transverse relaxation rates (R2) for the gels as a function of dose. The pooled data show that the dose response was highly reproducible over a wide range of doses. The dose response is well fitted by a straight line (92).

Additional experiments have shown that the response of the BANG gel can be adjusted by varying the concentration of cross-linker used per total amount of comonomer (93). Figure 5 demonstrates the relationship of R2 to dose for five different values of the weight fraction of cross-linker per total comonomer. Figure 5b shows that, in the linear region of gel response, the greatest sensitivity of the gel was achieved at 50% cross-linker concentration. Similar data have been shown more recently for several different polymer gel mixtures (94).

The temperature of imaging has a large effect on both the gel sensitivity and its dynamic range (93,95). Dose sensitivity (s 1 Gy 1) as a function of concentration of cross-linker is plotted in Fig. 6. Sensitivity is seen to reach a maximum at 50% cross-linker (as described above), but sensitivity at all concentrations increases as the temperature at the time of imaging is reduced.

Figure 7 shows that the maximum R2 achievable, and therefore the dynamic range of the gel, is dependent on the temperature at the time of imaging. While R2max increases with cross-linking, the dependence is enhanced by cooling the gel during NMR measurement.

For a number of gel compositions presently being evaluated, the fundamental chemistry and physics of response are well understood. Several gel compositions have been characterized in great detail (82,87,92,96–98). In polymer gels, for example, it is understood that the interaction of

R2 (s−1)

8.0

0.0

0.050.0 100.0 150.0 200.0 250.0 300.0 350.0

Dose (Gy)

(a)

Dose (Gy)

(b)

Figure 5. (a): Relationship between R2 and dose for five different concentrations of cross-linker per total comonomer.

(b) Lower dose region of the data from (a). (Reproduced with permission from Ref. 93.)

radiation with water produces free radicals, which trigger the cross-linking of monomers into polymer chains (81,99). The polymer chains bind water protons tightly causing a change in their paramagnetic properties that is detectable by magnetic resonance imaging (92,100). The relationship between dose and relaxation rate can be influenced by several additional factors, including accuracy of the calibration curve (101) and the aging characteristics of the gel (96,102,103).

The quality of the imaging process is affected by the homogeneity of the B1 field (104) and the presence of eddy currents (105). Some additional complications due to the distortion of MR imaging systems have been identified (106).

Optical Scanning of Polymer Gels

Dosimetric results with MR imaging have been encouraging, but the need to use expensive and often inaccessible imaging systems renders this technique somewhat impractical. In most compositions, polymerization changes the optical characteristics, and measurements of optical density can be related to absorbed dose (85,107–111).

Optical computed tomography (OCT) of polymer gels can be conducted in a similar manner to X-ray CT. To date,

0.0 20.0 40.0 60.0 80.0 100.0

%C (w/w)

Figure 6. Dependence of the dose sensitivity on the cross-linker content, for three different temperatures. (Reproduced with permission from Ref. 93.)

OCT has been limited to transmission measurements, although the potential exists for measurements of attenuation, fluorescence, scatter, polarization and refractive index changes (112). Optical computed tomography has been performed by several investigators (107,109–113), but in general, the techniques all require the use of a cylindrical vessel to hold the gel, a tank filled with a medium matching the refractive index of the gel, and a monochromatic light source. Several of these systems use parallelray geometry and filtered back projection to reconstruct the image. At least one system uses a diffuse white light and cone-beam geometry (111).

An optical imaging system employing He–Ne laser CT scanning of the gel has been described (107). The scanner operates in a translate-rotate geometry and is capable of producing stacks of planar dose distributions with pixel size and slice thickness as small as 100 mm (114).

Optical scanning of several gels has been conducted using a modified version of a 3D optical CT laser scanner that was developed recently at MGS Research, Inc. (107,108,115,116). (see Fig. 8.)

The scanner, which is PC controlled, operates in a translate-rotate geometry and utilizes a single He–Ne laser

%C (w/w)

Figure 7. Dependence of the maximum R2 on cross-linker fraction for three different temperatures. (Reproduced with permission from Ref. 93.)

488 RADIATION DOSIMETRY, THREE-DIMENSIONAL

Figure 8. An optical scanner developed for use with polymer gels. (Photograph by M. Heard.)

light source and a photodiode, together with a reference photodiode to account for fluctuations of the laser output intensity. The gel is mounted on a central turntable and is immersed in a liquid that matches the gel’s refractive index to minimize the loss of signal from projections at the edges of the gel. The platform on which the light source and the detector are mounted moves vertically. Isotropic resolution of 1 mm is achievable using this scanner, with scan times on the order of 8 min per plane. An image of a gel exposed to an 192Ir high dose-rate (HDR) source appears in Fig. 9.

Further evaluation of an OCT system has been performed, to determine the stability and reproducibility of the system (118). In addition, characterization of gels has been performed to determine the optimum sensitivity consistent with the dynamic range of the scanner (119).

X-Ray CT Scanning of Polymer Gels

The formation of polymer chains increases the physical density of the gel, and the resulting change in attenuation coefficient can be measured by measurements of X-ray transmission, such as by computed tomography (120– 125). While the change in density is small, it has been shown to vary proportionally with dose (122,126). This

Figure 9. The OCT image of a polymer gel exposed to an 192Ir HDR brachytherapy source. The central region was occupied by the source during irradiation and was replaced with irradiated gel for imaging (117).

Figure 10. The CT image of several vials of polymer gel irradiated to different doses. (Reproduced with permission from Ref. 128.)

change in density leads to a small change in CT number when irradiated gels are examined with CT. Recent data show that this change can be as much as 0.2 kg m3 Gy 1 (127). An image of tubes of gel irradiated to different doses appears in Fig. 10. Methods for improving the quality of X-ray imaging have been developed, and include the acquisition of multiple images, background subtraction, and filtering (126,127).

Ultrasound Imaging of Polymer Gels

Polymerization leads to changes in elasticity of the medium, and the corresponding changes in ultrasound absorption can be exploited (129–132). Ultrasound has been used to evaluate changes in density and elastic constant of a number of materials. Several different ultrasonic parameters can be measured and these can be used to characterize materials. The most commonly measured parameters attenuation and reflection coefficients, and the speed of propagation. A pulse-echo technique using one probe or a transmission technique using two probes is used to measure these parameters. These parameters can be related to structural properties of the sample including bulk density, elastic constants as well as sample inhomogeneities.

Vibrational Spectroscopic Imaging of Polymer Gels

Finally, vibrational spectroscopy can be used to demonstrate the conversion of monomers to polymer chains (133– 136). Fourier transform (FT)–Raman vibration spectroscopy of polymer gel dosimeters has been investigated as a means by which the fundamental structure and properties of the dosimeters might be better understood. Raman spectroscopy has also been used to investigate the track structures of proton beams in polymer gel dosimeters (137). This study illustrated the difficulty in using polymer gel dosimeters to extract quantitative dose maps when exposed to proton radiation. Further studies will be required to determine whether Raman microscopy can be used routinely in the evaluation of polymer gel dosimeters.

Ion chamber

Film

TLD

Gels

Gels (potential)

Figure 11. A spider plot, illustrating the capabilities of several common dosimetry systems, as well as gels, and the potential capabilities of gels. (Redrawn with permission from Ref. 110.)

CHARACTERISTICS OF GEL DOSIMETERS

Gel dosimeters have a number of characteristics that make them attractive for radiation dosimetry (138). A novel comparison of gel dosimeters with conventional dosimetry systems has been presented in the form of a spider plot (see Fig. 11, Ref. 110). This graphical presentation illustrates the relative performance of dosimeters, such as ion chambers, film, TLDs and gels by considering such parameters as accuracy, volume measured, cost, three-dimensionality, resolution, energy dependence, and time required for the measurement. Oldham has shown that gels compare favorably with the other detectors in most characteristics, including their relative accuracy, volumetric nature, inherent three-dimensionality, high resolution and lack of energy dependence over much of the important energy range (110). Methods for characterizing the response of gels have been found, and in particular, a technique for characterizing the dose resolution has been described (89,139,140).

However, today gels are still time-consuming and relatively expensive. Several dosimetric aspects have not yet been realized, including the absolute accuracy of measurement, and the ability to render a 3D dose distribution as opposed to multiple planes of data, although progress is being made rapidly on both aspects. In addition, the issues of cost and time required are being addressed. The availability of optical CT scanning and other imaging techniques are likely to drive down the cost of gel analysis, and improve the penetration of this modality into the clinic. At the same time, newer optical CT scanners equipped with more powerful computers are faster and can perform comprehensive imaging of gels in the time previously required for a single slice.

APPLICATIONS OF GEL DOSIMETRY

Potential applications of gel dosimetry have been summarized on several occasions in the recent past (97,138,141– 143) although the field is developing rapidly. Today it is considered by many that gel dosimetry has useful characteristics that can facilitate radiation therapy dosimetry, especially in situations that are not handled well by conventional dosimeters. These characteristics include the ability to measure complex 3D dose distributions; to integrate dose accurately without dependence on dose rate, at least over a fairly wide range; tissue-equivalence; high spatial resolution; and lack of energy dependence over most of the kilovoltage and megavoltage range. With most gels, data are stored permanently, making gels suitable for performance of dosimetry at remote locations (144). They also are relatively safe to manufacture and handle, although some components such as acrylamide are toxic and must be handled with appropriate protection until mixed.

Demonstrated applications of gel dosimetry to date include basic dosimetry (depth dose, penumbra, wedge profiles) in photon, electron, neutron, and proton beams; dose distributions from imaging procedures; conformal therapy, stereotactic radiosurgery, and intensity-modulated radiation therapy (IMRT); dose distributions around brachytherapy sources (low and high dose rate, and intravascular sources); internal dosimetry (131iodine doses); and evaluation of tissue heterogeneities. The advances made recently in these areas will be discussed.

Basic Dosimetry

Gel dosimeters have the capability to record and display the dose distribution throughout a 3D volume. This ability affords advantages over conventional dosimeters, even for basic dosimetry parameters such as percent depth dose in photon and electron beams (54,92,145). Gel dosimetry has been shown to be useful to validate simple multiple-field arrangements (146) and more complex anatomical situations including tangential breast treatment (147,148), conformal therapy (149) and scalp treatment with electron beams (150). Dynamic functions, such as a programmable wedge filter are difficult to measure with ionization chambers or diodes, and film is often used to provide data in a single plane. Gels have proven useful for capturing the dose distributions from programmable wedge filters, and allow distributions in multiple planes to be demonstrated from a single exposure (151).

Dose from Imaging Procedures

More recently, the use of gels to demonstrate dose distributions from imaging procedures has been explored (152,153). In a novel experiment, a high sensitivity gel was used to determine the dose from CT imaging. The benefit of this measurement is that the dose distribution throughout a patient volume can be estimated without requiring the use of numerous point dosimeters (e.g., TLD) and without averaging the dose along a line or throughout a volume (e.g., a pencil ionization chamber). These benefits may be most apparent in evaluating the dose distribution from helical CT scanners.

490 RADIATION DOSIMETRY, THREE-DIMENSIONAL

Figure 12. A BANG gel irradiated with a highly conformal dose distribution produced by a Gammaknife treatment unit. The distribution can be appreciated qualitatively without the need of imaging systems or processing. (Photograph by the author. See also Ref. 161.)

Evaluation of Conformal Dose Distributions

Stereotactic Radiosurgery. Gels have been used to demonstrate the dose distributions from stereotactic treatments both from dedicated multisource cobalt units and from linear accelerators (154–161). A clear benefit of gel dosimeters is that they can display a dose distribution, especially a highly conformal one as is produced by stereotactic radiosurgery techniques, so that it can be appreciated qualitatively in three dimensions without need of imaging systems or processing (see Fig. 12, Ref. 161).

In one series of measurements, gels were prepared in glass flasks chosen for their size and shape, which was comparable to that of a human head. Additional polymer gel material from the same batch was prepared in glass test tubes, for irradiation to selected doses, to generate a doseresponse curve. The gels were prepared in Guilford, CT, and were shipped to Lexington, Kentucky for irradiation and analysis (161).

A gel prepared in a 16 cm diameter flask was fitted with a radiosurgical head frame (Leksell, Elekta Corporation, Atlanta, GA), as shown in Fig. 13. This flask was also equipped with a glass rod extending to near the center of the flask, to be used as a target. The MR images were obtained and were transferred to a Gammaknife treatment planning computer (Gammaplan; Elekta Corporation), where a complicated dose distribution was planned using multiple target points. Once the plan was completed, the coordinates of the individual target points were determined, and the gel was moved to the Gammaknife irradiation unit. Treatments were delivered to each of the target points, in accordance with the treatment plan. A dose of 10 Gy was delivered to the 100% isodose line.

Dosimetric imaging of the flask and test tubes containing gel was performed between 25 and 36 h after irradiation. The flask was placed in the head coil of the imager and the test tubes irradiated for calibration purposes were placed around the flask. The images were transferred via network to a Macintosh computer, and the DoseMap program was used to compute the maps of transverse relaxation rate (R2).

Figure 13. Photograph of a glass flask filled with the BANG Polymer Gel dosimeter. A glass rod was inserted into the gel to provide a target around which to localize the dose distribution. The flask was fitted with a Leksell stereotactic head frame. The gel is shown as it appeared following irradiation. (Photograph by the author).

A dose-response calibration curve was obtained as described earlier. Images of the gel-filled test tubes were obtained, and R2 determined as a function of dose.

The calibration curve was then applied to R2 maps of the flask irradiated with the Gammaknife unit. The result yielded an image of the dose distribution, as shown in Fig. 14a and 14b. As all scans were performed with the head ring and localizer box in place, the coordinates of the image plane could be determined. These image planes were located 1 mm from each of the corresponding treatment plans shown in Fig. 14a and 14b. Finally, isodose curves were drawn (by the DoseMap program) by interpolating within the measured dose distribution.

The measured dose distributions were compared with the treatment plans prepared prior to irradiation by superimposing the two data sets. The superimposed data are shown in Fig. 15a and 15b. The calculated and measured dose distributions were registered by aligning the point representing the tip of the glass rod.

The measured dose distributions compare favorably with the calculated dose distributions. In fact, the dose

Figure 14. The R2 maps obtained from the irradiated gel

(a) Distribution in the axial plane. (b) Distribution in the sagittal plane. (Reproduced with permission from Ref. 161.)

Figure 15. Composite figures showing both the treatment plan prepared using a Gammaplan treatment planning computer (drawn in black, labeled in percent of maximum dose) and isodose curves measured by the technique described in the text (drawn in gray, labeled in Gy). (a) The distribution in the axial plane containing the 8 isocenters. (b) The distribution in a perpendicular sagittal plane. (Reproduced with permission from Ref. 161.

map taken in the plane of the target points (Fig. 14a) indicates regions of overlap not demonstrated by the treatment planning system. As shown in Fig. 15a and 15b, the measured isodose lines conform in shape quite well with the calculated data, but seem to show a shift away from the glass target rod. The dose images were obtained in planes that were shifted 1 mm from the planes of dose calculation, and this shift might account partially for the difference in size and shape of the isodose curves. However, Fig. 15a shows a shift in the lateral (X) direction away from the glass target rod, which cannot be explained by a difference in the axial (Z) coordinates of the planes of calculation and measurement. Instead, it appears more likely that the dose distribution was placed 1 mm further from the glass target rod than intended.

Evaluation of Repeat-Fixation Stereotactic Radiotherapy.

In recent years, fractionated stereotactic radiation therapy has been seen as a desirable method of delivering high dose radiation therapy to malignancies of the brain. Techniques developed for immobilizing the patient have also been applied more recently to intensity-modulated radiation therapy, in which conformal dose distributions are delivered through multiple fractions to one or more target volumes. In both techniques, reproducible positioning of the patient is critical, to ensure that the target volume receives the intended dose, and normal tissues are spared to the extent determined by treatment planning techniques. The BANG gel dosimeter has been used in a fractionated regimen to demonstrate the reproducibility of multiple setups under stereotactic position methods (158).

Intensity-Modulated Radiation Therapy (IMRT). Gels dosimeters have proven themselves to be valuable for evaluating and confirming IMRT dose distributions (146,162–169). Most investigations have been conducted in simple geometric phantoms (Fig. 16), but others have employed anthropomorphic phantoms in arrangements that allowed direct comparison with measurements using other techniques such as film and TLD (163,165,166).

Figure 16. A cylindrical flask containing a normoxic gel shortly after irradiation with an IMRT treatment. The dose distribution is clearly visible, demonstrating the change in optical density with dose. (Reproduced from Ref. 167, with permission.)

Beach developed a gel insert for an existing anthropomorphic phantom that had been developed with film and TLD dosimeters (170). The phantom design revision included converting the existing imaging/dosimetry insert from a block-style design to a cylindrical design (Fig. 17). This insert contained embedded structures that simulated a primary and secondary target volume as well as an organ at risk (OAR). An additional insert was then constructed to house the polymer gel dosimeter. This insert was specially designed using Barex plastic. Both the imaging insert and the gel insert had an image registration system incorporated into their construction.

Figure 17. An anthropomorphic head-and-neck phantom developed by the Radiological Physics Center (170) showing the modifications made to accommodate a gel dosimeter.

492 RADIATION DOSIMETRY, THREE-DIMENSIONAL

Figure 18. (a) A calculated dose distribution for an IMRT treatment, shown in a gray-scale format. (b) The measured dose distribution obtained from optical CT of a polymer gel, following irradiation with the treatment plan shown in (a). (From Ref. 165, with permission.)

X-ray CT images were obtained of the phantom with the imaging insert in place, and an IMRT treatment plan was developed. The phantom was then taken to the linear accelerator, the imaging insert was replaced with the gel insert, and the IMRT treatment was delivered.

A commercially available optical computed tomography (OCT) scanner (107) was commissioned for this project and future work with polymer gel dosimetry. The OCT scanner was used to image polymer gels before and after being irradiated. The preirradiation images were subtracted from the postirradiation images using a pixel-by-pixel subtraction method. The resultant images had net OD values that were directly proportional to the dose received by each given pixel. A comparison of the calculated dose distribution and the measured distribution is shown in Fig. 18.

Repeated measurements showed that a polymer gel imaged with optical CT was reproducible to within 1% (171). Repeated OCT imaging was shown to be consistent to within 1%. However, the results also showed that the techniques used to calibrate the gel (irradiation of a similar gel container with small-diameter beams delivering doses spanning the expected range) did not provide absolute dose measurements offering better agreement than 10% with the calculated data.

Duthoy compared the dose distribution measured with gels to the calculated distribution, for complex intensitymodulated arc therapy (IMAT) treatments in the abdomen (172). Vergote also examined IMAT with gels and observed a reproducible difference between calculations and measurements in low dose regions near steep dose gradients; a phenomenon also observed by Cadman et. al. and attributed to the failure of treatment planning systems to model the transmission of radiation through the rounded ends of multileaf collimator leaves (169,173).

Brachytherapy. Determining dose distributions and confirming the results of planning for brachytherapy treatment is historically difficult. No suitable methods of dosimetry have existed in the past to enable measurement and display of these 3D and complex distributions. Measurements around single sources have been possible only in a point-by-point fashion, such as with small ionization chambers or with thermoluminescence dosimeters

(TLDs), (174) or in planar fashion with film (175). These methods are quite unsatisfactory for anything other than distributions around single sources, or very simple source arrangements. In contrast, the BANG polymer gel dosimetry system has the capability to measure and display complex dose distributions from complicated source arrangements. It is necessary to immerse the applicator containing the sources into the gel, or arrange for its introduction into a catheter already placed in the gel.

The ability of gels to record and display dose distributions around a high dose rate (HDR) source was first demonstrated over a decade ago (92,176,177). Maryanski et al. showed the dose distribution around a single catheter into which a high dose rate (HDR) remote afterloader source had been positioned (178). The HDR unit was programmed to dwell the source at several locations in the catheter, to deliver an elliptical dose distribution. After irradiation, the gel was imaged with MR, and a map of the dose distribution was computed. The map is shown in Fig. 19, where the color intensity is proportional to dose. Isodose lines, determined from the dose map data, are superimposed on the intensity map. Points at which the dose was computed by the treatment planning system also are shown. Excellent agreement between the position of the calculated dose points and the corresponding measured isodose lines indicates the agreement between doses measured by the gel and computed by the treatment planning system.

More recently, measurements have been made in close proximity to HDR 192Ir sources (117,179) (see also Fig. 9). These measurements have shown that complications occur when measurements are made in the steep dose gradients close to an HDR source. Polymerization of the gel causes an increase in the gel density and a corresponding decrease in the volume filled by the gel. The change in density causes shrinkage of the gel in the vicinity of the source, distorting the resulting measured distribution. Changes to the composition of the gel to increase the concentration of gelatin

Figure 19. Use of the BANG gel to measure the dose distribution around an HDR source. The source was positioned in a catheter implanted in a BANG polymer gel. The figure illustrates a comparison between the dose distribution determined from a MRI image of the gel and the calculated dose distribution. (From Ref. 178.)

can mitigate the amount and effects of the density changes. Furthermore, there are suggestions that the high dose rates found near brachytherapy sources, particularly those of HDR afterloaders, can introduce temperature gradients that influences the polymerization of acrylamide monomer gels (87,93,180,181).

Efforts also have been made to characterize low dose rate (LDR) sources, such as prostate seeds (182–184), eye plaques (185), 137Cs afterloading sources (186,187) and intravascular sources (188). Studies have indicated that the diffusion of monomers (or ferrous and ferric ions in Fricke gels) across steep dose gradients can introduce errors in measurement (92,189). As the use of gels to measure dose distributions from LDR sources requires long exposure times, diffusion of monomers or ions could introduce significant errors, and gels exhibiting high diffusion rates should be avoided for these measurements.

A further problem withgel dosimetry for LDR brachytherapy has been demonstrated by recent studies indicating energy dependence at lower energies. Data show that a polymer gel dosimeter under responds to radiation in the 20–60 keV range (190). Others have shown differences in gel response from one formulation to another, and suggest that the MAGAT gel is most water-equivalent over a wide range of energies (191). Changes in mass attenuation coefficient of polymer gels during irradiation can also introduce errors in the dose distributions measured around low energy sources.

Internal Dosimetry

Gel dosimetry has shown promise in the determination of dose distributions from administrations of unsealed

radioactive sources (192). The authors embedded a vial of 131I into a flask of polymer gel and observed a distance-

dependent change in the T2 signal. They also mixed 131I into the gel and demonstrated a change in T2 signal that was dependent on distance from the concentration of activity. No more recent investigations have been located.

Measurement of Neutron Dose Distributions

Some developments have been reported in characterizing fast and epithermal neutron beams with gel dosimetry (193–195). Thin layers of Fricke-xylenol orange gels have been irradiated in phantoms composed of insensitive gel. Adding 10B or other nuclides with large cross-sections has increased the sensitivity of the gel dosimeter to neutrons. This technique has been used to determine the profiles of neutron beams used for boron neutron capture therapy. Some benefits of the use of gel dosimetry are the tissueequivalence of the dosimeter to these energies, and the ability to separate the components of dose.

Measurement of Particle Dose Distributions

Several investigators have demonstrated the use of polymer gel dosimeters to record the dose distributions produced by proton beams (88,137,196–198). However, several authors have noticed disagreements between measurements with gels and conventional dosimeters such as diodes in the peak region of the distribution. Gustavsson has suggested that the response of gels, as they are based

Figure 20. The variation in LET as a function of depth for a monoenergetic proton beam (dashed curve, left-hand scale) and the measured relative sensitivity for the gel dosimeter (full curve, right-hand scale). Also shown is the depth dose curve for the proton beam (dotted curve), normalized to 100% at the Bragg peak. (Reproduced with permission from Ref. 198.

on the formation of free radicals, is dependent on the LET of the radiation (197,198). As the LET of the beam increases in the peak region, the local ionization density increases. As the distance between the radicals formed in the gel decreases, the likelihood of recombination of radicals increases. A decrease in the production of radicals with increasing LET has been described previously (199). Consequently, significant differences appear between depth dose measurements with gels and those with detectors such as diodes (see Fig. 20, Ref. 198).

Jirasek et al. performed track energy-deposition calculations and raman spectroscopy and reported agreement between these techniques and gel measurements (137). Their conclusion also was that the high density of deltaray interactions close to the track of a proton resulted in high doses being delivered to the gel. These doses saturated the response of the gel by consuming the available monomer. This effect was greater near the end of the proton range, consistent with the results of other authors.

Gels have been used also to demonstrate the dose distribution produced by 12C ions (200). Similar effects associated with decreased radical formation at high LET were observed in the carbon beam.

Evaluation of Tissue Heterogeneities

A valuable feature of gel dosimeters is that they are very nearly tissue-equivalent, particularly at photon beam energies above 100 kV. Previous investigations have shown that the BANG gel, the MAGIC and MAGAS normoxic gels, as well as gels based on Fricke or vinyl solutions have electron densities within 1% of soft tissue, and effective atomic numbers in the range of 7.4 (190). However, several investigators have attempted to measure the effects of nonunit density tissues on external beam dose distributions. Early measurements were performed to estimate the dose distribution behind high atomic number heterogeneities, to simulate the presence of bone (201– 204). More recently, measurements have been made behind or adjacent to cavities filled with air or with lung-equivalent plastic (168). To attempt a measurement

494 RADIATION DOSIMETRY, THREE-DIMENSIONAL

in lung-equivalent gel, Olberg produced a foam of gel with the approximate density of lung tissue (205). Other investigators have evaluated the promise of gel dosimeters to simulate lung tissue, by introducing polystyrene foam beads into a gel mixture (206). While these measurements showed promise, there were several sources of error. First, the introduction of air, or air-containing polystyrene beads introduced the possibility of oxygen contamination. Purging the polystyrene beads with nitrogen, or using nitrogen rather than air to foam the gel addressed this problem. The introduction of air or polystyrene eliminated the possibility of evaluating the measured dose distribution by optical scanning, and MR imaging must be used. The presence of air may lead to partial volume imaging effects that could introduce errors into the measurement.

COMPLICATIONS TO BE ADDRESSED

As was suggested earlier in this article, there are a number of complications associated with gel dosimetry that remain to be addressed, and that are inhibiting the routine use of gels in the clinic. Some of these are listed below, with short descriptions of the causes of the problems, and possibilities for correcting them.

Imaging Artifacts

This article has discussed several methods of generating images of dose distributions using gels. Principal methods are MRI, OCT, and X-ray CT. Each of these imaging methods is prone to imaging artifacts, although the type of artifact and its causes are different with the different modalities. In MRI, for example, susceptibility artifacts can result from variations in the conductivity of the volume being imaged, and interference is likely when multiplanar imaging of adjacent planes is attempted. The presence of air or low-density structures can lead to partial volume effects or susceptibility artifacts.

In OCT, any structure that blocks the light beam is likely to cause a streak artifact, similar to those produced by high densities in X-ray CT images. In addition, the refraction of the light at interfaces between the gel and other materials can cause ring artifacts or distortion of the image. The artifacts found in OCT images have been described (110). An example of the artifact caused by high optical densities is shown in Fig. 21.

When X-ray CT is used, artifacts can result from the low signal to noise ratio that occurs because of the very small density differences present in the gel. These artifacts have been investigated in some detail previously (121).

Temperature Dependence

The existence of a dependence on temperature during irradiation of polymer gels was not recognized immediately, but it has since been shown that this dependence exists. Furthermore, the temperature dependence can be more pronounced for some polymer gel formulations than others. The polymerization that occurs as a result of irradiation of the gel is exothermic, and consequently can lead to a temperature rise that influences further polymeriza-

Figure 21. An optical CT scan of a normoxic gel irradiated with a low dose rate 125I brachytherapy source. The high optical densities close to the source completely attenuate the laser, resulting in a streak artifact.

tion of gel in response to continuing exposure. In extreme cases, this temperature rise can exceed several degrees Celcius (207).

Oxygen Sensitivity

The sensitivity of polymer gels to oxygen has been discussed extensively, and several investigators have responded by developing gels that contain oxygen scavengers, such as the MAGIC gel (86). The oxygen scavenger removes oxygen present in the gel at the time of manufacture, even if this is done in normoxic conditions. It can remove additional small amounts of oxygen, but ultimately will be overwhelmed if exposure to normal atmosphere is ongoing. While this problem has been addressed, it still creates minor inconvenience that might limit the successful introduction of gels into routine clinical use. The characteristics of several normoxic gel dosimeters have been investigated in detail (90,208).

Tissue Equivalence and Energy Dependence

Gels, both Fricke and polymer types, compete well when compared to other dosimeters in terms of their tissue equivalence and energy dependence. In comparison to thermoluminescence dosimeters (TLD), radiographic film, and even ionization chambers, for example, gels are considerably less energy dependent and are much more tissue equivalent (209). However, under extreme conditions of photon energies below 60 keV, and LET values greater than 2.5 keV m 1, gels show a dependence that remains to be fully characterized (190).

Simulation of Nonunit Densities

The benefits of gels discussed in the previous paragraph lead to the inability of gels to easily simulate nonunit density tissues. To date, limited efforts have been described to create low density gel mixtures, to simulate lung tissue. No attempts have been described to date to create high density mixtures and are unlikely to be with today’s emphasis on the use of gelatin-based formulations.

Diffusion of Monomer in Steep Gradients

The diffusion of monomer, and the shrinkage of gel proportional to the creation of long polymer chains, can be addressed through the development of better gel mixtures. Avoiding small monomers such as acrylamide can reduce the rate of diffusion in regions of steep dose gradient, such as the penumbra of radiation beams (210). Employing different concentrations of gelatin might reduce or eliminate problems associated with shrinkage of gels in high dose regions. Some normoxic gels may demonstrate decreased diffusion in regions of steep dose gradient (90,208).

SUMMARY AND CONCLUSIONS

The importance of quality assurance in radiation therapy is well documented. High quality, safe, and effective radiation therapy is dependent upon the proper operation of equipment, the accuracy of alignment devices, and the dependability of dosimetry procedures. The accurate delivery of the prescribed dose depends on procedures involving dosimetry systems. Properly functioning dosimeters are necessary to assure that the equipment is properly calibrated and that treatment planning procedures are conducted correctly.

A wide variety of dosimetry systems are available to medical physicists today. Choosing the correct dosimetry system for any given application requires an understanding of the operation of the device and its appropriateness for the intended circumstances. This presentation has reviewed a number of dosimetry systems presently available, their design and operation, and some of the uses for which they are valuable.

Gel dosimetry offers the promise of accurate and convenient dosimetry under a variety of circumstances. In most of the examples discussed above, gel dosimeters offer a number of advantages over conventional dosimeters. Chief among these is the ability to measure a complex dose distribution throughout a volume with a single radiation exposure. Additional advantages include tissue equivalence, high spatial accuracy, good dose precision, and reasonable convenience.

However, gel dosimetry continues to experience little acceptance in the clinic, largely because some aspects of promise have not been achieved, and because of a perceived lack of convenience. Members of the radiation physics community are apparently not convinced that the benefits of gels sufficiently outweigh conventional dosimeters such as film and TLD. It is incumbent on those of us working with gels to encourage more widespread use, by taking every opportunity to display the results of measurements with gels.

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