• State the reason why the Environmental Protection Agency (EPA) sets limits for radioactive contamination.

fore radioactive isotope of the element iodine. It has been used quite extensively since 2000 in the form of titanium­encapsulated cylindrical seeds (4.5 mm long and in cross­section about the diameter of an apple seed [Fig. 16.1]) to give a tumoricidal radiation equivalent dose to cancers that are confined within the prostate gland. With the aid of computerized treatment planning and real-time ultra­sound imaging, the seeds are permanently inserted into the gland in a calculated prescribed arrangement. The goal is to deliver 145 gray (Gy) to at least 90% of the prostate’s volume while limiting radiation dose as much as possible to adjacent structures such as the urethra, bladder, and anterior rectal wall. The insertion process is done in the operating room and typically takes about 2 hours. This is a same-day procedure, and the patient is usually discharged within 4 to 5 hours afterward.

the inner-electron shell vacancy by an outer low-energy electron. The nucleus now has one less proton, and thus the decay process has led to the formation of a different element called tellurium-125 (

where the • refers to an excited state and the symbol g signifies a gamma ray.

Because these radiation emissions have little penetrat­ing power, a very high percentage of the radiation energy remains concentrated in the prostate gland. Essentially all of the remaining radiation is absorbed by the patient ex­cept for some small but detectable amount on the pa­tient’s physique, which emerges. At a distance of 1 m, the radiation exposure rate for virtually all prostate seed im­plants is less than 0.5 milliroentgens per hour (mR/hr), increasing, however, to 15 to 20 mR/hr at the patient’s lower abdominal surface. In terms of international or SI units, these values correspond to equivalent dose rates of approximately 5 microsieverts/hr at 1 m and 150 to 200 microsieverts/hr at the abdominal surface.

(beta decay*), it generates both electrons with an ap­proximate mean energy of 192 keV and relatively high­energy assorted gamma rays (mean energy of 365 keV).

the chemical compound sodium iodide (NaI

), which is radioactive and can be orally administered in the form of tablets. For a patient who has thyroid cancer, it is desirable to strongly irradiate any residual thyroid tis­sue not removed by surgery to destroy any remaining cancerous areas while significantly sparing surrounding tissue and other organs. Because the thyroid gland tends to highly absorb any iodine in the blood, administration

of

I-labeled sodium iodide tablets is an efficient way of delivering a destructive radiation dose to the remain­der of the thyroid. The relatively low-energy electrons mainly cause the destruction.

conducted at that time. If any activity readings above background levels are found, the disposal must be de­layed until the reading is at background levels. A record of the storage and disposal of the residual radionuclide and contaminated items must be kept for review by regulatory inspectors.

56 neutrons. The beta decay of molybdenum produces technetium in a higher energy state than its ground state, and as a result it, too, is not stable. Most isotopes generated in this manner immediately shed their excess energy. However, some delay in doing so for a short period, and these relatively more enduring isotopes are given the designation m, which stands for metastable (meaning “more lasting”).

In Chapter 3, in which the pair production interaction is discussed, a diagnostic modality called positron

though this modality does not require the occurrence of x-ray induced pair production interactions, it does make use of annihilation radiation events. In the case of PET, the annihilation radiation is initiated by the ra­dioactive decay of the nucleus of an unstable isotope. The instability in this case is associated with too many protons residing within the nucleus. Nuclei such as

these usually spontaneously undergo a reaction in which the excess proton is transmuted into a neutron and a positively charged electron (positron). This con­serves electric charge because the neutron has none. To conserve energy as well, the process includes the emis­sion of an additional particle called the neutrino. A neutrino has no electric charge and negligible mass, but its energy of motion (kinetic energy) balances the energy of the reaction. Neutrinos very rarely interact with anything and are therefore exceptionally difficult to detect.*

essence a facility gains not only the ability to detect the presence of abnormally high regions of glucose metabo­lism, yielding evidence of cancer spread (metastasis) into other body areas, but also the means to obtain de­tailed information about the anatomic location and extent of these lesions or growths. Such a combined imaging device is called a PET-CT scanner (Fig. 16.2).

shielding at close distances could require up to 2.5 cm or an inch of lead. Therefore, the design of a PET-CT imag­ing suite is complex, involving significant radiation safety concerns. A discussion of the radiation safety design dif­ficulties concerning the PET-CT technologist for an oc­cupationally permissible facility follows.

high-energy annihilation photons emanating in all di­rections from the patient while having the PET-CT scan. Furthermore, the presence of yet a third source of radia­tion, also of high energy, must be considered. Every pa­tient who is to have a PET-CT scan requires what is called a “prep” time. During this time 18F, in the form of FDG with an initial activity usually of approximately 15 millicuries* (555 megabecquerels** [MBq]), is in­jected into the patient. The patient typically remains in the injection or prep room in a semireclining position for 45 to 60 minutes so that the FDG gets distributed throughout the body. This implies that to have a smooth-flowing, coordinated patient throughput, it will be necessary that while one “hot” patient is being scanned, a second “hot” patient is reclining in a nearby room waiting to be scanned. Therefore, during this pro­cedure the technologist and other personnel, as well as the general public, must be protected from at least two sources of high-energy photon radiation in addition to the scatter x-radiation from the CT scanner. Also, the change of activity during the prep and scan times due to the short half-life of most PET isotopes should be taken into account.

For simplification, just the requirements for the satis­factory radiation protection of the PET-CT radiographer in a new PET-CT suite will now be considered in detail. A workload for a busy facility with one scanner could be seven PET-CT patients per daily shift, which amounts to 35 patients per work week, with each patient receiving at the start of prep time an activity of 15 millicurie (mCi) of