Treatment planning

28 The successful treatment of cancer with x rays requires that the tumor and all of its microscopic extensions into normal tissue receive a radiation dose sufficient to kill the cancer cells. At the same time one must avoid the serious complications that result from overexposing nearby normal tissue.

29 The patient's survival probability as a function of radiation dose is the blue curve in figure 4. If the dose is low, the tumor is likely to recur. At high doses, on the other hand, the therapy itself is likely to cause unacceptable compli­cations. Between these extremes the physician seeks to maximize the patient's survival of both the cancer and the treatment. But if the patient's discomfort is too severe at the radiation dose D_opt that optimizes the product of the two competing factors, the physician may choose a somewhat lower dosage.

30 Radiation dose optimization in cancer therapy de­mands careful delineation of the margins of the tumor. That requires good medical imaging. The use of planar radiographs had several limitations. It was difficult to visualize the tumor clearly, and difficult to transcribe the radiographic information to the cross-sec­tional plane used to design the treatment plan.

31 CT and, more recently, magnetic resonance imaging have largely overcome these difficulties. Many treatment centers have a CT unit dedicated to treatment planning and monitoring. The images not only provide a cross-sectional representation of the patient's internal anatomy, but also yield an accurate representation of the body contour and, often, excellent visibility of the tumor and surrounding normal tissues. Because the data are digital, they can be entered directly into the treatment-planning computer so that proposed treatment plans can be super­imposed directly onto the CT images. Then the plan can be implemented on the treatment machine.

32 For such a process to succeed, however, the geometry used to acquire the CT data must correspond precisely to the patient's setup for each of the subsequent treatments. That is accomplished by laser alignment employing reference marks on the patient, and then periodic verification with a treatment simulator constructed to reproduce the proper geometry. The medical physicist must closely monitor the alignment of radiation-dose delivery with the tomographic input data and the computed treatment plan.

Figure 3

Digital radiographic system creates digital x-ray images that can be displayed in real time or stored. X rays that traverse the patient unscattered are recorded at the digital receptor screen and the data are passed on for processing and display. Scattered x-ray photons are kept from the receptor screen by an antiscatter grid.

Tomographic therapy

33 Most applications of x-ray CT information to radiation treatment planning rely on a commercial CT scanner and various external techniques to maintain the alignment of the scanner and the accelerator with the treatment plan. But a number of medical physicists have built CT scanners on treatment-simulator gantries. Usually such units compile image pro­jections by digitizing the output from a television camera that monitors the transmitted x-ray intensity imaged on an image-intensifier screen. Although these units do not provide resolution comparable to that from a diagnostic CT scanner, they duplicate the treat­ment geometry and yield images good enough for treatment planning.

34 The images from a CT unit are gray-scale representations of the ma­trix of attenuation-coefficient values across a planar section through the tumor. If the matrix could be meas­ured at the x-ray energies used in treatment, then the dose distribution could be corrected for the presence of nonhomogeneous structures in the ir­radiated tissue, and patient alignment could be monitored during the treat­ment. Several groups have success­fully developed detectors and com­puter algorithms to accomplish these objectives with megavolt treatment machines.

35 Most radiation treatments nowa­days involve the use of multiple fixed radiation fields converging on the tu­mor from different angles. Thus the dose can be concen­trated in the tumor while much lower doses are delivered to surrounding normal tissues. In many cases one can get an even better dose distribution by continually rotating the gantry of the x-ray accelerator around the patient during treatment so that only the tumor is constantly in the path of the beam. But this rotating-beam therapy is complicated by the fact that malignant tumors are noto­riously asymmetrical. Therefore the size of the radiation field has to be varied continuously as the beam direction through the tumor changes. The dose distributions could be improved even further by also varying the beam inten­sity during the rotation. This approach, called conformal therapy, is being pursued in several centers in the US and overseas. It requires detailed three-dimensional knowl­edge of the anatomy of the irradiated tissue. It also requires exquisite computerized positioning control of the accelerator, the gantry and the patient's couch.

36 A significant advance in conformal therapy would be the convergence of CT and megavoltage therapy in a single gantry, so that tomographic images could monitor align­ment and dose distribution continually during treatment. This hybrid approach remains to be developed in full. It presents a number of challenges, but the potential benefit is considerable.

37 The modern uses of x rays for clinical imaging and therapy are a prime example of “high-tech” medicine.They illustrate the contributions of physicists working with physicians to improve diagnosis and treatment. Much remains to be done, especially in refining the growing convergence of imaging with therapeutic irradiation. Medical physics will continue to other physicists many opportunities to apply their special knowledge to the healing arts.

IV. Reading for General Understanding.

1. Check the comprehension of the text “X-rays in Medicine” by choosing the answer,

which you think, is correct.

1) Half a million cancer patients each year receive x-ray treatments, about half of them

for curative purposes and the rest for pain relief.

1. yes,it’s true

2. it is mostly true. I think it’s more the than half a million.

3. No, it is not so. X-ray treatments are used only for pain relief.

2) The development of noninvasive imaging techniques of various kinds started in

Röntgen’s physics laboratory a hundred years ago.

1. I think this development started more than a hundred years ago.

2. No it’s not true. At that time there was imaging with radio-isotopes.

3. Yes, that’s right.

3) A major breakthrough was achieved in 1972 with the introduction of law-energy x-rays.

1. You are quite right.

2. No it is not so. The major breakthrough was achieved in 1982.

3. You are mistaken. The major breakthrough was achieved in 1972 with the

4. introduction of x-ray transmission computed tomography.

4) Film images are bulky to store and they must be transported physically from one

location to another.

1. You are mistaken. Film images are quite small and they are easy to store.

2. No, you are not right. Film images are bulky to store, but they are transmitted

by cables.

3. You are right. Film images are transported physically and they are bulky to store.

5) The beam may be scanned as a point or line across the patient.

1. The beam is not scanned. It is fixed.

2. The beam may be scanned under the patient.

3. The beam may be scanned across the patient as a point or line.

6) Dose rates are quite limited and the beam margins are rather unsharp.

1. You are mistaken. Dose rates are limitless and the beam margins are rather

unsharp.

2. You are not right either. Dose rates are quite limited and the beam margins are

rather sharp.

3. I think, I am right. Dose rates are limited and the beam margins are rather unsharp.

7) The use of implanted radioactive sources is an elaborate process requiring care

to protect the patient and attending personnel from excessive exposure

1. You are wrong. The use of implanted radioactive sources doesn’t require care

2. to protect the patient and attending personnel.

.

3. The use of implanted radioactive sources is a simple process that requires to

protect neither the patient nor attending personnel.

4. The use of implanted radioactive sources requires care to protect the attending

personnel and patient and it is not a simple process.

8) The use of planar radiographs had several limitations.

a) You are wrong. The use of planar radiographs had no limitations.

b) You are mistaken. It is the use of x-ray technique that has several

limitations.

c)You are quite right. It is the use of planar radiographs that has several

limitations.

9) Because the data are digital they can be entered directly into the treatment-

planning computer

a) Digital data cannot be entered directly into the treatment-planning computer

because they have several limitations.

b) Only digital data can be entered directly into the treatment-planning computer.

c) It is analog data that can be entered directly into the treatment-planning computer.

10)Therefore the size of the radiation field has to be varied continuously as the

beam direction through the tumor changes.

1. You are quite right. Since the beam direction through the tumor changes,

the size of the radiation field must be varied continuously.

2. You are mistaken. As the beam direction through the tumor does

not change, there is no need to vary continuously the size of the radiation field.

3. You are wrong. As the beam direction through the field remains fixed

therefore the size of the radiation field has to be varied continuously.

2. Pick out from the text all the word combinations with the following words (terms)

and give their Russian equivalents.

- image - effect

- imaging - radiation

- resolution - therapy

- scattering - accelerator

V. Reading for Detail and Language Study.

1.Find in the text “X-rays in Medicine” the English equivalents for the following

phrases: “Sectional imaging”

- методы аналоговой томографии

- трёхмерное распределение

- конкретные области применения

- преодолеть это ограничение

- предача данных под определённым углом

- основной прорыв

- сигнал с детектора х лучей

- коэффициент затухания

“Digital Imaging”

- фотографическая плёнка

- преодолеть эти ограничения

- цифровые технологии

- различные методы сбора данных

- обработка изображений

- постоянный поток фотонов

- цифровые методы

- режим диагностики в реальном времени

- дистанционная передача изображения

“X-ray Therapy”

- лечение

- кожа пациента

- гамма лучи

- линейный ускоритель стоячей волны

- свойства разрушения тканей

- лучевая терапия

- вживлённый радиоактивный источник

“Treatment Planning”

- доза облучения

- магнитный резонанс

- недопустимые осложнения

- плоскость поперечного сечения

- планы лечения

- ЭВМ, планирующая лечение

- аппарат для лечения

- опорные метки на пациенте

" Tomographic Therapy”

-разрешающая способность

-интенсивность луча во время вращения

-на пути луча

-распределение дозы облучения

- входные данные для томограммы

2. Explain the meaning of the following words and expressions.

• Noninvasive imaging techniques.

• X-ray energy

• Planar x-ray images

• Implanted radioactive sources

• CT scanner

• High-Tech medicine

3. Translate the text “Tomographic therapy” in written form.

VI. Oral Practice

1. Answer the questions on the text “X-rays in Medicine”

a) In what way are ordinary planar x ray images formed?

b) What happens to x rays while traversing the patient?

c) What forms a two-dimensional; projection of the transmission of x rays and when?

d) What kind of interactions predominates in x ray energies employed in medical imaging.

e) What energies do clinicians use for chest x rays.

6. Name different kinds of resolution and describe each of them.

2. Speak on the following:

1. Disadvantages of Cobalt-60.

2. Linear accelerators used for tumor treatment.

3. Implanted radioactive sources.