5. Applied Physics for Radiation Oncology

5. Applied Physics for Radiation Oncology - Applied Physics...

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Unformatted text preview: Applied Physics for Radiation Oncology Robert Stanton and Donna Stinson [Publishen Medical Physics Pubiishing Corporation (January 1996), ISBN-13: 978-0944838600] pages: 267 — 279. '6 Brachytherapy 0 Introduction 9 Radium o Radium Substitutes i Radioactive Sources * Permanent Implants ¢ Implant Dosimetry t Remote High-Intensity Afterioading . Specific Implant Techniques ¢ Radiation Safety with Implants o Applicators o Afterioading i Single Plane, Double Plane, and Volume Implants INTRODUCTION Previous chapters have dealt with teletherapy (external beam radiation therapy), including the design of equipment and the method of treatment planning. In teletherapy, fi‘actionated doses of 120 to 400 cGy are given multiple times sepa- rated by many hours or days in order to obtain the optimum therapeutic effect. This chapter will discuss brachytherapy, a technique in which sealed sources of ionizing radiation are placed within the patient, in or close to the diseased tis- sues. In contrast to teletherapy, brachytherapy sources may be left in place for a limited period of time to irradiate the tissues. Brachytherapy uses the rapid fallofi' 0f radiation intensity produced by the inverse square law to keep the dose to dis— tant healthy tissues low while giving high doses to the tumor. Depending 0n the half—life and activity of the material used, sources may be removed after a few minutes (HDR), days (LDR), or remain in the patient (permanent implants). S‘i—o no u 268 Applied Physics for Radiation Oncology The first uses ofbrachytherapy, suggested independently by Pierre Curie and Thomas Edison, were actually some of the most successful examples of radiation therapy of that time. By 1905, centers around the world were implanting radium tubes within tumors for limited periods of time and curing them. RADIUM 236'Radiurn is a radioactive element with a very long half—life (T1 ,9 of 1622 years, It is found in uranium deposits and is actually a decay product of the element mU (T1,,2 = 4.52 X 109 yr). Radium decays through 11 intervening steps (called a decay series) to a stable form of lead, 2“EPh. In the process, it emits alpha parti- cles, beta particles, and gamma rays. The alpha and beta particles have no clinical use because of their short range. The gamma rays emitted in the radium decay series, however, are of high enough energy (Em = 0.8 MeV) to penetrate deeply into tissues. RADIUM SUBSTITUTES Radium has been largely replaced in recent years by other radioactive nuclides because of its chemical properties and radiation properties. Chemically, radium behaves like calcium; therefore, any inadvertent leakage of the isotope from its sealed source can result in deposition in bone and a consequent high radiation Table 16. 1 Radionuclides Nuclide Efictive Photon Energy Mel/O Halfibfi F* 225Ra 024—22 1622 years 8.25 “CD 1.25 5.26 years 13.07 an 0.78 3.83 days 3.25 l"’7Cs 0.66 30 years 3.27 "’de 0.02 17 days 1.48 man 0.42 2.70 days 2.38 192k 0.37 74.2 days 4.64 all 0.023 60.2 days 1.451 13'I fi‘:0.3-0.3; 720.36 8.05 days 2-2 33P {311.71 max 14.3 days NA “Y [3123; 721.74 64 hours NA 9°51: [310.55 23.1 years NA *5pecific garruna ray constant defined at 1 cm &0m 1 mCi source: R 51113! mCi hr. NA = not applicable; no gamma emissions. i 2 E m {l 5 i—W_‘ Brachytherapy 269 Eyelets for thread Platinum case —-I‘- ' 7°13] Radium salt Total length length l . _ KM m Active Tube lengm Hive length Needle T" Figure 16.1 Radioactive sources- The diameter of the tube is about 3 to 4 mm, while that of the needle is about 2 mm. The total length includes the length of radioisotope (the active length) and the length of the welds and attachment points. dose. Radium’s high average gamma ray energy also results in high exposures to medical personnel working with the patient. Table 16.1 lists some commonly used radionuclides including some used in place of radium. RADIOACTIVE SOURCES Most of the radionuclides in table 16.1 are used in sealed sources. Sealed sources are illustrated in figures 16.1 and 16.2. In these sources, a specific amount of the isotope is encapsulated in two sealed layers of a heavy metal such as platinum or steel. The metal serves both to enclose the isotope and to prevent the dispersal of it or its daughter products and also to absorb any alpha and beta particles, if present. Figure 16.1 notes two lengths for the sources, the total length of the source (that seen on a radiograph) and the active length of the source. Sources are aVail— able in many different strengths. Their activity is usually between 0.1 and 30 mg radium equivalent‘ (or defined in terms of mCi; 1 mCi is equal to 3.7 X 107 Bq). 1. One milligram of 335R: is approximately 1 mCi. 270 Applied Physics for Radiaan Oncology Iridiummz\ % ere - Seeds Trocar in tube Figure 16.2 DiEerent configurations of 1931:. The wire or seeds in plastic ribbon are placed in the tumor using a hollow needle or trocar. The trocar is withdrawn fiom the tumor after insertion of the sources, leaving the sources behind. 193Ir is available in the form of flexible wires and seeds encased in 0.5 mm plastic tubes (also called ribbons), as shown in figure 16.2. Either of these forms can be easily inserted into patients. The technique used is to first implant hol— low needles, or trocars, into the patient’s tumor in surgery. Then radioactive wires or ribbons are threaded through the trocars. Afler the sources are in place, the trocars can be removed, leaving a somewhat flexible, less painfiil implant. Radi— ographs are taken to determine the location of the iridium, and the dosimetry calculations are performed. Intracavitary implants are those in which tubes are placed within body cav— ities such as the vagina and uterus. In interstitial therapy, needles are pushed directly into tumors that are accessible fiom outside the body, SuCll as those in the head and neck, the anus, etc. Both intracavitary and interstitial implants are limited to a few days, after which the sources must be removed. Such treatment is called temporary insertion or a temporary implant. APPLICATORS Radioactive sources are usually placed in applicators of various designs to main- tain the position of sources in the diseased area. Figure 16.3 shows several dif— ferent applicators often used with tubes in gynecologic cancer therapy. Many designs of gynecological applicators are available, each named after its designer. Delcos, Suit-Fletcher (sometimes called Fletcher—Suit), Heyman, Bmchytherapy 27 1 cervical clamp threaded cap \ hollow tube Figure 16.3a Tandem, intra~uterine applicator. This hollow device is inserted through the cervical as into the body of the uterus to radiate the surrounding tissues. The cervical clamp is placed at the location of the cervix. Radioactive sourCes are then afterloaded in the applicator cavity. Lateral View: Source 03” {inside ovoid) Anterior View: Sources (inside ovoids) Threaded caps Adjustable Source spacers rotating clamp (Ovoids) figure 16.3b Ovoids (or colpostats) are inserted into the vagina. They are usually used along with a tandem. but they may also be used alone. The ovoids and tandem are inserted into the patient with or without partial or general anesthesia. The radioactive sources are loaded after surgery and simulation. Body of applicator Screw I ' cap Sources in applicalor cavity Figure 16.3c press—sectional view ofa Burnett vaginal applicator. This device is pre—loaded and then Inserted into the patient's vagina for irradiation of adjacent tissues. 272 Applied Physics for Radiation Oncology Burnett, etc. are similar devices for positioning tubes in cavities. A tandem (fig. ure 16-3a) is a tube that is inserted through the us, the orifice of the cervix, into the uterus and into which a source can be afterloaded- Ovoids or colpostats (fig. ure 16.3b) are devices that, when placed in the cervical fornices of the vagina’ can hold sources in a good position to irradiate the tissues surrounding the cervix. Figure 16.3c shows a crossmsectional view of a Burnett vaginal applicator that is pre—loaded and then inserted into the vagina. Methods for calculating doses using these devices are well documented. Treatment with tubes in Suit—Fletcher applicators or other combinations of tandem and ovoids using the Manchester System has a long history. In this system the treatment parameters are the total number of milligrams of radium (or equiValent) in all the sources, their distribution in the tandem and Dvoids, and the total treatment time. The treatment unit is the milligram-hour of ufiRa. Currently, the standard dose unit is the cGy to a specific anatornical point or iso- dose line and is determined using tables of dose distributions around sources- The anatomical points usually used for cervical and uterine treatments are called points A and B. PointA is located two centimeters above the cervix in the uterus, two centimeters awayr fi'om the center of the uterine canal (cervical 05). Point B is defined as being 5 cm on either side of the nudline. Also of interest is the “pelvic side wall,” approximately 1 cm lateral to the medial aspect of the pelvic side wall. uterus tandem B A A B + + 1+ + cm \ ovoids cervical us figure 16.4 Illustration of points A and B, redrawn from T]. Godden. Brachytherapy 273 AFTERLOADING Because the sources used in brachytherapy are always “on,” they pose a signifiu cant radiation safety hazard for medical personnel. One approach to reducing un— necessary exposure is the use of the afterloading technique. In afterloading, empty applicators are placed in the patient and are carefiilly positioned with packing material (gauze, etc.) placed in the cavities along with the applicators to hold them firmly in position. The packing also pushes normal tissues away from the sources, reducing the dose to healthy tissue. Radiographs are taken (either in the operating room for verification, or preferably with a ra— diograpbic simulator for dosimetry, which is far more accurate) to verify and care— fiflly measure the position of the source applicators and document their location. During this process dummy or non—radioactive source replicas may be used to simulate actual source placement. Radiographs of the sources in place are criti- cal for determining the implant dosimetry. In order to aid dose calculations, pre— cise localization of the sources in three spatial dimensions is necessary. At least two radiographs are necessary for this. Usually two orthogonal films are taken at right angles (90°). Magnification factors are carefully determined for each film, often by placing a ring ofa known diameter on the approximate location of the implant. Once the source strength and dose distributions are determined, the pa— tient is ready for loading. The patient, who will be hospitalized for several days during therapy, is then returned to the hospital room, and the radioactive sources are inserted into the applicators. Afterloading techniques are also performed with radioactive wires or ribbons. Hollow plastic tubes are placed into the tumor through the trocars mentioned above. Then radiographs are taken with dummy sources. Real sources are inserted into the plastic tubes once dummy source positions are verified to be correct, and dosimetry calculations are performed. Interstitial iridium implants using the technique outlined above are often used to boost the dose to tumors already given with teletherapy beams. Examples are rumors of the breast, floor ofmouth, brain, lymph nodes, and gynecological lesions. SINGLE PLANE, DOUBLE PLANE, AND VOLUME IMPLANTS For a very small tumor, the insertion of a single small source might give an ac— ceptable dose to the tumor. Most clinical tumors, however, are at least a few cubic centimeters in volume and require more than a single source. Tumors that are thin in cross section can be treated with a singe row of sources, called a single 274 Applied Physics for Radiation Oncology Top View End View Iridium wires Flgure 16.5 Single plane implant. In this example, five strands of sources are placed through the tumor. On the left is a view along the strands; on the right, a View perpendicular to the strands. Note that each strand comprises four individual seeds. End View Edge View Iridium seeds \ Figure 16.6 Double plane implant. When large tumors are treated, volume implants comprising several planes of sources are used. plane implant. This is shown in figure 16.5. Larger tumors require more than one plane to treat the tissue adequately. Figure 16.6 shows two views of a dou- ble plane irnplant—one seeing the sources end—on, the other showing the source lengths. When more than two planes of sources are used, the implant is called a vol- ume implant. Even more complex techniques can be used with iridium sources. For example, templates (devices that guide the insertion of trocars) using up to 54 needles have been successfiflly used in the treatment of pelvic tumors. Brachythempy 275 Figure 16.? Examples of 1251 insertion tools. 115I seeds loaded into cartridges are placed into the gun, allowing quick introduction into the patient with minimal exposure to the operating room personnel. PERMANENT IMPLANTS When the treatment area is in a location that does not allow easy insertion and removal of temporary implants, permanent implants are sometimes used. In this technique radioactive sources (e.g., Gold—198, Iodine—125, or Palladium~103 seeds) are implanted directly into the lesion. The sources remain in the patient permanently, decaying and irradiating the patient. To deliver the proper dose in an acceptably short period, sources used in this technique have somewhat shorter half—lives than temporary implants. Radon, with a half—life of 3.8 days, was the first such source used. It was then replaced with lggAu, then by 1251, and more re— cently by 1'33Pd. All of these nuclides have relatively short half-lives, so the bulk of the radiation dose is delivered in a short period of time. As an example of permanent implantation, "—51 has been used in early stages of prostate cancer. Doses delivered to the tumor are hetWeen 100 to 200 Gy. Snurces of 125I of 0.5 mCi (1.85 X 107 Bq) are placed in the prostate using a spe- cial seed gun or seed inserter with very accurate spacing between sources in order to deliver the prescribed dose. IMPLANT DOSIMETRY The calculation of the exposure rate from a single source is straightforward. As mentioned above, the dose rate is largely a function of geometric factors only. At a distance d from a point source of radiation, the exposure rate is: 2.76 Applied Physics for Radiation Oncology I‘XA d2 X(d) = Where X(d) is the exposure rate in RI hr at a distance r (cm) from a point source of activity A (mCi), and F is the specific exposure or specific gamma ray con— stant, the exposure rate in R/hr at 1 Cm from a mCi (3.7 X 107 Bq) point source. Although actual sources are not points, the above equation can he used if the dis— tance from the source is significantly greater than the longest dimension of the source. For example, a 2 cm long source, while not a point, can be considered a point at distances well beyond 2 cm from the source’s center. The exposure rate at 5 cm fiom a 10 mCi Cesium—137 source is calculated as follows: FXA Xi?) = d2 R x cm2 mCi X hr (5 cm)2 X6 cm) 2 [3.27 ]x(10 mCi) _ 32.7 25 =1.31R/hr Doses to the patient’s tumor, however, must be calculated using more pre— cise and analytical algorithms and computer programs. One method used before the geueral availability of computers that guided radiation oncologists in their im- plant technique was the pre—calculated system of dosimetry. In this rigid ap- proach, tables of several dose distributions are used to determine the combined dose from a standard collection of sources. Doses can he expressed in many ways, including average and minimum doses Within the target volume. There are many techniques of source distribution. These systematic approaches include the Quirnby technique, which uses a uniform distribution of sources to give a non-uniform dose distribution (with a higher dose in the center of the im- planted volume than at its periphery) and the Paterson—Parker technique, which uses a non-uniform distribution of sources to give a uniform dose distribution throughout the implanted volume.2 2. The Paris system developed by Pierquin and Dun-en: is another system of dose calculation. It is used primarily for ME: wire linear implants and prescribes dose to what they call a “basal dose." (See reference 7.) w "bfifih—lh‘ r r-r‘H-I 3] 1]" as ra t; [2. Brachytherapy 277 REMOTE HIGH-INTENSITY AFTERLOADING In the 1960s high dose rate (I-IDR) remote afterloaders were introduced. These devices use high—intensity sealed sources of radioactive materials, remotely introduced into afterloading applicators. HDR. remote afterloader sources can be up to 1000 mg—radium equivalent, whereas the sources discussed previously are from 02—30 rug-radium equivalent.3 These devices combine the spatial advan- tage of fast dose fallofi' with distance associated with intracavitary therapy with the fractionation ofteletherapy. The afterloading applicators are inserted into cav— ities in the same manner as in conventional brachytherapy. After the applicator locations are verified, the patient is placed in a shielded room, and the applica— tors are connected to special tubes. Remote controlled mechanisms use computer control to introduce high—activity radioactive sources through the tubes into the applicators. With this technique, treatment times are a few minutes instead of days as in conventional low dose rate {LDR) brachytherapy. The use of a special shielded room reduces the exposure to support staff (nurses, housekeeping per- sonnel, physicians, etc.). Low—intensity remote afterloading devices are also avail— able. In these, the dose rates to the tumor approximate conventional brachytherapy. However, the remote devices allow easy removal of the sources from the patient during patient care activities, reducing exposures to staff. SPECIFIC IMPLANT TECHNIQUES High—intensity ‘251 and 192Ir sources have recently been used in temporary im— plants of brain tumors. This technique requires very accurate knowledge of brain anatomy and tumor location for accurate source placement. CT scanning and a special stereotactic frame attached to the patient’s skull (a BRW frame) are used to aid in source placement and the dosimetry of a brain implant. The only use of [3‘ sealed sources in brachytherapy is the 9°81“ eye applicator. The device contains a thin radioactive metal foil. The 9"Sr [3‘ decays into WY; which in turn undergoes 13‘ decay, emitting higher energy beta particles of 0.93 MeV average energy (227 MeV maximum energy) with a range of about 4 mm. This radiation is used in treating very superficial lesions on the surface of the eye. Since these [3‘ sources have such short ranges, there is no hazard unless there is direct contact with the source. Wooden storage boxes“ and the stainless steel source 3. According to ICRU Report 38, lowr dose rates are between .4 Gy/h to 2 Gy/h, medium dose rates are between 2 Gy/h to 12 Gy/h. and high dose rates are greater than 12 Gy/h. 4. Low 2. to minimize bremsstrahlung production fiom the electrons. 278 Applied Physics for Radiation Oncology holders, alongr EidEEbov—ers on the applicators, are sufficient to absorb all of the {3‘ emissions. In Europe, 2m"Ru eye applicators are also commonly used. Another use ofiridiurn plaques, which use a flat distribution of sources arranged in a customized mold placed on the surface of a patient’s le— sion. This approach can be used on any reachable tissue surface (e.g., buccal rnu— cosa, vaginal surface, and ocular surface). RADIATION SAFETY WITH IMPLANTS Special radiation safety precautions are required when patients are implanted with radioactive sources. More detailed discussion is given in chapter 17. All tempo- rary implant patients are hospitalized, and the radiation levels in and around their room are monitored to ensure safe levels for the personnel attending the patient. Patients with high—activity pennanth implants of 30 mCi (1.11 X 109 Bq) or greater are also hospitalized until radiation levels surrounding the patient decay to safe levels (either 30 mCi or 10 mR/ hr, whichever is less). Time and distance are conunonly used to limit personnel and visitor exposure. In addition, mov— able lead shields are sometimES used to lower excessively high radiation levels around implant patients. Finally, patients must be surveyed after source removal to verify that removal. I-IDR devices require extensive radiation safety protocols because of their potential dangers, including source separations, time setting errors, etc. (See reference 9.) PROBLEMS 1. What is the exposure rate at 1 cm flour a 10 mCi (3.7 X 108 Bq) source ofmCs? At 1 meter? What is the reason for afierloading? What are two reasons for replacing “Ra? Why is it important to use sources with short half—lives in permanent implants? What are orthogonal films? Why are they used in brachytherapy? P‘P‘PP’N What is the major difference between 1251 and 193Au for permanent implants? (See table 16.1.) 2‘4 When are multiplane implants used instead of single plane implants? 8. How much 60CD is equivalent in radiation level to 10 mCi (3.7 X 103 Bq) of 126Ra? 9. ‘What is the exposure rate at 17 meter from 10 mCi (3.7 X 103 Bq) of “'21:? 10. What is the half-life ofml? I751? l 1 $ 1‘ l 1 Bmchytherapy 279 BIBLIOGRAPHY 10. 11. Sch-11311,]. The Basic Physics of Radiation Wimpy 3rd edition. C.C. Thomas, Chicago, Illinois, 1990. Shearer, D.R., ed. Recent Advances in Braclzytlzerapy Physics, American Association of Physicists in Medicine, New York, 1981. US. Department of Radiological Health Handbook, Revised Edition, Washing— ton, DC, 1970. “Interstitial Iridium Template Techniques” in 8.1-1. Levitt and N. Tapley, Téclmologi— cal Basis of Radiation Wierapy: Practical Clinical Applications, Lea and Febiger, Philadel— phia, Pennsylvania, 1984. Stanton, L. Basic Medical Radiation Physics, Appleton—Century—Crofts, New York, 1969, pp. 402-435. Hall, EJ. Rodi'obfologyfiar the Radiologist, 3rd edition. JB. Lippincott, Hagerstown, Maryland, 1988, pp. 116—136. Khan, EM. The Physics of Radiation Therapy, 2nd edition. Williams 86 Wilkins, Baiti- more, Maryland, 1994, pp. 418-473. Godden, Physical Aspects of Brathytlieropy, Medical Physics Handbook #19, Hilger IOP Publishing Ltd, 1988. Stanton, R. et al., "Radiation Safety Program for a High Dose Rate Remote Af— terloacler," Radiation Protection Management, Volume 11, #4 Sept/ Oct 1994, pp. 67- 80. Subir Nag, Cd., High Dose Rate Brarlzytlierapy: A Textbook, Futura Publishing Co., Inc, 1994. International Commission on Radiation Units and Measurements, Report 38, Dose and Pfilume Specg‘imtfons for Reporting Introtaw'tory Therapy in Gynecology (ICRU), Bethesda, MD, 1985. 17 ...
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