Radiation Physics

1251Seed

Dose Tables for

Implants1

V. Krishnaswamy, Ph.D.

Dose tables are given for 125 1implants, as well as detailed information about the dose distribution which is calculated for different square planar and cubic volume implants using recently published depth dose data. Minimum peripheral and maximum central doses are evaluated for different areas and volumes for seeds with 1 mCi (37 MBq) equivalent activity and spaced 1 cm apart. The dose delivered at any specific point in a permanent implant is the dose rate at that point multiplied by the mean life (2,080 hrs) and the activity per seed (1 mCi). The effect of spacing is studied and a spacing rule is suggested. INDEX TERMS:

Dosimetry, radionuclide • Radiations, measurement • Therapeutic radiology, interstitial and intracav-

itary

Radiology 132:727-730, September 1979

a low energy, photon emitting isotope, is presently being used in interstitial radiotherapy. Its low energy range of 27-35 kev allows much better shielding than the high energy sources; it has a half life of 60.2 days; and the radiations emitted have a half value layer of 0.025 mm in lead. Excellent results have been achieved clinically using 1251 in the form of seeds encapsulated in titanium in permanent implants (6). Dillman (4) recently compiled the radiation properties for this source, which has an exposure constant in air of 1.45 R cm 2/mCi-hr (1.01 X 10- 2 [mC/kg] cm 2 [MBq hr]-1). For the encapsulated 1251 sources, activity (mCi or MBq) is usually given in terms of equivalent or compensated activity, which includes the effect of self-absorption within the seed. This paper presents an independent implantation system for the 1251 seed source based on recently published two dimensional depth dose data (7).

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MATERIALS AND METHODS

Depth Dosimetry

Depth dose distribution around a 1251 source in tissue has been calculated and verified by measurements with thermoluminescent dosimeters (7). The absorbed dose constant that includes attenuation and scatter in tissue is 1.32 rad cm 2/mCi-hr (3.57 X 10- 2 Gy cm 2/MBq-hr). This dose constant is for the isotope and is not an "effective" value (i.e., integrated over 47r). For the data presented in this paper, the isotopic dose constant is appropriate, since direction of dependencies is handled explicitly by measurement. This is in close agreement with the data given by Chen and Heller (3). The radiographic gold marker at the center of the source produces nonuniform dose distribution, particularly along the length.

Implantation System

1251 interstitial implants are performed using the Dimension Averaging Method. Source activity is obtained from the average dimension and a multiplicative constant of 5 (5). The spacing required for the amount of available activity per seed must be evaluated using a spacing nomograph developed by Anderson (1). This approach delivers a specific dose to the tumor volume without giving detailed information about the dose distribution. Alternatively, one can use the high energy seed source table developed by Laughlin (8). Since the depth dose characteristics of 1251 seeds are different from those of high energy sources, this method introduces an error range of -7 % to + 12 %. It would be useful to have a table with the minimum possible errors that is easy to apply in practice. However, more detailed information could be obtained by radiographic localization of seeds and by using digital computers to evaluate the dose distribution for a specific implant. Localization of the seeds is rather difficult due to their small size. For a simple planar and volume implant, a table of minimum peripheral and maximum central dose is useful. Radiographic localization is necessary, however, to check the accuracy of implant placement.

Tables for Implants

Dose distributions at 0.5 em above a planar implant and bounded by the peripheral seeds, midway between different planes in volume implants defined earlier as target planes (2), have been calculated at 0.5 em grids for square planar and cubic volume implants of different areas and volumes. The seeds were assumed to have an equivalent activity of 1 mCi (37 MBq) and were spaced 1 em apart,

1 From the Department of Radiotherapy, Hospital of the Albert Einstein College of Medicine, Bronx, New York. Received Nov. 7, 1978; revision requested Jan. 4, 1979; accepted Apr. 13. as

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Dose tables for 125I seed implants.

Radiation Physics 1251Seed Dose Tables for Implants1 V. Krishnaswamy, Ph.D. Dose tables are given for 125 1implants, as well as detailed informat...
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