Development of an 241Am applicator for continuous low-dose-rate irradiation of the rat sarcoma BA1112

An 241Am applicator for continuous low-dose-rate irradiation of the rat sarcoma BA1112 has been developed. The irradiator consists of two disc sources, each containing 800 mCi of 241Am, an isotope which emits primarily 60 keV photons. The disc sources are held in a specially-designed light-weight helmet which surrounds the tumor on the head of the rat. Dose distributions produced by these sources have been measured using an ionization chamber, thermoluminescent dosimeters and Fricke dosimeter. A computerized treatment planning system has been modified to compute dose distributions from 241Am sources, to optimize the design of this applicator. Computed and measured dose distributions for several values of separation between the 241Am discs are presented. A survival curve for cells from tumors irradiated in vivo with this applicator has been determined by an in vitro colony formation technique. The mean lethal dose DO was found to be 720 cGy for an average tumor dose rate of 95 +/- 7 cGy/hr. The major advantages of the 241Am applicator in comparison with the 192Ir applicator used previously for continuous low-dose-rate studies are: a considerably smaller half value layer thickness and the longer half life of the radionuclide. These features make it more suitable for long-term tumor cure studies because of the lower whole body dose to the animal, the availability of relatively constant dose-rate irradiators for many years, the decreased shielding requirements for the animal care facility and the diminished exposure to laboratory personnel involved with the implants on the animals.