A dose computation model for 241Am vaginal applicators including the source‐to‐source shielding effects

Abstract

A dose computation model has been developed for the determination of dose distributions around vaginal plaque applicators containing encapsulated 241Am sources. Encapsulated sources of 241Am emit primarily 60-keV photons which have a half-value layer thickness of 1/8 mm of lead. This makes possible highly effective in vivo shielding of normal tissues at risk, by placing thin lead shields at appropriate places on the applicator. However, self-absorption of photons in the source material itself is intense, requiring bulky sources of about 1 cm diameter. These sources also produce considerable source-to-source shielding which must be taken into account in dose calculations. Our dose computation model for a single source employs three-dimensional integration of dose contributions from volume elements of the source including the effects of absorption and scattering of photons in the source material, titanium encapsulation, and water. An empirical correction to Berger's data on buildup factors of point, isotropic sources is made to account for the effects of anisotropic photon emission by cylindrical 241Am sources. The second part of our dose computation model takes into account source-to-source shielding effects on both primary and scattered photons for the vaginal plaque geometry. The results of the model have been verified for accuracy by comparisons with extensive dosimetry measurements using lithium fluoride thermoluminescent dosimeters.