On the validity of the superposition principle in dose calculations for intracavitary implants with shielded vaginal colpostats

Intracavitary vaginal applicators typically incorporate internal shielding to reduce dose to the bladder and rectum. While dose distributions about a single colpostat have been extensively measured and calculated, these studi es neglect dosimetric perturbations arising from the contralateral colposta t or the intrauterine tandem. Dosimetric effects of inhomogeneities in brac hytherapy is essential for both dose-based implant optimization as well as for a comparison with alternate modalities, such as intensity modulated rad iation therapy. We have used Monte Carlo calculations to model dose distrib utions about both a Fletcher-Suit-Delclos (FSD) low dose-rate system and th e microSelectron high dose-rate remote afterloading system. We have evaluat ed errors, relative to a Monte Carlo simulation based upon a complete appli cator system, in superposition calculations based upon both precalculated s ingle shielded applicator dose distributions as well as single unshielded s ource dose distributions. Errors were largely dominated by the primary phot on attenuation, and were largest behind the shields and tandem. For the FSD applicators, applicator superposition showed differences ranging from a me an of 2.6% at high doses (>Manchester Point A dose) to 4.3% at low doses (< Manchester Point A dose) compared to the full geometry simulation. Source-o nly superposition yielded errors higher than 10% throughout the dose range. For the HDR applicator system, applicator superposition-induced errors ran ging from 3.6%-6.3% at high and low doses, respectively. Source superpositi on caused errors of 5%-11%. These results indicate that precalculated appli cator-based dose distributions can provide an excellent approximation of a full geometry Monte Carlo dose calculation for gynecological implants. (C) 2001 American Association of Physicists in Medicine.