Effect of superposition assumption on Monte Carlo dose calculation about shielded gynecological colpostats

The Fletcher-Suit-Delclos (FSD) intracavitary vaginal applicator typically incorporates tungsten shielding to reduce dose to the bladder and rectum. Dose distributions about a single colpostat have been extensively measured and calculated. However, these measurements do not consider perturbations from the contralateral colpostat or the intrauterine tandem on the dose distribution. Accurate accounting for all dosimetric effects in brachytherapy is essential for both dose based implant optimization and for comparison with alternate modalities, such as intensity modulated radiation therapy. We have used Monte Carlo calculations to evaluate the accuracy of the superposition principle as it applies to both single shielded applicator dose distributions and single source dose distributions. Preliminary analysis indicates differences ranging from 2-5%, warranting further investigation.