Comparison of high dose-rate and low dose-rate dose distributions for vaginal cylinders

Purpose: The identification of appropriate high dose-rate parameters required to produce a "uniform" dose distribution on the surface of a vaginal cylinder. The high dose-rate dose distribution is then compared to the traditional low dose-rate dose distributions obtained with Burnett cylinders. Methods and Materials: Dose distributions were calculated for 2,3, and 3.5 cm diameter Burnett cylinders with and without crossing sources. Three models for the high dose-rate cylinders were developed and compared. High dose-rate dose distributions were calculated for 2,3, and 3.5 cm diameter cylinders with and without anisotropic corrections for various dose specifications points. Results: Low dose-rate distributions are not uniform over the surface of the applicator. The exact distributions depend upon cylinder diameter and upon the exact source loading. High dose rate dose distributions can be cinfigured to provide for a "uniform" dose on the surface, if an apex dose specifications point is used together with dose specification points on the surface of the applicator opposite each dwell position. Conclusions: The conversion of low dose rate techniques to high dose rate techniques for vaginal cylinders involves an appreciation of the details of dose distributions of both approaches. The comparison between traditional low dose-rate distributions and high dose-rate distributions shows that, unlike the low dose-rate distributions, a relatively uniform high dose-rate distribution can be obtained independent of cylinder diameter. The clinical significance of the differences in the low dose-rate and high dose-rate distributions remains to be determined by long-term follow up of patients treated with high dose-rate techniques.