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Effects of prescription depth, cylinder size, treatment length, tip space, and curved end on doses in high-dose-rate vaginal brachytherapy.

Li S, Aref I, Walker E, Movsas B

Department of Radiation Oncology, Henry Ford Health System, Detroit, Michigan 48202, USA. sli1@hfhs.org

PURPOSE: To determine the effects of the prescription depth, cylinder size, treatment length, tip space, and curved end on high-dose-rate vaginal brachytherapy (HDR-VBT) of endometrial cancer. METHODS AND MATERIALS: Treatment plans were prescribed and optimized based on points at the cylinder surface or at 0.5-cm depth. Cylinder sizes ranging from 2 to 4 cm in diameter, and treatment lengths ranging from 3 to 8 cm were used. Dose points in various depths were precisely defined along the cylinder dome. The given dose and dose uniformity to a depth of interest were measured by the mean dose (MD) and standard deviation (SD), respectively, among the dose points belonging to the depth. Dose fall-off beyond the 0.5 cm treatment depth was determined by the ratio of MD at 0.75-cm depth to MD at 0.5-cm depth. RESULTS: Dose distribution varies significantly with different prescriptions. The surface prescription provides more uniform doses at all depths in the target volume, whereas the 0.5-cm depth prescription creates larger dose variations at the cylinder surface. Dosimetric uncertainty increases significantly (>30%) with shorter tip space. Extreme hot (>150%) and cold spots (<60%) occur if no optimization points were placed at the curved end. CONCLUSIONS: Instead of prescribing to a depth of 0.5 cm, increasing the dose per fraction and prescribing to the surface with the exact surface points around the cylinder dome appears to be the optimal approach.

Published 5 March 2007 in Int J Radiat Oncol Biol Phys, 67(4): 1268-77.
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