Optimization in high dose rate vaginal cylinder for vaginal cuff irradiation

Aim: The aim of this study is to evaluate the influence of high dose rate (HDR) brachytherapy source step size, source dwell position, dose prescription depth, dose specification points and optimization technique on dose distribution around Microselectron HDR brachytherapy vaginal cylinders and to evaluate the influence of distal dwell position and optimization technique on rectal and bladder dose of patients treated for vaginal cuff irradiation. Materials/Methods: Orthogonal radiographs of vaginal cylinders of diameter 2.0, 2.5, 3.0 and 3.5cm form the basis of the study. Dose distribution using the PLATO brachytherapy treatment planning system (version 14.1) was generated. Two different HDR cylinder models, namely the non-curved dome model (NCDM) and curved dome model (CDM), were studied. To evaluate bladder and rectum dose in the patients NCDM was used. Results: CDM gives more uniform dose distribution around the cylinder than NCDM. Dose prescription at 5mm depth from the surface results in very high dose to apex and dome as compared with the surface dose prescription. Dose prescription depth and dwell positions influence the length of prescription isodose. Optimization method and dwell positions affect the bladder and rectal dose of the studied patients. Conclusions: Uniform dose distribution can be obtained for HDR vaginal cylinders by appropriately selecting dose specification points and optimization method. Dose distribution can be configured to provide a uniform dose on the surface, if the apex and curved surface of the cylinder are considered for dose specification and optimization. Appropriate HDR parameters are identified to minimize the dose to the apex of the vaginal cylinder, essential to reduce the dose to overlying small bowel and reduce the dose to rectum and bladder.