Dosimetric assessment of an air-filled balloon applicator in HDR vaginal cuff brachytherapy using the Monte Carlo method

Purpose As an alternative to cylindrical applicators, air‐inflated balloon applicators have been introduced into high‐dose‐rate (HDR) vaginal cuff brachytherapy to achieve sufficient dose to the vagina mucosa as well as to spare organs at risk, mainly the rectum and bladder. Commercial treatment planning systems which employ formulae in the AAPM Task Group No. 43 (TG 43) report do not take into account tissue inhomogeneity. Consequently, the low‐density air in a balloon applicator induces different doses delivered to the mucosa from planned by these planning systems. In this study, we investigated the dosimetric effects of the air in a balloon applicator using the Monte Carlo (MC) method. Methods The thirteen‐catheter Capri™ applicator by Varian™ for vaginal cuff brachytherapy was modeled together with the Ir‐192 radioactive source for the microSelectron™ Digital (HDR‐V3) afterloader by Elekta™ using the MCNP MC code. The validity of charged particle equilibrium (CPE) with an air balloon present was evaluated by comparing the kerma and the absorbed dose at various distances from the applicator surface. By comparing MC results with and without air cavity present, dosimetric effects of the air cavity were studied. Clinical patient cases with optimized multiple Ir‐192 source dwell positions were also explored. Four treatment plans by the Oncentra Brachy™ treatment planning system were re‐calculated with MCNP. Results CPE fails in the vicinity of the air‐water interface. One millimeter beyond the air‐water boundary the kerma and the absorbed dose are equal (0.2% difference), regardless of air cavity dimensions or iridium source locations in the balloon. The air cavity results in dose increase, due to less photon absorption in the air than in water or solid materials. The extent of the increase depends on the diameter of the air balloon. The average increment is 3.8%, 4.5% and 5.3% for 3.0, 3.5, and 4.0 cm applicators, respectively. In patient cases, the dose to the mucosa is also increased with the air cavity present. The point dose difference between Oncentra Brachy and MC at 5 mm prescription depth is 8% at most and 5% on average. Conclusions Except in the vicinity of the air‐mucosa interface, the dosimetric difference is not significant enough to mandate tissue inhomogeneity correction in HDR treatment planning.