Dose perturbation due to the polysulfone cap surrounding a Fletcher‐Williamson colpostat

We conducted a Monte Carlo evaluation of the dosimetric impact of the polysulfone cap used with the Fletcher‐Williamson (FW) colpostat forI192 r high‐dose rate and pulsed‐dose rate intracavitary brachytherapy. Polysulfone caps with diameters of 30 mm, 25 mm, 20 mm, and 16 mm (mini‐ovoid) were simulated, and the absorbed dose rate in surrounding water was calculated and compared to the dose rate calculated for a bareI192 r source in water. The dose perturbation depended on the cap diameter, distance away from the cap surface, and angular position around the cap. The largest dose rate reductions were found to be where the cap is thickest, in the general direction of the tumor bed. The range of perturbation mainly from the polysulfone cap over all depths and cap diameters was + 2.8 % (dose enhancement) to − 6.8 % (dose reduction); this perturbation is separate from the well‐documented attenuation of the rectal and bladder tungsten shields of the colpostat. Based on the impact of the polysulfone cape, the FW colpostat cap's material composition should be modified to reduce this dosimetric effect, or brachytherapy treatment planning dose algorithms should be improved to account for this perturbation. PACS numbers: 87.10.Rt 87.53.‐j 87.53.Bn 87.53.Jw