Time-dependent dose distribution changes in prostate brachytherapy with iodine seeds.

Abstract

115 Background: Prostate brachytherapy with iodine seeds has a lasting dose delivery due to a long half life of I125 (i.e. 2 months). To assess changes in dose metrics over time in low-dose rate prostate brachytherapy with iodine seeds and its relationship with rectal distension. Methods: One hundred and twenty-one post-implant CT scans performed every 2 weeks over 2 months after the treatment procedure were analyzed for dosimetry. Each CT-scan was performed without and with rectal enema. The following rectal parameters were collected to evaluate the influence of rectal distension over time: rectal volume, cross-sectional surface area (CSA) and relative CSA calculated from the first CT-scan chosen as reference (CTref). Results: Mean V150% and V200% of the prostate volume increased significantly over time (p < 0.0001) and were significantly higher on postimplant CT scan performed at day 45 (CT45) (p = 0.024 and p = 0.003, respectively) and CT scan performed at day 60 (CT60) (p = 0.0005 and p < 0.0001, respectively) compared with CTref. All dosimetric parameters for the rectum were significantly increased over time. No rectal parameters were significantly different after rectal enema and there was no difference in doses delivered to the prostate and the rectum after rectal emptying. Conclusions: Increased hot spots in the prostate and the rectum during the 2 months following the procedure with iodine seeds may be related to prostate shrinkage rather than rectal distension. We hypothesize that radioelements with shorter half-life (Pd103 or Cs131) or HDR may give less toxicity, as dose distribution is less time-dependent.