Rectal dosimetry following prostate brachytherapy with stranded seeds – Comparison of transrectal ultrasound intra-operative planning (day 0) and computed tomography-postplanning (day 1 vs. day 30) with special focus on sources placed close to the rectal wall

Abstract Background and purpose The aim of the study was to compare intra-operative and postplanning at different intervals with special focus on sources placed close to the rectal wall. Materials and methods In 61 consecutive patients, CT scans were performed on day 1 and day 30 after an I-125 implant with stranded seeds. The number of sources ⩽7 mm to the rectal wall was determined, and displacements were analyzed. The angulation of strands relative to rectal wall was compared between intra-operative transrectal ultrasound (TRUS) and both postplanning CT scans. Results Sources close to the rectum on day 1 ( n  = 204) have been the most apical in a strand in 98.5% ( n  = 201). By comparing day 1 and day 30 data, significant inferior source displacements (mean 3.6 mm; p  = 0.02) relative to pelvic bones and a decreasing distance to the rectal wall (mean 1.2 mm; p 30°). Conclusions Posterior preplanning margins around the prostate should be particularly limited at the level of the prostate apex.