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

Background and purposeThe 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 methodsIn 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.ResultsSources 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 < 0.01) – consequentially increasing rectal dose – were determined only for sources initially ⩾3 mm to the rectum. In contrast to an almost parallel arrangement of the needle track and the rectal wall in TRUS, strands and rectal wall converged towards the apex in the postplanning CT scans (mean >30°).ConclusionsPosterior preplanning margins around the prostate should be particularly limited at the level of the prostate apex.