Evaluation of source displacement and dose–volume changes after permanent prostate brachytherapy with stranded seeds

Background and purposeThe aim of the study was to analyze source displacements and dose–volume changes in the first month after a permanent implant.Materials and methodsIn 51 consecutive patients, CT scans were performed at the postoperative day (day 1) and one month (day 30) after an 125I implant with stranded seeds. Seed positions were determined relative to pelvic bones for five seeds at the base and five seeds at the apex for each patient (n = 510) and compared. To verify these results, treatment margins (TM = distance of prescription isodose to prostate) and displacements of the prostate surface (anterior/posterior/right/left/superior/inferior) relative to pelvic bones were measured.ResultsSeed positions have moved significantly between day 1 and 30 in the posterior (mean 1.0 mm; p < 0.001) and inferior (mean 3.8 mm; p < 0.001) directions. TM increased particularly at the posterior (mean 2.2 mm; p < 0.001) and apical (median 3.0 mm; p < 0.001) prostate contour with decreasing oedema. With a stable apex position and a mean inward posterior surface displacement of 1.1 mm (p < 0.001) relative to pelvic bones, seed displacements could be well correlated with prescription isodose displacements (Pearson correlation coefficients ⩾0.81; p < 0.001).ConclusionsBoth changes of prostate volume and seed displacements need to be considered to explain dosimetric changes after permanent prostate brachytherapy.