Needle applicator displacement during high-dose-rate interstitial brachytherapy for prostate cancer.

PURPOSE To introduce an effective ambulatory technique in high-dose-rate interstitial brachytherapy (HDR-ISBT) for prostate cancer, we investigated the displacement distance using our novel calculation method. METHODS AND MATERIALS Sixty-four patients treated with HDR-ISBT as monotherapy were examined. Of these, 4, 17, and 43 patients were administered treatment doses of 38 Gy (3 days), 49 Gy (4 days), and 54 Gy (5 days), respectively. For dose administration, we used 776 flexible applicators with a removable template (ambulatory technique). Using CT images, we calculated the relative coordinates of the metal markers and applicators. From these coordinates, to analyze displacement during treatment, we measured the distance between the tip of the needle applicator and the center of gravity of the markers along the average applicator vector. RESULTS The median displacement distance for all applicators was 7 mm (range, -14 to 24), and that of each treatment schedule was 4, 6, and 9 mm for 38, 49, and 54 Gy, respectively. Of the 776 applicators, displacement of >10 mm was seen in 198 (26%) applicators and >15 mm in 57 (7%) applicators. Body height (p<0.0001) and anticoagulant usage (p<0.0001) were significant factors influencing displacement. CONCLUSIONS We investigated needle applicator displacement using our unique method. Additional cranial margins are necessary if there is no repositioning of the dwell position. CT scanning should be performed daily during treatment for checking the position of the applicator to detect and rectify the issue of displacement.