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Purpose: To evaluate adaptive planning for permanent prostate brachytherapy and to identify the prostate regions that needed adaptation. Methods and materials: After the implantation of stranded seeds, using real-time intraoperative planning, a transrectal ultrasound (TRUS)-scan was obtained and contoured. The positions of seeds were determined on a C-arm cone-beam computed tomography (CBCT)-scan. The CBCT-scan was registered to the TRUS-scan using fiducial gold markers. If dose coverage on the combined imagedataset was inadequate, an intraoperative adaptation was performed by placing remedial seeds. CBCT-based intraoperative dosimetry was analyzed for the prostate (D 90 , V 100 , and V 150 ) and the urethra (D 30 ). The effects of the adaptive dosimetry procedure for Day 30 were separately assessed. Results: We analyzed 1266 patients. In 17.4% of the procedures, an adaptation was performed. Without the dose contribution of the adaptation Day 30 V 100 would be < 95% for half of this group. On Day 0, the increase due to the adaptation was 11.8 AE 7.2% (1SD) for D 90 and 9.0 AE 6.4% for V 100 . On Day 30, we observed an increase in D 90 of 12.3 AE 6.0% and in V 100 of 4.2 AE 4.3%. For the total group, a D 90 of 119.6 AE 9.1% and V 100 of 97.7 AE 2.5% was achieved. Most remedial seeds were placed anteriorly near the base of the prostate. Conclusion: CBCT-based adaptive planning enables identification of implants needing adaptation and improves prostate dose coverage. Adaptations were predominantly performed near the anterior base of the prostate.

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