Y Khouj*, T Harris, D O'Farrell, E Neubauer Sugar, P Nguyen, P Orio, M King, I Buzurovic, Dana-Farber/Brigham and Women's Cancer Center, Harvard Medical School, Boston, MA

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  Y Khouj

PO-GePV-T-17 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To present the dosimetry analysis of low dose rate (LDR) prostate brachytherapy patients treated over a three-year period. To compare dosimetry on the implant day (D0) and post-implant dosimetry at the nominal optimal dosimetry time (D30) for patients implanted with 125I.

Methods: A total of 118 patients diagnosed with prostate cancer were treated with LDR brachytherapy using I-125 seeds (activity 0.371mCi at implant) between 2018-2020. 67% patients received 145 Gy while 33% received 108/110 Gy boost prescription. Transrectal Ultrasound (TRUS)-based intraoperative planning was used to obtain real time images for dosimetry on D0. Prostate (target) and organs-at-risk (OAR) volumes were contoured based on MR-TRUS registration or TRUS-only. The planning was performed while the patient was under anesthesia in real-time using Oncentra Prostate treatment planning system based on inverse-planning to optimize and analyze target and OAR dose metrics. After plan approval, patient specific needles were built using QUICKLINK® seed loader (BARD). Day 30 post-implant dosimetry was performed using MR-CT or CT-only for contouring, and CT-based planning. Target and OAR dosimetric parameters were reported.

Results: D0 target dosimetry was as followed: D90 111.7±3.7%, V100 96.1±2.8%, V150 48.6±5.7%. The corresponding D30 target dosimetry was D90 108.9±7.3%, V100 94.0±3.3%, V150 49.7±9.4%. The urethra D0 V125 0.03±0.08cm3 and V150 0.00±0.01cm3 shifted to D30 V125 0.14±0.18cm3 and V150 0.02±0.04cm3. Rectum D0 metrics were V100 0.05±0.11cm3 and V69 1.43±0.62cm3. Rectum D30 values settled at V100 0.23±0.43 cm3 and V69 1.25±1.30 cm3.

Conclusion: D30 dosimetric parameters revealed high consistency in dosimetry compared to D0. Post-implant dosimetry complied with the American Brachytherapy Society recommendations. The intraoperative technique takes advantage of the real-time images to allow for accurate needle placement, seed localization, and dose distribution optimization, all of which may contribute to dosimetric stability from D0 to D30.

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