Effectiveness of Rotating Shield Brachytherapy for Prostate Cancer Dose Escalation and Urethral Sparing

Purpose: To compare single-fraction Gd-153-based rotating shield brachytherapy (RSBT) for prostate cancer with conventional Ir-192-based high-dose-rate brachytherapy (HDR-BT) in a planning study that radiobiologically accounts for dose rate and relative biological effectiveness. RSBT was used for planning target volume (PTV) dose escalation without increasing urethral dose for monotherapy, or for urethral sparing without decreasing PTV dose as a boost to external beam radiation therapy. Methods and Materials: Twenty-six patients were studied. PTV doses were expressed as equivalent dose delivered in 2 Gy fractions (EQD2), accounting for relative biological effectiveness (1.00 for Ir-192 and 1.15 for Gd-153), dose protraction (114-minute repair half-time), and tumor dose response (alpha/beta of 3.41 Gy). HDR-BT dose was prescribed such that 90% of the PTV received 110% of the prescription dose of 19 Gy for dose escalation and 15 Gy for urethral sparing, corresponding to EQD2(90%) values (minimum EQD2 to the hottest 90% of the PTV) of 93.9 Gy(EQD2) and 60.7 Gy(EQD2), respectively. Twenty 90.95 GBq Gd-153 RSBT sources and one 370 GBq Ir-192 HDR-BT source were modeled. Results: For dose escalation with fresh sources, RSBT increased PTV EQD2(90%) by 42.5% +/- 8.4% (average +/- standard deviation) without increasing urethral D-10%, with treatment times of 216.8 +/- 28.9 minutes versus 15.1 +/- 2.1 minutes. After 1 half-life (240.4 days for Gd-153 and 73.8 days for Ir-192), EQD290% increased 20.5% +/- 9.1%. For urethral sparing with fresh sources, RSBT decreased urethral D-10% by 26.0% +/- 3.4% without decreasing PTV EQD2(90%), with treatment times of 133.6 +/- 16.5 minutes versus 12.0 +/- 1.7 minutes. After 1 half-life, urethral D-10% decreased 20.2% +/- 4.8%. Conclusions: RSBT can increase PTV EQD(90%) or decrease urethral D-10% relative to HDR-BT at the cost of increased treatment time. Source aging reduces RSBT benefit, but RSBT remains theoretically superior to HDR-BT by > 20% after 1 half-life has elapsed. (C) 2018 Elsevier Inc. All rights reserved.