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

Abstract

To compare single-fraction 153Gd-based rotating shield brachytherapy (RSBT) for prostate cancer with conventional 192Ir-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. Twenty-six patients were studied. PTV doses were expressed as equivalent dose delivered in 2Gy fractions (EQD2), accounting for relative biological effectiveness (1.00 for 192Ir and 1.15 for 153Gd), dose protraction (114-minute repair half-time), and tumor dose response (α/β of 3.41Gy). HDR-BT dose was prescribed such that 90% of the PTV received 110% of the prescription dose of 19Gy for dose escalation and 15Gy for urethral sparing, corresponding to EQD290% values (minimum EQD2 to the hottest 90% of the PTV) of 93.9 GyEQD2 and 60.7 GyEQD2, respectively. Twenty 90.95 GBq 153Gd RSBT sources and one 370 GBq 192Ir HDR-BT source were modeled. For dose escalation with fresh sources, RSBT increased PTV EQD290% by 42.5%±8.4% (average±standard deviation) without increasing urethral D10%, with treatment times of 216.8±28.9minutes versus 15.1±2.1minutes. After 1 half-life (240.4days for 153Gd and 73.8days for 192Ir), EQD290% increased 20.5%±9.1%. For urethral sparing with fresh sources, RSBT decreased urethral D10% by 26.0%±3.4% without decreasing PTV EQD290%, with treatment times of 133.6±16.5minutes versus 12.0±1.7minutes. After 1 half-life, urethral D10% decreased 20.2%±4.8%. RSBT can increase PTV EQD90% or decrease urethral D10% 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.