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

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 2 Gy 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 3.41 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 ¹⁵³Gd RSBT sources and one 370 GBq ¹⁹²Ir HDR-BT source were modeled.

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 ¹⁵³Gd and 73.8 days for ¹⁹²Ir), EQD2_90% 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%.

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.