Room: Davidson Ballroom A
Purpose: To compare single-fraction Â¹â?µÂ³Gd-based rotating shield brachytherapy (RSBT) for prostate cancer to conventional Â¹â?¹Â²Ir-based high-dose-rate brachytherapy (HDR-BT) in a planning study which radiobiologically accounts for dose rate and relative biological effectiveness (RBE). 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 radiotherapy.
Methods: Twenty-six patients were studied. PTV doses were expressed as equivalent dose delivered in 2 Gy fractions (EQD2), accounting for RBE (1.00 for Â¹â?¹Â²Ir and 1.15 for Â¹â?µÂ³Gd), dose protraction (114 minute repair half-time), and tumor dose response (Î±/Î² 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 EQD2â‚‰â‚€% values (minimum EQD2 to the hottest 90% of the PTV) of 93.9 GyEQD2 and 60.7 GyEQD2, respectively. Twenty 90.95 GBq Â¹â?µÂ³Gd RSBT sources were modeled and one 370 GBq Â¹â?¹Â²Ir HDR-BT source.
Results: For dose escalation with fresh sources, RSBT increased PTV EQD2â‚‰â‚€% by 42.5% Â± 8.4% (average Â± standard deviation) without increasing urethral Dâ‚?â‚€%, with treatment times of 216.8 Â± 28.9 minutes versus 15.1 Â± 2.1 minutes. After one half-life (240.4 days for Â¹â?µÂ³Gd and 73.8 days for Â¹â?¹Â²Ir), EQD2â‚‰â‚€% increased 20.5% Â± 9.1%. For urethral sparing with fresh sources, RSBT decreased urethral Dâ‚?â‚€% by 26.0% Â± 3.4% without decreasing PTV EQD2â‚‰â‚€%, with treatment times of 133.6 Â± 16.5 minutes versus 12.0 Â± 1.7 minutes. After one half-life, urethral Dâ‚?â‚€% decreased 20.2% Â± 4.8%.
Conclusion: RSBT can increase PTV EQD2â‚‰â‚€% or decrease urethral Dâ‚?â‚€% 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 one-half-life has elapsed.
Funding Support, Disclosures, and Conflict of Interest: This work was funded by NIH ROI EB020665 and NIH 1R41 CA210737-01. Ryan T Flynn has financial interest in pxAlpha, LLC, whic his developing a Rotating Shield Brachytherapy product.