ESTRO 2025 - Abstract Book

S4023

Radiobiology - Tumour radiobiology

ESTRO 2025

Purpose/Objective: Treatment duration in stereotactic radiosurgery (SRS) can reduce the biological effective dose (BED) for a given fractionation, due to bi-phasic, sub-lethal damage repair (SLDR) processes unaccounted for by the linear-quadratic (LQ) model. Many Gamma Knife (GK) studies suggest outcomes are better predicted using BED rather than physical dose, however little data exists that provides guidance between GK, from which modern dose schedules derive, and faster linear accelerators 1,2 . We demonstrate the critical need to consider treatment time when prescribing dose for vestibular schwannoma (VS) with Gamma Knife, Cyberknife (CK) or Truebeam (TB). Material/Methods: 977 VS patients were treated between 2001 and 2024 on either GK Model B (219), Perfexion (344), Cyberknife VSI (193), or Truebeam (221) with one- (636) or multiple- (198) fraction regimens of 11/1, 12/1, 12.5/1, 13/1 or 14/1 Gy or 18/3, 25/5, or 27.5/5 Gy. Truebeam patients received VMAT (130) or DCA (91), and with GTV-to-PTV margins of 0 (150), 0.5 (43) or 1 mm (28). No GK or CK cases used margins. BEDs, using Jones’ A9 formalism 3 , were calculated using treatment parameters and radiobiological parameters α/β (2.47 Gy), fast and slow repair half-times of 11.4 and 129.6 minutes, respectively. These were compared to values using the LQ model. Results: Mean treatment times were 55.5, 44.3, 30.1 and 8.3 minutes for Model B, Perfexion, Cyberknife and Truebeam, respectively. GK mean BEDs (mBEDs) (55 Gy 2.47 ) were the primary comparator for Cyberknife and Truebeam mBEDs. With no margin, single fraction mBEDs increased by 7.3% and 25.5%, at 12 Gy and 6.4% and 33.7% at 13 Gy for Cyberknife and Truebeam, respectively, while at 18 Gy/3, Cyberknife mBEDs decreased 1% and increased 12.2% for Truebeam. At 25 Gy/5 and 27.5 Gy/5, Truebeam mBEDs increased 42.9% and 67.6%, respectively, and were 13.9% and 33.6%, respectively, higher than the 12 Gy/1 Truebeam cohort. GTV-to-PTV margins resulted in large mBED increases on Truebeam. At 12 Gy/1 and 0.5 mm margin, mBED increased 42%, and for 25 Gy/5 and 27.5 Gy/5 with 1 mm margin, mBEDs increased 56.3% and 83.1%, compared to GK.