Dosimetric Comparison Between Brass Aperture-Enhanced Proton Pencil Beam Scanning and Photon Stereotactic...

The current study aimed to quantify potential dosimetric advantages of a novel, brass aperture-enhanced, proton-based stereotactic radiosurgery (SRS) technique compared to conventional photon linac-based SRS in patients with cerebral arteriovenous malformations (AVMs).

A retrospective review was performed of patients treated with proton SRS for cerebral AVMs at our institution between 2023 and 2025. Volumetric modulated arc therapy linac-based SRS plans were generated for dosimetric comparison and normalized to match the clinical target volume (CTV) prescription coverage of the delivered proton plan. SRS plan quality was evaluated by Radiation Therapy Oncology Group conformity and homogeneity indices (CI, HI). The V12 Gy representing the normal brain volume receiving ≥ 12 Gy including the CTV (total brain) was measured.

Fourteen patients were included, and most had no prior hemorrhage (64%), embolization (79%), or AVM-directed radiotherapy (86%). Three patients received staged radiation, totaling 17 treatment plans for review. Mean AVM size was 8.8 cc (0.4-30.6). SRS dose ranged from 17 to 20 Gy with 3 larger AVMs treated using volumetric staging. For the 4 AVMs ≤2.0 cc in size, the average CI was 1.89 and 1.81 for photons and protons, respectively. The average CI for AVMs >2 cc was 1.15 and 1.17 for photons and protons, respectively. A statistically significant difference was observed with HI (P = .0034) and V12 Gy (P = .0017). Fourteen of 17 (82.4%) HI were lower with protons. There was on average a 5.86 cc improvement in V12 Gy with protons.

Brass aperture-enhanced proton PBS SRS improved V12 Gy in 15 of 17 plans (88.2%), including small volume AVMs and AVMs close to the brainstem. These findings are hypothesis-generating and must be interpreted cautiously. Maturation of our clinical outcome data (AVM obliteration and symptomatic radionecrosis rates) will yield further insights.