Use of apertures in single-energy pristine Bragg peak FLASH radiotherapy

Background: Proton single-energy Bragg peak (SEBP) FLASH delivery can achieve dosimetric distributions equivalent to conventional intensity-modulated proton therapy (IMPT). However, range-pulling and field compensator devices enlarge the proton pencil beam spot size, increasing lateral penumbra and compromising dose conformality and high-dose-rate distribution. Apertures are employed to mitigate these effects, enhancing dose metrics while preserving ultra-high dose rate performance. Purpose: To investigate the efficacy of apertures in sharpening lateral dose falloff and enhancing dose conformity in proton pencil beam scanning (PBS) Bragg peak (BP) FLASH radiotherapy (RT), addressing increased lateral dose spillage caused by universal range shifter (URS) and range compensator (RC) usage. Methods: PBS Single-energy BP (SEBP) FLASH treatment plans with and without brass apertures were optimized using an in-house planning system. Dose and dose rate characteristics were simulated using MCSquare. Penumbra reduction was assessed in a water phantom for 3 and 5 cm square fields under varying pullback (10 cm, 20 cm. 30 cm) and air gap (5 cm, 10 cm, 15 cm) conditions. Aperture effects were evaluated at the entrance, midpoint, and BP positions. The rGBM cancer plan was optimized using SEBP with various dose thresholds (0, 2, and 5 Gy) applied to analyze dose and ultra-high dose rate (V_40Gy(RBE)/s) performance. Results: Use of aperture significantly reduced lateral penumbra across all spatial positions. The degree of penumbra reduction increased significantly with the increased pullback, and the reduction effect at the BP was generally comparable to that at the entrance. The 5 cm field generally showed greater penumbra reduction than the 3 cm field. In the rGBM case, apertures improved gross tumor volume (GTV) dose conformity and reduced organ-at-risk (OAR) exposure but decreased ultra-high dose rate coverage for both GTV and OARs. The ultra-high dose rate coverage of GTV was not affected by the dose threshold, while the ultra-high dose rate coverage of the brain increased with the increasing dose threshold. Conclusion: Apertures effectively reduce lateral penumbra and dose spalliage to OARs, improving target dose conformity in range shifter-based SEBP FLASH-RT. They also can reduce ultra-high dose-rate exposure to critical OARs in the low-dose region while maintaining the FLASH ratio in the high-dose region.