Intra-Field Interruption In Pencil Beam Scanning Proton Flash Radiotherapy: Dosimetric Impact and Clinical Solutions
Abstract
Purpose
Ultra-high-dose-rate (UHDR) proton irradiation may spare normal tissue (FLASH effect), but PBS proton dose rate is limited by machine constraints (minimum MU/spot and dwell time). Intra-field interruptions can leave sub-threshold MU for remaining delivery, degrading planned UHDR coverage. We quantified this impact and developed real-time mitigation strategies for Varian ProBeam.
Methods
Multi-field, single-energy Bragg-peak FLASH plans (minimum MU/spot = 500) were generated for five consecutive lung patients (34 Gy/fraction). Interruptions were simulated by pausing at different spots, producing remaining MU <500 for the interrupted spot. Four real-time strategies were tested: (1) skip the interrupted spot and redistribute its remaining MU to neighboring spots; (2) resume the interrupted spot at 500 MU with MU compensated from neighbors; (3) skip the interrupted spot and discard remaining MU; (4) resume the interrupted spot with the remaining MU. For each interruption scenario, the optimal strategy was selected and compared using target dose coverage and UHDR coverage metrics. Experimental validation used a 7×7 spot map (5 mm spacing, 1200 MU/spot).
Results
Optimal choices depended on spot position and remaining MU: late-sequence interruptions favored strategy 4, while earlier interruptions favored strategy 1 (low remaining MU) or strategy 2 (high remaining MU). Compared with discarding MU (strategy 3), the optimal strategy better preserved target coverage (ΔiCTV D95%: −0.07% vs −0.55%, p<0.05). Compared with resuming with remaining MU (strategy 4), it better preserved UHDR coverage (ΔBODY V40Gy/s: 0.21% vs −3.83%, p<0.05), with similar trends for OAR metrics. Measurements agreed with simulations for dose-rate distributions and deviations.
Conclusion
A practical real-time mitigation approach for intra-field interruptions in PBS FLASH proton therapy on Varian ProBeam was developed, substantially preserving planned dose and UHDR coverage and validated experimentally.