Optimizing Transmission FLASH Radiation Therapy for Large-Field Postmastectomy Breast Treatment

Ahmal Jawad Zafar; Sunil William Dutta; Matthew Joseph Case; Zachary Diamond; Duncan Bohannon; Reshma Jagsi; Xiaofeng Yang; Jun Zhou

doi:10.1016/j.ijrobp.2026.04.095

Optimizing Transmission FLASH Radiation Therapy for Large-Field Postmastectomy Breast Treatment

Int J Radiat Oncol Biol Phys. 2026 May 8:S0360-3016(26)00692-9. doi: 10.1016/j.ijrobp.2026.04.095. Online ahead of print.

Authors

Ahmal Jawad Zafar¹, Sunil William Dutta¹, Matthew Joseph Case¹, Zachary Diamond¹, Duncan Bohannon¹, Reshma Jagsi¹, Xiaofeng Yang¹, Jun Zhou²

Affiliations

¹ Department of Radiation Oncology and Winship Cancer Institute, Emory University, Atlanta, Georgia.
² Department of Radiation Oncology and Winship Cancer Institute, Emory University, Atlanta, Georgia. Electronic address: jun.zhou@emory.edu.

PMID: 42107782
DOI: 10.1016/j.ijrobp.2026.04.095

Abstract

Purpose: We investigated the effect of scanning speed, beam configuration, and dose-rate modeling on the FLASH effect in postmastectomy proton transmission beams planning and evaluated the potential of spot scanning path optimization for enhancing the FLASH effect.

Methods and materials: Five patients with left-sided postmastectomy breast cancer (32 Gy/5 fractions) were retrospectively replanned with single-energy (249 MeV) tangential transmission beams, supplemented by a clinical en face beam for dose homogenization. FLASH evaluation employed 2 models: Krieger's FLASH effectiveness model (FEM) and Folkerts' average dose-rate (ADR) framework. Plans were simulated under conventional pencil beam scanning, split-field, and optimized spot sequences (using genetic algorithm [GA]), with vertical scan speeds varied from 10 to 20 mm/ms. FLASH effect in normal tissues was quantified by the percentage of voxels meeting the threshold (≥4 Gy at ≥40 Gy/s). A dose adjustment factor of 0.67 was applied to voxels meeting FLASH criteria to compute FLASH-weighted dose metrics in normal tissues within the chest wall target, whereas the physical dose to tumor cells remained unchanged.

Results: The FLASH effect showed high sensitivity to scanning patterns and model selection. Increasing vertical scan speed from 10 to 20 mm/ms increased the FLASH in clinical target volume (CTV) by 22% (ADR) and 12% (FEM), whereas in skin, it rose from 41.4% to 58.8% (ADR) and 8.4% to 13.1% (FEM). Split-field delivery improved the temporal distance between the vertical columns of the spot scanning pattern, yielding a superior FLASH effect, which is up to a 9.2 Gy reduction in CTV (FLASH-corrected) D_mean_{(mean dose)} with the ADR model. GA-based optimization shortened scan time and provided FLASH comparable with split-field delivery, with a CTV D_mean reduction of 7.87 Gy (ADR GA), with skin D_mean reductions of 2 to 3 Gy.

Conclusion: This study demonstrates that FLASH outcomes are highly sensitive to scanning trajectory, scan speed, and model selection. Beyond these parameters, optimizing spot delivery using a path minimizer, such as GA, can further improve the dose-rate distribution in healthy voxels across all scenarios.