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Spread-Out Bragg Peak FLASH Radiotherapy for Head and Neck Reirradiation: A Treatment Planning Study
Marc Alomar1,2, Arnaud Pin3, Rasmus Nilsson4
1Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS, USA.
International Journal of Particle Therapy
|February 20, 2026
Summary
Proton FLASH radiotherapy shows promise for re-irradiating head-and-neck cancers, offering better normal tissue sparing. This study found spread-out Bragg peak (SOBP) FLASH plans provide dosimetric advantages over standard intensity modulated proton therapy (IMPT), especially for organs at risk.
Area of Science:
- Radiation Oncology
- Medical Physics
- Oncology
Background:
- Proton FLASH radiotherapy utilizes ultra-high dose rates to potentially improve normal tissue sparing while maintaining tumor control.
- Re-irradiation of recurrent head-and-neck (HN) cancer presents challenges due to proximity of critical structures and prior radiation exposure.
Purpose of the Study:
- To investigate the dosimetric advantages of spread-out Bragg peak (SOBP) FLASH proton therapy compared to standard intensity modulated proton therapy (IMPT) for re-irradiation in recurrent HN cancer.
- To assess the potential for enhanced normal tissue protection using FLASH techniques in a challenging clinical scenario.
Main Methods:
- Retrospective analysis of eight recurrent HN cancer cases treated with hypofractionated proton therapy (5 × 8 Gy fractions).
- Development of FLASH plans using a single energy layer and patient-specific modulation, with robust optimization for setup and range uncertainties.
- Application of a biologically effective dose model with a FLASH-modifying factor (FMF) of 0.7 for normal tissues to evaluate therapeutic potential.
Main Results:
- SOBP-FLASH plans showed reduced target homogeneity and slightly lower dose conformity compared to IMPT, particularly for targets near the neck.
- Robust target coverage was maintained in FLASH plans (D95% > 96.6% ± 1.2%) under uncertainty scenarios.
- FLASH plans demonstrated a reduction in maximum dose to organs at risk (OARs) near the target, e.g., larynx maximum dose decreased from 29.9 Gy (IMPT) to 25.4 Gy (FLASH).
Conclusions:
- SOBP-based FLASH proton therapy preserves the distal normal tissue sparing benefits of IMPT while offering potential reductions in high-dose exposure to nearby OARs.
- FLASH plans provide enhanced normal tissue protection under uncertainty, representing a significant advantage over conventional radiotherapy techniques.
- The use of an FMF of 0.7 in the effective dose model indicates a favorable dosimetric profile for FLASH in re-irradiation scenarios.
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