Spread-Out Bragg Peak FLASH Radiotherapy for Head and Neck Reirradiation: A Treatment Planning Study

Proton FLASH radiotherapy offers the potential to enhance normal tissue sparing while maintaining tumor control. This study investigates the dosimetric advantages of spread-out Bragg peak (SOBP) FLASH compared to standard intensity modulated proton therapy (IMPT) for re-irradiation in recurrent head-and-neck (HN) cancer.

Eight recurrent HN cancer cases were retrospectively analyzed using hypofractionated proton therapy plans (5 × 8 Gy fractions). FLASH plans were designed using a single energy layer and patient-specific modulation devices implemented in the RayStation. Robust optimizations accounted for setup (±3 mm) and range (±3.5%) uncertainties. A biologically effective dose model incorporating a FLASH-modifying factor (FMF) of 0.7 for normal tissues meeting dose (≥5 Gy) and dose rate (≥40 Gy/s) thresholds was used to assess the therapeutic potential of FLASH. Dosimetric parameters such as target coverage, homogeneity index (HI), conformity index (CI), and organ-at-risk (OAR) sparing were compared between FLASH and standard IMPT plans.

Compared to standard IMPT, SOBP-based FLASH plans demonstrated reduced target homogeneity (p < .05) and slightly lower dose conformity, with the effects more pronounced for targets near the neck region due to larger center-to-axis distances to avoid collisions. Despite these limitations, FLASH plans maintained robust target coverage (D95% > 96.6% ± 1.2%) in robustness scenarios. The FLASH effective dose model indicated a reduction in the maximum dose to OARs within 2 cm of the target volume; for instance, the maximum dose to the larynx decreased from 29.9 Gy (IMPT) to 25.4 Gy (FLASH).

SOBP-based FLASH plans preserved the dosimetric advantages of IMPT for sparing distal normal tissues while offering potential reductions in high-dose exposure to OARs near the target volume. Incorporating an FMF of 0.7, these plans showed comparable dosimetric profiles to IMPT with the added benefit of enhanced normal tissue protection under uncertainty, a unique advantage over conventional radiotherapy.