KEY POINTS
- The investigators tested hybrid ultra-high-dose-rate/conventional-dose-rate irradiation in two murine models: localized hindlimb irradiation in BALB/c mice for skin toxicity and whole-abdominal irradiation in C57BL/6 mice for gastrointestinal toxicity. Ultra-high-dose-rate irradiation was delivered with prototype electron FLASH systems.
- Hybrid treatment consisted of an uninterrupted FLASH boost followed after a one-minute beam-off interval by escalating conventional-dose-rate radiation. Skin experiments used 10- or 20-Gy FLASH boosts, while abdominal experiments used 10- or 14-Gy boosts, each compared with dose-matched conventional/conventional split-dose controls.
- For skin toxicity, a 20-Gy FLASH boost shifted the median toxic dose from 24.5 Gy with conventional irradiation to 30.9 Gy, giving a dose-modifying factor of 1.26 (95% CI 1.18–1.34). Significant survival advantages were observed at cumulative doses of 23, 26 and 29 Gy.
- Reducing the skin FLASH boost to 10 Gy weakened but did not eliminate protection: the dose-modifying factor fell to 1.14 (95% CI 1.07–1.22), with median toxic doses of approximately 27.0 versus 23.7 Gy. At 23 Gy, survival was 90% versus 50%, although this comparison did not reach significance (p=0.063).
- In the gastrointestinal model, a 14-Gy FLASH boost retained measurable sparing with a dose-modifying factor of 1.09 (95% CI 1.05–1.13) and significantly better survival at cumulative doses of 16 and 17 Gy.
- By contrast, reducing the abdominal FLASH boost to 10 Gy eliminated measurable sparing: dose-response curves overlapped and the dose-modifying factor was 1.00 (95% CI 0.96–1.04). Thus, the ability of hybrid delivery to preserve the FLASH effect appeared strongly dependent on tissue type and the size of the ultra-high-dose-rate component.
- Continuous FLASH irradiation in the authors' previous experiments produced larger dose-modifying factors than any hybrid regimen, confirming that adding a conventional-rate component attenuates normal-tissue protection. Crucially, the present study included no tumor model, tumor-control endpoint or clinical treatment, so therapeutic-ratio preservation remains unproven.
CLINICAL TAKEAWAY
Hybrid delivery could offer a practical route around one of FLASH RT's main engineering problems: using conventional-rate radiation for part of a conformal treatment without losing all normal-tissue sparing. The effect was dose- and tissue-dependent, however, and the concept remains far from clinical validation because tumor control and real-world conformal delivery were not tested.