Long-term Musculoskeletal and Marrow Sparing with Proton FLASH in Juvenile Mice: Implications for Pediatric...

Osteosarcoma is the most common primary bone malignancy in children and adolescents. Radiotherapy is limited by intrinsic radioresistance and the risk of severe long-term musculoskeletal toxicities. FLASH radiotherapy, delivered at ultra-high dose rates (>40 Gy/s), has demonstrated normal tissue sparing in preclinical models, but its effects on the developing skeleton and marrow remain poorly defined. This study evaluated the chronic normal tissue effects of proton FLASH in juvenile mice, modeling the pediatric context.

Juvenile C57BL/6 mice (3-4 weeks old) were randomized to receive 11 Gy FLASH (≈200 Gy/s) or conventional proton irradiation (0.2 Gy/s average), or sham treatment to the left hind leg using a synchrotron-based proton beamline. Mice were followed for 10 weeks post-treatment. Bone toxicity was assessed with microCT (bone mineral density, bone volume fractions, trabecular indices) and histology. Bone marrow cellularity was quantified on H&E-stained sections, and muscle fibrosis was assessed using Masson's trichrome.

FLASH-treated mice exhibited significant preservation of bone microarchitecture compared with conventional treated mice, with higher bone mineral density and bone volume fractions (p < 0.05). Trabecular numbers were maintained, while structure model index indicates a mechanically favorable trabecular structures in the FLASH group. Bone marrow cellularity was preserved in FLASH mice (5.3% reduction vs. sham) compared with conventional (11.3 % reduction, p < 0.05). Muscle fibrosis was significantly lower in FLASH group (fibrosis positivity 2.6 % vs. 3.0% for FLASH vs. conventional CONV, p < 0.05). No severe immobility or weight loss was observed across groups.

Proton FLASH significantly reduces long-term bone, marrow, and muscle toxicities in juvenile mice. These findings provide the first demonstration of musculoskeletal sparing in a synchrotron proton FLASH platform and highlight its translational potential for pediatric osteosarcoma.