Musculoskeletal and Marrow Sparing With Proton FLASH Radiation Therapy in Juvenile Mice.

PURPOSE: Radiation‑induced musculoskeletal toxicities are one of the major limitations in pediatric patients whose skeletons are still developing. FLASH radiation therapy, delivered at ultrahigh dose rates (>40 Gy/s), has demonstrated normal-tissue sparing in preclinical models, but its effects on the developing skeleton and marrow remain incompletely characterized. This study evaluated the normal-tissue effects 10 weeks after proton FLASH irradiation in juvenile mice, modeling the pediatric context.

METHODS AND MATERIALS: Juvenile C57BL/6 mice (3 to 4 weeks old) were randomized to receive 11 Gy protons with FLASH (≈200 Gy/s), conventional dose-rate irradiation (CONV, 0.2 Gy/s average), or sham treatment (SHAM) to the left hind leg using a synchrotron-based proton beamline. Mice were followed for 10 weeks posttreatment. Bone toxicity was assessed with micro-computed tomography, including bone mineral density, bone volume fraction, trabecular indices, and limb length. Bone marrow cellularity was quantified on hematoxylin and eosin-stained sections, and muscle fibrosis was assessed using Masson's trichrome staining.

RESULTS

Proton FLASH irradiation preserved bone microarchitecture compared with conventional irradiation, with higher bone mineral density and bone volume fractions (P < .05). Trabecular number was maintained, while the structure model index indicated a mechanically favorable trabecular structure in the FLASH group. Proton FLASH also partially preserved longitudinal bone growth, with tibial length ratios of 0.94 (FLASH) versus 0.91 (CONV) versus 1.00 (SHAM). Bone marrow cellularity was preserved in FLASH mice (5.3% reduction vs SHAM) compared with CONV (11.3% reduction; P < .05). Muscle fibrosis was significantly lower in the FLASH group (fibrosis positivity 2.6% vs 3.0% for FLASH vs CONV; P < .05). No severe immobility or weight loss was observed across groups.

CONCLUSIONS

Proton FLASH reduced musculoskeletal and marrow toxicities at 10 weeks postirradiation in juvenile mice. These findings extend prior proton FLASH adult mice studies by demonstrating tissue sparing in a juvenile model and support further investigation of FLASH as a strategy to reduce normal‑tissue injury during radiation therapy of pediatric patients.