Localized FLASH Radiotherapy Reduces Long-Term Skin and Muscle Damage While Preserving Systemic Homeostasis

Radiotherapy (RT) is a cornerstone treatment for nearly 50% of cancer patients, yet its efficacy remains limited by cumulative damage to healthy tissues over time. Delivering RT at ultra-high dose rates (FLASH-RT) represents a transformative strategy, as it appears to maintain tumor control, while sparing the surrounding normal tissue—a phenomenon termed FLASH effect. Here, we compared electron FLASH-RT and conventional RT (CONV-RT) on skin and muscle tissue of naïve mice assessing tissue integrity and gene expression. Bulk RNA sequencing revealed striking differences: FLASH induced minimal transcriptional disruption in skin and muscle, whereas CONV-RT triggered thousands of differentially expressed genes, including massive activation of fibrosis, inflammation, cell death-related pathways in skin, and broad dysregulation of genes linked to muscle function, remodeling and the unfolded protein response. Histological and ultrastructural analyses corroborated the findings, showing reduced immune infiltration in the skin and preserved tissue architecture both in skin and muscle following FLASH. In conclusion our study not only confirms the protective nature of FLASH but also provides novel mechanistic insights into the cascade linking local injury to systemic dysfunction under CONV-RT, reinforcing the translational potential of FLASH to expand the therapeutic window of radiotherapy. Overall design: C57BL/6J mice were exposed to 35 Gy electron irradiation to the left hindlimb by either FLASH-RT or conventional (CONV-RT) modality. Sham, not irradiated mice served as controls. 70 days post treatment, skin and muscle tissues from the irradiated area were collected and the trascriptomic profile investigated by RNA-Seq.