Spatial dose distribution modulates normal tissue response beyond peak-to-valley dose ratio in spatially fractionated radiotherapy

PURPOSE: Spatially fractionated radiation therapy (SFRT) enables delivery of high peak doses while sparing normal tissue, yet the relative contributions of peak dose, valley dose and spatial dose distribution remain unclear. This study investigates whether spatial dose organization contributes to normal tissue response beyond conventional dosimetry descriptors. Methods: Acute and late skin responses were evaluated in a murine hind-limb model after single-fraction irradiation with clinically pertinent Mini-GRID and planar beam configurations that allow for peak and valley dose effects to be decoupled. Longitudinal clinical scoring, histopathological evaluation and quantitative proteomic analysis were performed up to 90 days following irradiation. Results With the peak dose held constant, Mini-GRID irradiation resulted in reduced toxicity compared with planar irradiation delivering higher valley doses. In contrast, conditions with similar valley doses but substantially different peak doses showed comparable outcomes. These findings suggest that maximal dose alone does not act as a predictor of normal tissue response. Distinct molecular signatures associated with spatial dose heterogeneity were identified through a proteomic analysis corresponding to spatial dose heterogeneity, which was consistent with regulated tissue remodeling and resolution of inflammation. Conclusion: These findings suggest that spatial dose distribution contributes significantly to normal tissue response beyond peak and valley dose magnitude alone. While valley dose defines baseline tissue injury, spatial dose organization modulates repair capacity and long-term outcome. This work provides a radiobiological framework for SFRT optimization and supports the incorporation of spatial dose descriptors, beyond conventional metrics such as PVDR, into treatment design. .