Combined minibeam and conventional radiotherapy reduces severe skin toxicity and improves tumor control

Radiotherapy (RT) is an important tool for cancer treatment, enabling the delivery of curative doses to tumors while minimizing exposure to surrounding healthy tissue. Advances in beam delivery techniques and precise image-guidance systems have significantly reduced radiation exposure to normal tissues; however, treatment-related side effects remain a major limitation. To address these challenges, novel preclinical strategies are being explored. This study investigated minibeam radiotherapy (MBRT), which employs narrow (>100 microns) parallel beams of X-rays that produce a distinctive peak-and-valley dose distribution. We aimed to assess the efficacy and skin toxicity of MBRT compared to conventional radiotherapy (CONV), and to examine the role of macrophages in treatment response. We used a syngeneic glioblastoma mouse model, created by subcutaneously injecting [Formula: see text] GL261 cells into the flank, and applied single-fraction treatments of MBRT, CONV, or a combined minibeam-conventional regimen (MBRT-CONV). Irradiation was performed using a dedicated, image-guided small animal radiotherapy system, combined with a custom minibeam collimator. We evaluated tumor-specific survival and acute radiation-induced skin damage at macroscopic and microscopic levels through histopathological analysis to compare treatment outcomes. Our results showed that MBRT led to significantly reduced incidence and severity of skin ulceration compared to CONV at high single doses (25-30 Gy), although MBRT alone provided less effective tumor control. Additionally, we evaluated the effects of these treatments on healthy skin to assess the tissue-sparing potential of MBRT. Specifically, we investigated the impact of RT on macrophage recruitment in healthy skin. Based on the rapid inflammatory resolution observed in healthy skin, we implemented a delayed combined treatment strategy to enhance tumor suppression, by administering a CONV dose fraction seven days after the initial MBRT. Among the tested regimens, the 30 Gy MBRT + 5 Gy CONV combination achieved the most pronounced tumor growth delay and survival benefit, while maintaining a low incidence of toxicity.