Pre-clinical studies investigating the combination of hypofractionated radiation with hyperthermia in a murine tumor and normal skin

Introduction: This preclinical study evaluated the effects of combining hypofractionated radiation (HFRT), either as X-ray photons or protons, with hyperthermia on tumor response and normal tissue damage in mice. Methods: The tumors were C3H mammary carcinomas implanted in the right rear foot of male CDF1 mice, while non-tumor-bearing mice were used to assess normal foot skin. HFRT was delivered in three fractions (5, 10, or 15 Gy) at 3 to 4-day intervals. Hyperthermia (40.5–42.5 °C) was applied once for 60 min, either 30, 90, or 180 min after the final radiation dose. Endpoints included tumor growth delay and moist skin desquamation. Mechanistic studies assessed DNA damage (γ-H2AX foci) 24 h after 3 × 10 Gy, with or without hyperthermia (42.5 °C for 1 h, administered 30 min post-RT), and tumor hypoxia (pimonidazole staining) measured 1 h after the last radiation fraction. Results: Animals responded similarly to X-ray photons and protons in the tumor and skin. Hyperthermia enhanced the response to X-ray photons in both tissues, with temperature and time-interval dependency, showing the greatest effects at higher temperatures and shorter intervals. Protons combined with hyperthermia showed similar results, although with less decay in time-interval effects at 42.5 °C. DNA damage assessments revealed no significant difference between radiation types, but a significant enhancement was seen when tumors were heated at 42.5 °C. Tumor hypoxia was reduced after 3 × 10 Gy irradiation with either radiation type. Conclusions: Combining HFRT with hyperthermia yielded effects comparable to single-dose studies for both tumors and normal tissues. These responses were similar for both X-rays and protons.