Abstract 1335: Comparison of X-ray and gamma-ray irradiators for in vitro and in vivo biological effectiveness

Abstract Objective: The Cesium Irradiator Replacement Program (CIRP) was established in 2014 to replace radioisotope-based irradiators with non-radioactive alternatives, such as X-rays. As most dosimetry is based on 137Cs irradiators, transitioning to X-ray systems has presented challenges in radiobiological research. A key obstacle stems from the inaccurate assumption of dose as a directly transferable constant across different irradiation types. To mitigate this issue, the University of Wisconsin-Madison, as a CIRP participant, has undertaken a study to establish consistency in dose measurement and biological response, both in vitro and in vivo, with a specific emphasis on the relative biological effectiveness of X-rays compared to cesium. Methods: Radiation dose was calibrated using experimental phantoms and thermoluminescent dosimeters. To determine dose response in vitro, HeLa cells were treated with 0 to 8 Gy of radiation in 2 Gy increments delivered from a 137Cs, 60Co, or 195 kVp X-ray source. Colony formation and comet assays were used to evaluate clonogenic survival and DNA damage, respectively. For in vivo studies, C57BL/6 mice received a single 8 Gy whole-body radiation dose from a 137Cs, 60Co, 220 kVp, or 300 kVp X-ray source. Lethal Dose50/30 (LD50/30) was used as the primary end point. Histopathological analysis of the spleen and bone marrow was performed to identify primary radiation-induced organ damage. Quantitative assessment of tissue injury was conducted using Halo image analysis. Hematology and blood chemistry were measured to monitor radiation-induced toxicity and subsequent recovery. Results: The 195 kVp x-ray source produced the highest number of DNA double-strand breaks, followed by 137Cs and 60Co. This was most evident as the total radiation dose increased and translated to a greater impairment of clonogenic survival, particularly evident at 4, 6, and 8 Gy for 195 kVp X-rays. While 137Cs and 60Co had similar effects at lower doses, 137Cs induced a more pronounced reduction in colony formation at 6 and 8 Gy. In animal studies, 60Co exhibited the highest LD50/30 (8.5 Gy), followed by 137Cs (8.1 Gy), 300 kVp (7.3 Gy), and 220 kVp (6.2 Gy), respectively. Conclusion: The differences were found in the biological effects between radioisotope-based and X-ray irradiators when delivered the identical physical dose. In animal models, radiation from a 60C source demonstrated a weaker biological effect than 137Cs, while 220 kVp X-rays showed the strongest biological effect. Given the differences in the energy spectra of the radiation sources, we will further investigate how dose-energy distribution influences biological effects in radiation-sensitive organs, particularly superficial tissues and hematopoietic systems. Citation Format: Caroline Mae Mork, Shrey Ramesh, Keith Kunugi, Kwangok P. Nickel, Larry Dewerd, Nina Zitzer, Randall Kimple. Comparison of X-ray and gamma-ray irradiators for in vitro and in vivo biological effectiveness [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1335.