Abstract 5349: Proton FLASH radiation spares normal skin and soft tissues of the murine leg from radiation-induced damage while being equipotent with standard proton radiation in controlling sarcoma growth
Authors: Velalopoulou Anastasia , Karagounis Ilias V. , Verginadis Ioannis I. , Goia Denisa , Kim Michele , Shoniyozov Khayrullo , Cengel Keith , Diffenderfer Eric , Dong Lei , Metz James , Busch Theresa , Maity Amit , Koumenis Costas 
Published: September 2020

Purpose: To investigate the potential normal tissue sparing effects of proton FLASH radiation on skin injury, leg contraction, and soft tissue inflammation while maintaining tumor cell killing efficacy for orthotopic murine fibrosarcoma tumor.

Materials and Methods: C3H/HeJ and C57BL/6J mice (n= 6) received 45 Gy of proton FLASH (&gt;85Gy/sec) or conventional radiation (CONV) (1Gy/sec), at the right hind leg. Normal tissue responses were evaluated by recording the skin reaction and the leg contraction over time as acute and chronic macroscopic observations of radiation-induced damage. The skin reactions were recorded using a scoring system of 10 grades, ranging from 0.5 to 3.5, with breakdown of irradiated skin area with moist exudate to be defined as 3. Leg contraction assay examined the extensibility of the leg using a jig and leg contracture was defined as the difference in length between the irradiated and non-irradiated hind leg. Neutrophil infiltration, measured by myeloperoxidase (MPO) activity, and persistent inflammation have been associated with the development of fibrosis, as a long-term effect post radiation. MPO activity was quantified by using intravenously administered luminol and chemiluminescence IVIS imaging at several time points post radiation. We also compared the antitumor efficacy of proton Flash radiation with Proton Standard radiation using syngeneic mice bearing orthotopic fibrosarcoma tumors on the hind leg. Tumors were established by intramuscularly injecting radiation-induced fibrosarcoma (RIF) cells in C57BL/6J mice. Tumor growth was monitored by tumor volume measurements, using a caliper.

Results: Acute effects of radiation on skin began with erythema, edema, pigment changes and depilation and was followed by complete loss of the epidermis, constant fibrinous exudates and edema. Skin reaction of C3H/HeJ mice exposed to proton FLASH radiation blindly received a 60% lower score than the CONV-irradiated mice at day 24 post IR, whereas C57BL/6J mice received 60% lower score at day 39, as they appear to be more radiosensitive. Leg contracture was clearly observed by day 68 in the C3H/HeJ whereas in C57BL/6J appeared much earlier, at 10 days post-radiation. Proton FLASH-irradiated group of C57BL/6J mice presented 50% less leg contracture as compared to the CONV-irradiated group, by day 46. MPO activity peaked for both strains at day 18 post radiation and FLASH-irradiated mice showed significantly higher levels of MPO activity compared to the CONV-irradiated mice. The MPO activity was still significantly different between proton FLASH and CONV groups up to day 27 in C57BL/6J mice. Moreover, proton FLASH and CONV radiation inhibited fibrosarcoma tumor growth to the same extent, and there was no significant difference in tumor growth delay or overall survival between the two groups.

Conclusions: Proton FLASH radiation causes significant less skin and soft tissue injury as well as lower levels of inflammation post-radiation compared to proton CONV radiation whereas tumor growth inhibition is indistinguishable between the two modalities.

Citation Format: Anastasia Velalopoulou, Ilias V. Karagounis, Ioannis I. Verginadis, Denisa Goia, Michele Kim, Khayrullo Shoniyozov, Keith Cengel, Eric Diffenderfer, Lei Dong, James Metz, Theresa Busch, Amit Maity, Costas Koumenis. Proton FLASH radiation spares normal skin and soft tissues of the murine leg from radiation-induced damage while being equipotent with standard proton radiation in controlling sarcoma growth [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5349.