Effects on hematopoietic and immune system of ultra-high dose rate radiotherapy (FLASH-RT).

Abstract

166 Background: Ultra-high dose rate radiotherapy, so-called FLASH radiotherapy (FLASH-RT) has shown to reduce radiation-induced damage including lung fibrosis, brain damage, skin fibrosis, etc. We investigated the effect of FLASH-RT compared of conventional dose rate radiotherapy (CONV-RT) at 5Gy of TBI on survival of hematopoietic system, peripheral blood cells, and immune-related responses. Methods: Mice were divided into 3 groups: control (0 Gy), FLASH-RT (109Gy/s), and CONV-RT (0.067Gy/s). FLASH-RT was performed with DIRAMS LINAC producing 6 MeV electron beams. Irradiated mice were sacrificed at 1, 2, 4, 7, 14, 21 and 28 post-TBI of 5 Gy and cellularity of peripheral blood and bone marrow, histological evaluation of immune organ such as spleen and thymus, and T and B lymphocytes in mouse spleen by flow cytometry were investigated. Results: In peripheral blood cell counts, RBC, neutrophil, lymphocyte counts were not significantly different between FLASH-RT and CONV-RT groups from day 1 to day 28 post-TBI. However, FLASH-RT initially induced more apoptosis of bone marrow cells, platelets, and eosinophils of peripheral blood compared to CONV-RT. FLASH-RT and CONV-RT groups showed similar histological changes in the thymus and spleen after 5Gy of TBI, but T lymphocytes of CONV-RT group in day 28 were less recovered compared to those of FLASH-RT group. Our results indicated that FLASH-RT affected more radiation-induced damage of hematopoietic system in early stages compared to CONV-RT, but FLASH-RT showed higher recovery of T lymphocytes after irradiation. Conclusions: These results show that FLASH-RT can induce similar damage and recovery pattern on hematopoietic system with CONV-RT in first week, but FLASH-RT might affect less T-lymphopenia in long-term than CONV-RT.