Abstract

PURPOSE: To investigate whether the vascular collapse in tumors by conventional dose rate (CONV) irradiation (IR) would also occur by the ultra-high dose rate FLASH IR.METHODS AND MATERIALS: Lewis lung carcinoma (LLC) were subcutaneously implanted in mice followed by CONV or FLASH IR at 15 Gy. Tumors were harvested at 6 or 48 hr post-IR and stained for CD31, phosphorylated myosin-light chain (p-MLC), gammaH2AX, intracellular reactive oxygen species (ROS), or immune cells such as myeloid and CD8alpha T cells. Cell lines were irradiated with CONV IR for Western blot analyses. ML-7 was intraperitoneally administered daily to LLC-bearing mice for 7 days prior to 15 Gy CONV IR. Tumors were similarly harvested and analyzed as above.RESULTS: By immunostaining, we observed that CONV IR at 6 hr post-IR resulted in constricted vessel morphology, increased expression of phosphorylated myosin light chain (p-MLC), and much higher numbers of gammaH2AX (surrogate marker for DNA double strand break)-positive cells in tumors, which were not observed with FLASH IR. Mechanistically, we found that MLC activation by reactive oxygen species (ROS) is unlikely since FLASH IR produced significantly higher ROS than CONV IR in tumors. In vitro studies demonstrated that ML-7, an inhibitor of MLC kinase abrogated IR-induced gammaH2AX formation and disappearance kinetics. Lastly, we observed that CONV IR when combined with ML-7 produced some effects similar to FLASH IR including the reduction in the vasculature collapse, fewer gammaH2AX-positive cells, and increased immune cell influx to the tumors.CONCLUSIONS: FLASH IR produced novel changes in the tumor microenvironment that were not observed with CONV IR. We believe that MLC activation in tumors may be responsible for some of those microenvironmental changes differentially regulated between CONV and FLASH IR.

View details for DOI 10.1016/j.ijrobp.2020.11.012

View details for PubMedID 33186615