Flash Irradiation Inhibits Breast Onset and Spares Radiation-Induced Liver Injury

Abstract

<h3>Purpose/Objective(s)</h3> To clarify the efficacy of FLASH radiotherapy (FLASH-RT) in inhibiting mammary tumorigenesis and normal tissue sparing. <h3>Materials/Methods</h3> We used a spontaneous mammary tumor model of MMTV-PyMT mice for the study. Prior to the formation of mammary tumor, MMTV-PyMT mice (3–6-week-old) were treated with a single fraction dose of 9-16 Gy FLASH-RT or conventional radiotherapy (CONV-RT) using a mice-applicator designed targeting five pair of mammary glands. Tumor growth was detected by thorough palpation of all 10 mammary glands twice a week. Liver function, H&E staining and TUNEL staining of liver tissues were performed to determine the irradiation-induced liver injury. Inflammatory cytokines of TNF-a, IFN-g, IL-6 and IL-10 in the liver tissue and in the serum on Days 6 (D6), 18 (D18) and 31 days (D31) post-irradiation (IR) were tested by ELISA. DHE staining in frozen sections of tumor tissue and liver tissue at 4 weeks post-IR were performed, respectively. <h3>Results</h3> Median days for first tumor occurrence were 65, 62, 58.5 and 52 days of age, while median days for the involvement of 10th mammary gland were 79, 76, 76 and 63 days of age for FLASH (14-16 Gy), FLASH (9-12 Gy), CONV (9-12 Gy) and Control (0 Gy) groups, respectively. Tumor weight at 12 weeks after birth in FLASH and CONV were significantly lower than controls. On 18 days after IR, serum ALT level was significantly increased in the CONV compared with the samples in FLASH. H&E staining of liver in 7 weeks after 9-12 Gy IR revealed that FLASH caused less histological abnormalities in comparison to the samples from CONV, which was similar to the tissue of control. The number of TUNEL-positive hepatocytes in the FLASH was less than that of the CONV but similar to that of the control. In liver tissue, TNF-a, IFN-g and IL-6 significantly increased in CONV compared with FLASH and control; however, IFN-g and IL-6 also significantly increased in FLASH compared with control, while IL-10 significantly decreased in CONV compared with control. Serum IL-6, IFN-g and TNF-a levels in FLASH did not markedly change on D6, D18 and D31 post-IR compared with control. Serum IL-6, IFN-g and TNF-a levels were elevated on D6 and continuously increased on D18 and D31 in CONV, but began to increase on D6, and then recovered to the control level on D18 or D31 in FLASH. Serum IL-10 levels decreased at D6, D18 and D31 in CONV, but began to decrease at D6, and then recovered to the control level at D18 or D31 in FLASH. FLASH induced reactive oxygen species (ROS) accumulation in the tumor tissue but not in normal tissue; while absence of ROS accumulation in either tumor or normal tissue under CONV. Notably, the ROS level of normal liver tissue did not differ under CONV or FLASH. <h3>Conclusion</h3> We found that mammary tumorigenesis was inhibited by FLASH without significant liver injury compared with CONV-RT. The alteration of serum and liver inflammatory cytokines induced by CONV-RT was mildly altered by FLASH. This unique benefit can by partially explained by the accumulatio of ROS in tumor tissue after FLASH.