Abstract
<h3>Purpose/Objective(s)</h3> The present study investigated the anti-tumor effect, immune response, and intestinal injury in breast cancer-bearing mice after ultra-high dose rate radiotherapy (FLASH-RT) and conventional dose rate radiotherapy (CONV-RT). <h3>Materials/Methods</h3> Six-week-old female C57BL/6 mice were inoculated subcutaneously with Py8119 and Py230 breast tumor cells in the inguinal mammary gland, and received 10 Gy abdominal 6 MeV X-ray FLASH-RT (>100 Gy/s) or CONV-RT (0.2 Gy/s) 15 days after tumor inoculation. EBT3 radiographic films were used to confirm the dose. Tumor samples were obtained 2 weeks post-irradiation (PI) in Py8119 tumor-bearing mice, spleen tissues were collected 4 weeks PI in Py230 tumor-bearing mice. Flow cytometry (FC) was performed with tumor and spleen tissues and immunohistochemistry (IHC) staining was performed in tumor tissues to test CD3+, CD4+, and CD8+ T lymphocytes. The number and length of remaining crypts were measured to evaluate acute intestine damage 3 days PI. <h3>Results</h3> Both FLASH-RT and CONV-RT significantly reduced the tumor weights. FC showed increased CD8+/CD3+ and CD8+/CD4+ ratios in tumor tissues and IHC confirmed a similar CD8+ T cells percentage 2 weeks PI, but there is no significant difference between FLASH and CONV groups. Surprisingly, significantly smaller spleen weight and substantially higher CD8+/CD3+ and CD8+/CD4+ ratios in the spleen in FLASH compared with non-irradiated control and CONV groups 4 weeks PI, indicating FLASH might stimulate a systematic immune response. FLASH irradiated mice had more remaining crypts and significantly longer crypt length than CONV irradiated mice, indicating a normal tissue sparing effect of FLASH-RT. <h3>Conclusion</h3> FLASH-RT and CONV-RT effectively suppressed tumor growth and promoted CD8+ T cell influx into the tumor. FLASH-RT resulted in a smaller weight and a higher proliferative rate of CD8+ T cells in the spleen than CONV-RT, which might indicate a different systematic immune effect between FLASH-RT and CONV-RT. FLASH-RT significantly spared the intestine from early damage.
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