Author: Ning Cao, Sunan Cui, Danielle P. Johnson Erickson, Juergen Meyer, Ramesh Rengan, Ben A Shaver, Jing Zeng 👨🔬
Affiliation: Department of Radiation Oncology, Fred Hutchinson Cancer Center, University of Washington, University of Washington and Fred Hutchinson Cancer Center, University of Washington 🌍
Purpose: To investigate the immune response in primary irradiated tumors and non-irradiated tumors in a mouse melanoma xenograft model, comparing 18Gy of conventional (CONV) and ultra-high dose rate (FLASH) proton radiation at 24-hour and one-week post-treatment.
Methods: YUMM3.2 melanoma cells were implanted in the right and left thigh of 6-week-old female C57BL/6 mice 10-12 days and 8 days prior to irradiation, separately. Mice were divided into 3 groups: 1) control/sham (n=13 for both time points); 2) CONV (n=14 for both time points); 3) FLASH (n=13 for 24-hour, n=16 for 1-week). Irradiation was with a 48.7MeV proton beam on a Scanditronix MC50 compact cyclotron, and the beam can be shaped by adjustable vertical and horizontal collimators. Using the proton beam entrance region, the right tumor in group 2 and 3 were irradiated to 18Gy at CONV (0.6–1 Gy/s) or FLASH (80-100 Gy/s) dose rates. Both the right and left tumors were harvested at 24-hour or 1 week post radiation. The tumors were digested and stained in CD3, CD4, CD8, and CD45 for florescence-activated cell sorting (FACS).
Results: At 24-hour post radiation, both CONV and FLASH groups showed reductions in CD4+ and CD8+ cells in the irradiated tumors, with a significant decrease in CD8+ cells in the FLASH group (p=0.1). By one week, both groups had increased CD4+ and CD8+ cells in the irradiated tumors, notably with a significant increase of CD8+ cells in the FLASH group (p=0.1). In unirradiated tumors, CD4+ cells significantly increased in the CONV group, while CD8+ levels rose in both (p=0.07 for FLASH and p=0.1 for CONV).
Conclusion: CONV and FLASH proton radiation differ in their effects on the immune response in tumors, with FLASH correlating with a higher presence of cytotoxic T cells (CD8+). Further studies are necessary to explore these immune responses in more depth.