Abstract
Recent reports have shown that very high dose rate radiation (35–100 Gy/second) referred to as FLASH tends to spare the normal tissues while retaining the therapeutic effect on tumor. We undertook a series of experiments to assess if ultra-high dose rate of 35 Gy/second can spare the immune system in models of radiation induced lymphopenia. We compared the tumoricidal potency of ultra-high dose rate and conventional dose rate radiation using a classical clonogenic assay in murine pancreatic cancer cell lines. We also assessed the lymphocyte sparing potential in cardiac and splenic irradiation models of lymphopenia and assessed the severity of radiation-induced gastrointestinal toxicity triggered by the two dose rate regimes in vivo. Ultra-high dose rate irradiation more potently induces clonogenic cell death than conventional dose rate irradiation with a dose enhancement factor at 10% survival (DEF10) of 1.310 and 1.365 for KPC and Panc02 cell lines, respectively. Ultra-high dose rate was equally potent in depleting CD3, CD4, CD8, and CD19 lymphocyte populations in both cardiac and splenic irradiation models of lymphopenia. Radiation-induced gastrointestinal toxicity was more pronounced and mouse survival (7 days vs. 15 days, p = 0.0001) was inferior in the ultra-high dose rate arm compared to conventional dose rate arm. These results suggest that, contrary to published data in other models of radiation-induced acute and chronic toxicity, dose rates of 35 Gy/s do not protect mice from the detrimental side effects of irradiation in our models of cardiac and splenic radiation-induced lymphopenia or gastrointestinal mucosal injury.
Highlights
Recent reports have shown that very high dose rate radiation (35–100 Gy/second) referred to as FLASH tends to spare the normal tissues while retaining the therapeutic effect on tumor
Unintentional irradiation of these lymphocyte compartments has been implicated in radiation-induced lymphopenia (RIL), which in turn, has been increasingly recognized as an independent predictor of inferior overall survival (OS) in gliomas, pancreatic cancer, lung cancer, and hepatocellular carcinoma, and inferior progression free survival (PFS) in esophageal cancer and head and neck cancers[8,9,10,11,12,13]
Our interest in FLASH Radiation therapy (RT) stemmed from an attempt to find potential ways to prevent/reduce radiation-induced lymphopenia in esophageal, lung, and pancreatic cancer patients
Summary
Recent reports have shown that very high dose rate radiation (35–100 Gy/second) referred to as FLASH tends to spare the normal tissues while retaining the therapeutic effect on tumor. Favaudon et al recently reported an entirely new paradigm that has the potential to dramatically broaden the therapeutic window allowing substantial dose escalation to the tumor and/or considerable reduction in acute and late toxicity from incidental normal tissue irradiation This approach used ultra-high dose rate electron irradiation (4.5 MeV electrons at ≥40 Gy/s, termed FLASH radiation). Whereas most studies demonstrate RIL as a poor prognostic factor in cancer treatment outcomes, the direct correlation between dose to lymphocyte-rich organs and RIL has been most recently described in pancreatic cancer patients receiving unintentional splenic RT and esophageal cancer patients receiving unintentional cardiac and splenic RT10,17,18 Using irradiation of these two lymphocyte-rich organs as our model system for RIL, we studied the potential normal tissue sparing ability of ultra-high dose rate RT. We surmised that the benefit of such a technique of RIL reduction could impact a wide range of clinical scenarios including treatment of esophageal, lung, breast, pancreatic and hepatobiliary cancer patients, where RIL is associated with inferior OS and/or reduced complete pathological response rates[11,19,20,21,22]
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