Rat model of fractionated (2 Gy/day) 60 Gy irradiation of the liver: long-term effects
Introduction: Radiotherapy of liver tumors is limited by supposed sensitivity of liver tissue possibly leading to radiation-induced-liver disease. However in rats, high single dose irradiation (HDI) showed only mild effects. Consequences of fractionated irradiation (FI) in such an animal model have not been studied.
Materials and methods: Rats were exposed to selective liver FI (total dose 60 Gy, 2 Gy/day) or HDI (25 Gy) and were sacrificed after three-months or additionally after 1 to 48h (HDI, only). Blood cells, serum, and livers of the animals were preserved. Tissue samples were used for immunohistochemical studies, for protein and RNA isolation. Sera were used to monitor blood cells and tissue damage (leukocytes count, AST, ALT, AP, LDH).
Results: Three-months after irradiation, no differences of FI or HDI were found for macroscopically detectable small “scars” on the liver surface, and for an increased number of neutrophil granulocytes distributed in the portal-fields and through the liver parenchyma. As well, no changes in HE-stained tissues or clear signs of fibrosis were found around the portal vessels. Differences were seen for the number of bile ducts being increased in FI- but not in HDI-livers. Serum levels indicative of liver damage were determined for AP, AST, ALT, yGT, and LDH, and were increased for AP only after FI while HDI led to increases of all serum parameters tested. By performing RT-PCR, we detected up-regulation of MMP-2, MMP-9, MMP-14 and of TIMP-1, TIMP-2, and TIMP-3 shortly after HDI but not at three months after FI or HDI.
Conclusion: Selective FI and HDI of rat liver showed no severe long-term consequences. Both induce a diffuse formation of small scars at the liver surface. Lack of “provisional clot”-formation and almost complete absence of recruitment of mononuclear phagocytes could be one explanation for scar formation as incomplete repair response to irradiation. Mechanisms responsible for serological, cellular, and histological changes have to be further explored.