Delayed radiation necrosis of brain evaluated positron emission tomography

In four patients highly suspected of having delayed radiation necrosis of the brain, regional cerebral blood flow and metabolism were studied by using positron emission tomography (PET) with the 15O-labelled gases, 18F-fluorodeoxyglucose (18FDG). All four patients showed marked decreases in glucose and oxygen consumption in the lesion with that suspected pathology in comparison with those in normal brain parenchyma. Two of them were histologically proved to have radiation injuries (one was radiation necrosis and another was early delayed reactions). But one patient was histologically proved to have a recurrent tumor. Although metabolic study with PET using 18FDG is generally an excellent procedure for differentiation between radiation injuries and recurrent brain tumor, these results imply that in some cases the differential diagnosis is difficult even with 18FDG. The present study demonstrated considerably more pronounced reduction in the metabolism than the decrease of the blood flow in the lesion with radiation necrosis. This result would indicate that the primary effect causing the radiation necrosis is a direct effect of irradiation on neural tissue rather than the capillary damage.