Gamma knife radiosurgery for neuroepithelial tumors: radiological and histological changes

Gamma knife radiosurgery (RS) has been introduced as a modern therapy for brain tumors. However, the effects of RS for neuroepithelial tumors are still obscure. The present study investigates the radiological and histological changes after RS to elucidate the biological effect. There were seven cases (two males and five females), ranging from 4 to 71 years with a mean age of 33 years. Two cases were located in the brainstem, another two in the cerebellum, and one each in the thalamus, the hypothalamus, and the frontal lobe. Histologically, two cases had gangliogliomas, four astrocytomas (1 pilocytic, 1 fibrillary, 2 anaplastic), and one glioblastoma. RS was performed after surgery with a central dose of 30-36 Gy. All cases were evaluated radiologically on MRI before and after RS. Four cases (3 astrocytomas and 1 glioblastoma) which neurologically deteriorated after RS were reoperated. These cases were examined using HE and immunohistochemical studies with antibodies of CD34, alpha-smooth muscle actin (SMA), p53, p21 and MIB-1 on the sections before and after RS. MRI demonstrated perifocal edema and intratumoral hypointensity on T2 weighted imaging (T2WI), suggesting radionecrosis in most of the cases within 6 months after RS. In the central part of the RS, destructive changes were observed in the tumor cells and endothelial cells: decrease in the tumor cell population, coagulation necrosis, and fibrinoid degeneration of vascular walls were revealed. In the peripheral part, however, some tumors contained viable tumor cells intermingled with blood vessels showing endothelial and pericytic proliferations. The increase of MIB-1 staining index was found in only one case. The p21 immunoreactivity was increased in endothelial cells, although the p53 immunoreactivity was unchanged. These results suggested that radionecrosis occurred earlier and more frequently in neuroepithelial tumors after RS than after conventional radiation.