Objective: To study histopathologic changes and the role of the microglia/macrophage cell in the therapeutic effect of I-125 interstitial brachytherapy on the cerebral gliomas.

Methods: Out of a series of 60 cases with cerebral astrocytomas and other brain tumors treated with I-125 interstitial brachytherapy, autopsy materials were available in ten cases 0.75 and 60 months after irradiation. The patients were treated with the maximum dosage (60 Gy) on the tumor periphery. Besides the routine hematoxylin-eosine and Mallory's PTAH trichrome staining, immunohistochemical reactions were carried out for CD15, CD31, CD34, CD45, CD68, CPM, HAM56 and HLR-DR antigens on paraffin sections to study immunologic phenotypic characteristics of the reaction cell population around gliomas after I-125 treatment.

Result: One month after irradiation, a necrotic zone developed around the I-125 seeds within the 72 Gy isodose curve. Histologically, there was a fresh coagulation necrosis in the center of the lesion. Reactive zone has not yet developed but scattered interstitial and perivascular CD68 positive macrophages were present in the surrounding brain tissues. Six months after the I-125 isotope treatment, a reactive zone developed: a microglial rim around the necrosis tissue, and a broad area of proliferating vessels and glial fibrillary acidic protein (GFAP) positive astroglial cells which contained CD68 positive activated microglial and macrophage cells. Fifty-four months after I-125 interstitial irradiation, the necrotic center became colliquative and cystic. The microglial rim was replaced by round end stage (HLR-DR and CD31 positive) macrophages. The reactive zone was characterized by astrocytic gliosis but vascular proliferation and macrophages were lacking.

Conclusion: Results of the present immunohistochemical study suggest that the early lesions are characterized by migrating macrophages apparently concerned with the removal of necrotic debris. The established phase of reactive zone around the necrotic center is characterized by a narrow inner rim of microglial accumulation and a broad outer area characterized by astrocytic gliosis, vascular proliferation, activated microglia and infiltration by macrophages. In the burned-out phases of I-125 interstitial brachytherapy of gliomas, the necrosis undergoes liquefaction and the microglial rim is replaced by astrocytic gliosis which can be considered as equivalent to the scar tissue formed around necrosis outside the central nervous system.