SPDR-2 Histopathological investigation of the oligodendroglial tumors resected following alkylating agent chemotherapy

Oligodendrogliomas, i.e., lower grade gliomas with 1p/19q codeletion, are often responsive to chemotherapy, however, those tumors eventually recur and life-limiting in the majority of patients despite initial chemotherapeutic response. We have been treating those patients with upfront chemotherapy and subsequent resection following tumor volume decrease since 2006. This study aimed to elucidate the histological changes and the mechanism of recurrence after alkylating agent chemotherapy in oligodendrogliomas. Fifteen oligodendrogliomas (Grade 2: 12, Grade 3: 3) resected following tumor volume decrease after alkylating agent chemotherapy were included and compared with their pre-chemotherapy specimens. Histological changes were investigated using hematoxylin-eosin staining, and changes in proliferative activity, status of glioma stem cells (GSCs), and tumor-infiltrating macrophages were assessed using immunohistochemistry. The frequent histological findings following chemotherapy included a sparse glial background, abundant foamy cell infiltration, gliosis, calcification, and nuclear degradation. The Ki-67/MIB-1 index decreased and the number of macrophages increased after chemotherapy. Moreover, the ratio of GSCs to total tumor cells increased after chemotherapy. GSCs and macrophages constitute the mechanism of resistance to and recurrence after alkylating agent chemotherapy in oligodendrogliomas.