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Introduction: Chromosomes 1p/19q co-deletion is a robust molecular marker for the diagnosis of oligodendroglial tumors, and has been included in the 2016 WHO modified classification. Although treatment for oligodendroglioma is controversial, upfront chemotherapy is regarded as one of the treatment option for low-grade tumor. We have treated all the 1p/19q co-deleted oligodendrogliomas, both grades II and III, with upfront chemotherapy without conventional radiotherapy for 20 years. The clinical experience from this trial may be suggestive for understanding of the biological features of oligodendroglioma with 1p/19q co-deletion toward precision medicine., Methods: This is a long-term retrospective data of the non-selected patients with 1p/19q co-deleted oligodendrogliomas uniformly treated with up-front chemotherapy. Seventy consecutive patients (48 with grade II and 22 with grade III tumors) were included., Results: The median follow-up period was 13 years. The 5-, 10-, and 15-year progression-free survival (PFS) rates were 85.7%, 54.8%, and 31.5%, respectively, and the median PFS was 146 months. In most cases, tumor recurrence was remained local and could be controlled by salvage surgery and/or chemotherapy. The 5-, 10-, and 15-year overall survival (OS) rates were 96.8%, 88.7%, and 80.0%, respectively, and the median OS was not reached. These survival data compared favorably with previous large clinical studies employing radiotherapy. Tumor grades based on World Health Organization classification, extent of surgery, and age affected neither PFS nor OS. Most patients were able to return to their premorbid social life., Conclusions: The long-term results drawn from 20-years of single institution experience show that the patients with 1p/19q co-deleted oligodendrogliomas can be successfully treated with up-front chemotherapy alone without compromising OS.

Despite accumulating knowledge regarding molecular backgrounds, the optimal management strategy for low-grade gliomas remains controversial. One reason is the marked heterogeneity in the clinical course. To establish an accurate subclassification of low-grade gliomas, we retrospectively evaluated isocitrate dehydrogenase-1 (IDH1) mutation in clinical specimens of diffuse astrocytomas (DA) and oligodendroglial tumors separately. No patients were treated with early radiotherapy, and modified PCV chemotherapy was used for postoperative residual tumors or recurrence in oligodendroglial tumors. Immunohistochemical evaluation of IDH status, p53 status, O(6)-methylguanine methyltransferase expression, and the MIB-1 index were performed. The 1p and 19q status was analyzed with fluorescence in situ hybridization. Ninety-four patients were followed for a median period of 8.5 years. For DAs, p53 was prognostic for progression- free survival (PFS) and IDH1 was significant for overall survival (OS) with multivariate analysis. In contrast, for oligodendroglial tumors, none of the parameters was significant for PFS or OS. Thus, the significance of IDH1 mutation is not clear in oligodendroglial tumors that are homogeneously indolent and chemosensitive. In contrast, DAs are heterogeneous tumors including some potentially malignant tumors that can be predicted by examining the IDH1 mutation status.