Clinicopathological and genetic characteristics of extraventricular neurocytomas

Extraventricular neurocytoma (EVN) shares histological features with central neurocytoma, but has a wide morphological spectrum. Little is known regarding its clinicopathologic nature, biological behavior and genetic abnormalities. The aim of this study is to examine the diagnostic criteria, genetic abnormalities and biological behavior of EVN. Clinicopathological and molecular genetic studies were performed in seven EVNs. Among them, three cases showed atypical histology. Immunohistochemically, synaptophysin was robustly positive, but neuronal muclear antigen was positive in only half the cases (4/7cases). Isocitrate dehydrogenase enzyme isoform 1 (IDH1) (H09 immunostaining), α–internexin and p53 were negative in all cases. One case was positive for galectin‐3. None of the cases showed IDH1 R132 and IDH2 R172 mutation by direct sequencing. One case showed high polysomy of the epidermal growth factor receptor (EGFR) gene; however, O6‐methylguanine‐DNA methyltransferase (MGMT) promoter methylation and 1p/19q co‐deletion were not detected. Array‐based comparative genomic hybridization (CGH) study was performed in two cases, revealing different profiles, with loss and gain of multiple chromosomal loci. Two children (18%) had tumor recurrence after initial surgery, and one of them showed worse histology at recurrence and EGFR high polysomy. One patient died from the disease at 18.5 months after surgery. From our study, we concluded that EVNs were characterized by the absence of p53 overexpression, α‐internexin positivity, MGMT promotor methylation and IDH1/IDH2 mutation. Oligodendrocyte transcription factor 2 expression was seen in a scattered positive pattern but quite large numbers of tumor cells were negative. EVN is a WHO grade II tumor but some cases (2/7 cases in our series) can show late recurrence but mortality is low (1/7 cases in our series). CGH study suggested genetic heterogeneity of EVNs and unknown subclassification, which requires verification in more cases.