Extraventricular neurocytomas: a morphological and histogenetic consideration. A study of six cases

Aims: Various molecular markers have been used for diagnosis, management and prognostication of gliomas. Neurocytomas are close morphological mimics of oligodendrogliomas. While combined 1p/19q deletion has been used as a molecular signature of oligodendroglial tumours, it has also been variably reported to occur in neurocytomas, especially those in extraventricular locations (EVN). In recent studies, presence of IDH1 mutation has shown immense prognostic significance in glial tumours including oligodendrogliomas, but its role in neurocytoma pathogenesis remains unexplored. In this study, EVN cases were analysed for histomorphological features, IDH1 mutation using an antibody for specifically detecting mutant IDH1(R132H) protein, and 1p/19q deletion by fluorescence in situ hybridisation (FISH) assay. Results: Over a period of 10 years (2000-2009), 60 cases of neurocytoma were diagnosed, of which six were EVN. These six cases were assessed for histomorphology, IDH1 mutation and 1p/19q deletion. Five cases showed atypical histological features. While none showed mIDH1(R132H), four of the five atypical cases harboured 1p/19q deletion either in isolation or in combination. The only case which was well-differentiated (typical) did not show 1p/19q loss. Conclusions: EVNs are more commonly associated with aggressive histological features. IDH1 mutations, although frequent in oligodendrogliomas, are not seen in EVN. However, similar to oligodendrogliomas, 1p/19q deletion is found in these tumours. Thus, a potential histogenetic link between oligodendrogliomas and EVN remains debatable. This molecular alteration may also have prognostic connotations, being associated with atypical morphological features. Due to the rarity of these tumours, multicentric pooling of larger studies is needed to have an insight into the impact of these molecular aberrations on their biological behaviour.