Rosette‐forming glioneuronal tumors: a report of three cases.

Abstract

Rosette‐forming glioneuronal tumor of the fourth ventricle (RGNT‐4th) is a newly recognized entity by the 2007 WHO classification. Similar rosette‐forming neuropil islands have been described in infiltrative gliomas at sites other than the 4th ventricle (RGNT). In the latter, rosette formation does not seem to impart the same favorable prognosis as in the WHO grade I RGNT‐4th. We describe three examples of RGNTs occurring outside the 4th ventricle. Tumors were located in the right frontal lobe, cerebellar vermis away from the 4th ventricle and cervical spinal cord. Radiographically, the tumors appeared as well‐circumscribed, solid masses with focal contrast enhancement. All three tumors were characterized by extensive well‐formed rosette formation surrounding neuropil islands. The latter stained intensely with synaptophysin and negatively for GFAP. MIB‐1 labeling was virtually undetectable. The glial component was characterized as low‐grade astrocytic in two cases, and as infiltrative oligodendroglial‐like in another. The oligodendrolial component did not show 1p/19q co‐deletion on FISH analysis. Ultrastructural examination showed some of the cells to have secretory granules, while others had processes filled with glial filaments. All three patients remain recurrence free approximately two years following surgery. The striking morphologic resemblance between RGNT‐4th and RGNT suggests that these are probably the same entity; even if one chooses to designate them both as mixed glioneuronal tumors or alternatively as glial neoplasms with divergent differentiation.