Rosette-forming glioneuronal tumor of the fourth ventricle with advanced microvascular proliferation--a case report.

Rosette-forming glioneuronal tumor (RGNT) of the fourth ventricle is a recently described novel type of primary brain tumor that was included into the current WHO classification of CNS tumors. It is a very rare, slowly growing, mixed neoplasm at cerebellar localization with distinctive morphological pattern. We present an unusual case of a 20-year-old patient with RNGT of the fourth ventricle with advanced microvascular proliferation. MRI revealed the solid-cystic tumor mass largely involving the cerebellar vermis and left hemisphere with compression of the fourth ventricle. Microscopically, the tumor showed classical architectural pattern with two distinctive components. The main component consisted of neurocytic rosettes formed by round, isomorphic nuclei arranged around eosinophilic, fibrillar cores with strong synaptophysin expression. The perivascular rosettes with cell arrangement along blood vessels were observed only sporadically. The second neoplastic component consisted of spindle or stellate astroglial cells with piloid process and Rosenthal fibers, strongly resembling pilocytic astrocytoma. Focally, the astroglial cells showed increased cellularity but without marked nuclear atypia. The glial part of the tumor revealed advanced proliferation of microvessels. The vessels of glomeruloid type exhibited multilayered endothelial proliferation and marked mitotic activity. MIB1 labelling index was generally low; however, in areas exhibiting microvascular proliferation its expression was significantly increased up to 20%. This report demonstrates the unique case of RGNT with conspicuous microvascular proliferation of glomeruloid type and extensive endothelial proliferation. As there is still limited clinical experience with RGNT, further studies are necessary to evaluate the biology of this type of tumor.