Rosette-forming Glioneuronal Tumor: A Rare Posterior Fossa Tumor in an Adolescent.

Rosette-forming glioneuronal tumor (RGNT) is a rare variety of slow growing mixed glioneuronal tumor involving primarily fourth ventricular region and occurring predominantly in young adults. We present a case of a 16-year-old boy who presented with dizziness and occipital headache. On radiological evaluation, a fairly large hypodense posterior fossa mass lesion in relation to the left side of the vermis, with a large cystic component was found. Surgical resection of the tumor was performed. Histopathological examination showed a biphasic tumor composed of bland neurocytic cells, arranged in the form of neurocytic rosettes along with glial areas resembling low-grade glioma. The neurocytic rich region shows strong synaptophysin positivity in the neuropil-rich core of the rosettes. Methylation-inhibited binding proliferative index was low (<1%). Based on these features, a diagnosis of RGNT was made. RGNT of the fourth ventricle should be considered in differential diagnosis of posterior fossa lesions, especially in relation to fourth ventricle and vermis in young adults.