Molecular Analyses of Rosette-forming Glioneuronal Tumor of the Midbrain Tegmentum: A Report of Two Cases and a Review of the FGFR1 Status in Unusual Tumor Locations

Background: Rosette-forming glioneuronal tumor (RGNT) is a rare tumor that arises primarily in the posterior fossa, with molecular features of mutation. A previous study reported that brainstem RGNT accounts for only 2.7% cases; therefore, midbrain RGNT is infrequent.

Case Description: The authors encountered two cases of RGNT located in the midbrain tegmentum (Case 1: 23-year-old woman and Case 2: 18-year-old boy), both exhibiting similar cystic components with gadolinium-enhanced cyst walls on preoperative magnetic resonance imaging, surgically resected through the occipital transtentorial approach. Histological findings in both cases comprised two characteristic architectures of neurocytic and glial components, typical of RGNT. Molecular assessment revealed no mutation in the initial specimen, but revealed K656E mutation in the recurrent specimen in Case 1 and showed no mutation but showed C228T mutation in Case 2. Neither case revealed K27, G34, or K27 mutations. DNA methylation-based classification (molecularneuropathology.org) categorized both cases as RGNT, whose calibrated scores were 0.99 and 0.47 in Cases 1 and 2, respectively.

Conclusion: Midbrain tegmentum RGNTs exhibited typical histological features but varied statuses with mutation. RGNT in rare locations may carry different molecular alterations than those in other common locations, such as the posterior fossa.