LG-40: UNUSUAL NON-PILOCYTIC HISTOPATHOLOGICAL FEATURES OF PEDIATRIC INFRATENTORIAL GLIOMAS WITH BRAF FUSION GENE: A REPORT OF 2 CASES

Genomic profiling studies have unraveled distinct molecular changes in different pediatric glioma entities. High grade gliomas commonly harbor H3F3A mutations or PDGFRA amplifications, diffuse low grade gliomas frequently display mutations in the FGFR1 or MYB1 gene, and pilocytic astrocytomas are characterized by MAPK pathway alterations including BRAF fusions and mutations. We report two infratentorial tumors with unusual morphological features. Patient 1 is a 15-year-old girl with a 6 months history of headache and a 4.5x4.5x6 cm cerebellar cystic tumor with contrast enhancement. A near gross total resection was performed. Histopathologically, the tumor was partly clear-celled but showed focally also some rosettes reminiscent of a RGNT. Vascular proliferations in a non-pilocytic tumor raised the suspicion of a malignant glioma. Patient 2, a 4-year-old girl presented with cephalea/strabism and had a cystic 2,8 x 2,9 x 2,5 cm tumor in the pineal region, radiologically compatible with a pilocytic astrocytoma. A gross total resection was performed. Histopathologically, ependymoma-like features with perivascular pseudorosettes, clear cells and vascular proliferations were present. In both cases immunhistochemical expression pattern confirmed a glioma without H3F3A K27M and BRAF V600E mutation, however classification and grading were difficult. Sequencing revealed in both tumors a KIAA1549– BRAF fusion. As this alteration is predominantly but not exclusively found in pilocytic astrocytomas and histopathologically no clear-cut signs of malignancy were present, no adjuvant treatment was administered. Both patients show no tumor recurrence/progression 32 and 21 months postoperatively. These cases emphasize the importance of molecular analyses in tumors with unusual morphological feature.