Histopathologic "Evolution" in Pediatric Primary Intracranial High-Grade Sarcoma - a Key that Unlocked the Correct Diagnosis

Central nervous system (CNS) tumors are the second most common in the pediatric age group, accounting for 3.5% of overall mortality. The 2021 World Health Organization (WHO) classification of pediatric CNS tumors has given insight into their molecular biology. Correct diagnosis of high-grade intracranial sarcomas is a well-known challenge because of their histopathological variation, presence of heterologous elements, and haphazard pattern of growth. We present a case of a 13-year-old female with a right-sided frontal hemorrhagic mass. Pathological work-up revealed an intra-cranial high-grade sarcoma, not otherwise specified (NOS). Despite receiving chemo-radiation, the lesion recurred after 9 months. This time, the sarcoma had evolved, showing distinct focal rhabdomyoblastic differentiation. Next-generation sequencing (NGS) based assay revealed variants p.E1705V, p.Y1417Ter in DICER1, and other mutations in KRAS and TP53 genes. The lesion was then diagnosed as spindle cell sarcoma with rhabdomyosarcoma-like features, DICER1 mutant. We propose that upfront molecular studies in pediatric undifferentiated high-grade sarcomas are indicated for precise diagnosis and classification.