SMARCB1-deficient sinonasal carcinoma in a pediatric patient: Imaging findings and differential diagnosis of a rare entity.

SMARCB1-deficient sinonasal carcinoma (SDSC) is a recently characterized and aggressive sinonasal malignancy described predominantly in adults, with only rare reports in patients under 18 years of age and limited radiologic characterization in young children. We present the case of an 8-year-old boy with new-onset vision loss found to have a large, heterogeneously enhancing central skull base mass demonstrating diffusion restriction, internal sheet-like calcification, extensive osseous destruction, and multicompartment skull base extension involving the cavernous sinuses, orbital apices, and internal carotid arteries. The imaging differential diagnosis included rhabdomyosarcoma, esthesioneuroblastoma, poorly differentiated chordoma, craniopharyngioma, germ cell tumor, and NUT carcinoma. Endoscopic biopsy demonstrated complete loss of INI1/SMARCB1 expression on immunohistochemistry, consistent with SMARCB1-deficient sinonasal carcinoma. Despite multimodal therapy including chemotherapy, radiation, immunotherapy, and subtotal resection, the tumor demonstrated progressive local and metastatic disease with subsequent vascular complications including internal carotid artery encasement and subsequent cerebral infarction. This case highlights the aggressive imaging appearance of SDSC in a pediatric patient and underscores the importance of including this entity in the differential diagnosis of destructive pediatric skull base masses to prompt appropriate immunohistochemical evaluation and ensure accurate pathologic classification.