SMARCB1 deficient sinonasal carcinoma: An emerging entity with a diagnostic challenge.

SMARCB1 deficient sinonasal carcinomas are rare neoplasms, classified under sinonasal undifferentiated carcinomas by the fourth edition of the World Health Organization (WHO) classification of head and neck tumors. It is characterized immunohistochemically by loss of SMARCB1(INI1) expression. We are reporting the case of a 63-year-old man who was evaluated for nasal stuffiness of 3 months duration in another hospital where a radiological evaluation showed a polypoidal soft tissue lesion in the right maxillary sinus extending to the right nasal cavity and spheno-ethmoidal sinus. He underwent excision biopsy which was reported as non- keratinizing nasopharyngeal carcinoma. He was referred to our center with residual disease in spheno-ethmoidal recess for which radiotherapy was given. After completion of radiotherapy, the primary site had no residual disease, but while on follow-up he developed left sided neck nodes within 4 months of completion of treatment. Excision of the lesion was done and histopathological and immunohistochemical analysis revealed it to be metastasis from SMARCB1 deficient sinonasal carcinoma and not nasopharyngeal carcinoma as diagnosed from the other center. This case is being reported to highlight the diagnostic challenge associated with this rare entity.