SMARCB1 (INI-1) - Deficient sinonasal carcinoma: Report of two cases.

SMARCB1 (INI-1)-deficient sinonasal carcinoma is a rare, poorly differentiated carcinoma defined by complete loss of tumor suppressor gene SMARCB1 (INI-1) within the neoplastic cell nuclei demonstrated by the immunohistochemical stain. SMARCB1 (INI-1) gene inactivation has been implicated in the pathogenesis of a diverse group of malignant neoplasms that tend to share "rhabdoid" morphology. SMARCB1 (INI-1)-deficient sinonasal carcinoma was first reported by Agaimy et al. in 2014. These tumors are often basaloid with focal rhabdoid differentiation, prominent necrosis, increased mitotic activity, and aggressive behavior. Other than being INI-1 and NUT negative, they are positive for pancytokeratin and express variable immunoreactivity for squamous markers like p63 and neuroendocrine markers like synaptophysin. Most patients present with locally advanced disease and hence a combination of chemotherapy, radiotherapy, and surgery is usually recommended.