Oral involvement of sinonasal undifferentiated carcinoma: A case report and immunohistochemical study of a challenging case

Introduction: Sinonasal undifferentiated carcinoma (SNUC) is a rare tumor highly aggressive most frequently arise in the maxillary sinus and nasal cavity. Oral involvement is extremely rare.& nbsp;Case report: A 62-years-old male presents a large infiltrative mass involving the hard palate and left alveolar ridge. Computed tomography showed bone destruction and invasion of paranasal sinuses and orbits. Histology revealed a malignant neoplasm consisting of small round cells with minimal cytoplasm and hyperchromatic nuclei without any connection with the oral mucosal epithelium. Immunohistochemical analysis showed epithelial origin (CK-7+, CK-20+, AE1/AE3+, EMA+) and lacked strong evidence of squamous and neuroendocrine differentiation (p63-, 34 beta E12- , NSE-/+, chromogranin-, synaptophysin-). TTF-1 negative ruled out the metastatic origin. A diagnosis of SNUC subtype positive for SMARCB1 (INI1) was reached. The patient was submitted to concurrent radiotherapy and chemotherapy without signs of recurrence after 2 years.& nbsp;Conclusion: SNUC involving the oral cavity is a rare malignancy that may mimic symptoms of dental infection or sinusitis. A careful correlation of clinical, microscopic, and immunohistochemical characteristics is mandatory for early diagnosis.