Metastatic SMARCA4-deficient undifferentiated tumor involving the gingiva

BACKGROUND: SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is a rare and highly aggressive malignancy characterized by loss of BRG1 expression. Oral involvement is exceptionally uncommon and may pose significant diagnostic challenges because of its nonspecific clinical and pathological features. CASE PRESENTATION: A 62-year-old man presented with a rapidly enlarging painful gingival mass in the right posterior mandible. He had a history of a clinically diagnosed hepatic malignancy without histopathological confirmation. Imaging revealed local alveolar bone destruction and multiple bilateral pulmonary nodules suspicious for metastatic disease. Histopathological examination demonstrated a highly undifferentiated malignant tumor with focal rhabdoid features. Immunohistochemistry showed complete loss of BRG1 expression with retained INI-1 expression, focal positivity for cytokeratin and vimentin, and a Ki-67 index of approximately 90%. Based on the clinicopathological findings, the lesion was diagnosed as metastatic SMARCA4-deficient undifferentiated tumor involving the gingiva. No adjuvant therapy was administered, and no definite local recurrence was observed during the 5-month follow-up. CONCLUSION: SMARCA4-UT should be considered in the differential diagnosis of rapidly enlarging oral masses with undifferentiated morphology. Accurate diagnosis relies on the integration of histopathological features, immunohistochemistry, and clinical imaging, with BRG1 immunostaining playing a pivotal diagnostic role.