A Case of Incidentally Followed Thoracic SMARCA4‐Deficient Undifferentiated Tumor: Insights Into the Tumor Origin

Thoracic SMARCA4-deficient undifferentiated tumor (UT) is a malignant neoplasm characterized by undifferentiated and/or rhabdoid morphology. This report describes a case of SMARCA4-deficient UT that was incidentally followed by computed tomography. The patient was a man in his 70 s who was undergoing regular chest computed tomography for postoperative surveillance of renal cell carcinoma. Twenty months before diagnosis of SMARCA4-deficient UT, a pulmonary cyst was noted adjacent to the chest wall and increased in size over time. A right upper lobectomy with partial pleurectomy was performed for diagnostic and therapeutic purposes. Macroscopically, we identified a 30-mm mass located predominantly in the chest wall. Histological examination revealed a proliferation of atypical polygonal cells or partly spindle-shaped with prominent nucleoli and irregular nuclear membranes in a nested pattern within the chest wall and lung tissue and partially within striated muscle tissue. Immunohistochemically, the tumor cells showed loss of expression of BRG1/SMARCA4 and BRM/SMARCA2. The extrapulmonary component was positive for CD34 and had a Ki-67 index that was higher than that in the intrapulmonary component. The radiological, pathological, and molecular findings led to a diagnosis of SMARCA4-deficient UT of intrapulmonary origin.