Usefulness of cell block examination for the cytological diagnosis of thoracic SMARCA4‐deficient undifferentiated tumor: A case report

Abstract SMARCA4‐deficient undifferentiated tumor (SMARCA4‐UT) is a high‐grade malignant neoplasm showing undifferentiated or rhabdoid morphology that significantly involves the thorax of adults. It has been reported as SMARCA4‐deficient thoracic sarcoma or SMARCA4‐deficient non‐small cell lung carcinoma according to the findings of immunohistochemical and genetic studies. We report a case of thoracic SMARCA4‐UT for which cell block analysis and immunohistochemical staining were useful for the final diagnosis. A 51‐year‐old man had a chief complaint of left back pain and visited our hospital for further examination. Cytological examination of a left pleural effusion was performed and we also made a cell block of the pleural effusion. Cytological examination revealed polyhedral to round tumor cells. The tumor cells appeared singly or formed loosely cohesive clusters. The nuclei were round to oval, enlarged, and sometimes eccentric with prominent nucleoli with irregular borders. The nuclear chromatin was unevenly distributed. The cytoplasm was vacuolar to eosinophilic. There were no characteristic structures of tumor cells. The cell block revealed many single or loosely cohesive round to epithelioid cells. Some tumor cells often exhibited eccentrically located nuclei and lightly eosinophilic cytoplasm, showing a rhabdoid morphology. On immunohistochemistry, the tumor cells were positive for SOX‐2 and they demonstrated significantly reduced SMARCA4 (BRG1) expression; SMARCA2 (BRM) and SMARCB1 (INI1) expression were retained. Accordingly, we made a diagnosis of SMARCA4‐UT. This case demonstrates the importance of performing histological and immunohistochemical analysis using cell blocks for immediate diagnosis.