SMARCB1/INI1‐deficient intrathoracic neoplasm with rhabdoid/plasmacytoid cytomorphology in a patient with plasma cell myeloma: A case report

SMARCB1 (INI1) is one of the switch/sucrose nonfermentable (SWI/SNF) complexes whose loss is associated with several tumors. SMARCB1 (INI1)‐deficient intrathoracic neoplasms are extremely rare and known to be highly malignant and lethal. This report presents the case of a patient diagnosed with SMARCB1 (INI1)‐deficient intrathoracic neoplasm during chemotherapy for plasma cell myeloma. A 77‐year‐old male patient complained of cough, bloody sputum, and fever with an enlarged right lung mass and pleural effusion. His cytological examination revealed undifferentiated epithelioid and rhabdoid/plasmacytoid cells with bi‐ or multinucleation, vacuolization, mitosis, and pleomorphism. However, it was difficult to distinguish the relapse of plasma cell myeloma as atypical plasmacytoid cells were detected. Immunohistochemically, the neoplastic cells showed a loss of SMARCB1 (INI1) expression in the cell block of pleural fluid and in the right lung of the autopsy specimen. Further, the patient was diagnosed with SMARCB1 (INI1)‐deficient intrathoracic neoplasm of the right lung based on histological and autopsy findings. To the best of our knowledge, this is the first report of cytomorphology in a SMARCB1 (INI1)‐deficient intrathoracic neoplasm.