Cytopathology of Thoracic SMARCA4-Deficient Undifferentiated Tumor: A Case Report.

Thoracic SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is an aggressive tumor with a dismal prognosis, most commonly involving the mediastinum and lungs. The diagnosis of SMARCA4-UT is still challenging, both clinically and pathologically, due to its non-specific clinical findings, a broad list of differential diagnoses, and rarity of cases reported in the literature, especially in terms of cytopathology. Most patients present at an advanced stage of disease at the time of diagnosis, and fine needle aspiration (FNA) cytology specimens and/or core biopsies are often the only diagnostic material available. SMARCA4-UT tumors are strongly associated with a smoking history; however, approximately 10% of the patients are never smokers. We report a case of a 78-year-old male with a remote minimal smoking history who presented with a large mediastinal mass and hilar lymphadenopathy. FNA of the mass demonstrated a moderately cellular aspirate with two populations of loosely cohesive malignant cells in a background of necro-inflammatory debris and bare nuclei. The majority of malignant cells showed enlarged nuclei with prominent nucleoli and scant cytoplasm, with a few malignant cells showing rhabdoid features. Immunohistochemical staining of the cell block showed the malignant cells were patchy positive for INSM-1 and PAX-8; however, negative for keratin markers, TTF-1, Napsin A, and p40. SMARCA4-UT was diagnosed with the loss of SMARCA4 (BRG1) expression in the tumor cells.