Fine-needle aspiration and effusion cytology of thoracic SMARCA4-deficient undifferentiated tumor and SMARCA4-deficient non-small cell lung carcinoma: A multi-institutional experience with 27 patients.

Background: Thoracic switch/sucrose nonfermentable-related, matrix-associated, actin-dependent regulator of chromatin, subfamily A, member 4 (SMARCA4)-deficient (SD) malignancies, including SD undifferentiated tumor (SD-UT) and SD non-small cell lung carcinoma (SD-NSCLC), have been recently described. The cytologic features of these neoplasms in fine-needle aspiration (FNA) and effusion specimens have rarely been reported in the literature. This study aimed to describe and compare the spectrum of cytologic, immunohistochemical, and clinical features of these high-grade malignancies recently encountered at the participating institutions.

Methods: This study documented clinical and imaging characteristics of tumors from 27 patients. Sixteen cytomorphologic features and immunohistochemical findings were compared between SD-UT and SD-NSCLC samples.

Results: Twenty three FNAs, two bronchial brushings, and two pleural fluids were evaluated, including 17 SD-UT cases (mean patient age, 70 years) and 10 SD-NSCLC cases (mean patient age, 62 years). Both malignancies presented with large thoracic masses and/or hilar/mediastinal lymphadenopathy. All SD-UT cytologic samples had a discohesive or mixed cohesive-discohesive architecture, and most (13 of 17) showed predominant rhabdoid or mixed rhabdoid-epithelioid features. Most SD-NSCLC cytologic samples (nine of 10) were either cohesive or mixed cohesive-discohesive and had a predominantly epithelioid morphology (eight of 10). Keratins and claudin-4 were negative or focally positive in SD-UT samples, whereas they were diffusely positive in SD-NSCLC samples. Both malignancies were negative for TTF-1 and p40/p63 and showed loss of expression of SMARCA4.

Conclusions: Although there is considerable clinical and cytopathologic overlap between SD-UT and SD-NSCLC, some key features allow for their distinction. SD-UT is mostly discohesive with rhabdoid or mixed rhabdoid-epithelioid features, whereas SD-NSCLC often has cohesive epithelioid morphology. The combination of clinical presentation, cytomorphology, and immunohistochemistry is essential for a definitive diagnosis.