Metastatic <b><i>SMARCB1</i></b> (<b><i>INI-1</i></b>)-Deficient Sinonasal Carcinoma Diagnosed by Endobronchial Ultrasound-Guided Fine-Needle Aspiration (EBUS-FNA): A Potential Diagnostic Pitfall and Review of the Literature

<i>SMARCB1</i> (<i>INI-1</i>)-deficient sinonasal carcinoma is a rare entity within the subgroup of poorly differentiated sinonasal tract carcinomas. As there are only two papers describing the cytologic features of this entity, herein we describe the unique cytomorphologic features of a pulmonary metastasis of this tumor and include the differential diagnosis based on tumor location. The patient was a 53-year-old male who initially presented with sinus congestion and vision changes including left-eye proptosis and diplopia. The initial biopsy of the ethmoid-centered sinonasal mass was non-keratinizing squamous cell carcinoma based on strong immunoreactivity with p40 and absence of immunoreactivity for chromogranin, synaptophysin, p16, and EBER. However, the final diagnosis of the surgical resection was amended to <i>SMARCB1</i> (<i>INI-1</i>)-deficient sinonasal carcinoma after additional immunohistochemical stains were performed. Post-primary resection, follow-up computed tomography imaging revealed significant interval progression of a solitary, initially indeterminate 1-cm lung nodule in the left upper lobe. Endobronchial ultrasound-guided fine-needle aspiration with concomitant core-needle biopsy was performed. Rapid on site evaluation of cytologic smears revealed a hypercellular specimen consisting of sheets of epithelioid cells with very scant to absent cytoplasm, ill-defined cell borders, enlarged fragile nuclei, and areas of nuclear molding. Mitotic figures were present. Other areas showed tumor cells with spindled to elongated nuclei and scant to ill-defined wispy cytoplasm. Both cytology cell block and core-needle biopsy histopathologic material showed the tumor cells to be negative for INI-1 nuclear staining as well as CK5/6, CAM5.2, p40, p63, CK7, AE1/3, and TTF-1. <i>SMARCB1</i> (<i>INI-1</i>)-deficient sinonasal carcinoma can have a spectrum of morphologies and may mimic “small-round-blue-cell” and spindle-cell tumors on cytology preparations. Given the pulmonary location of the aspirate, familiarity with the cytomorphologic spectrum of <i>SMARCB1</i> (<i>INI-1</i>)-deficient sinonasal carcinoma, inclusion of this entity within the differential diagnosis, and performance of immunohistochemistry will aid in arriving at the correct diagnosis.