Three shades of an unusual mediastinal tumour.

Thoracic SMARCA4-deficient undifferentiated tumour (SMARCA4-UT) is an unusual and aggressive tumour. While there are approximately 100 cases of this tumour reported in the literature, there are very few detailed descriptions of its cytomorphologic characteristics, and only rare cases in which primary diagnosis was made on cytologic material. Herein we present a case with a detailed description of the appearance on three specimen types: transbronchial needle aspiration (TBNA) cytology, transbronchial needle biopsy (TBNB) and effusion cytology. Thoracic SMARCA4-UT is an important diagnosis to clinch in modern pathology because of its prognostic and therapeutic implications. We discuss an integrated approach to clinching the diagnosis with reference to clinical, radiographic, morphologic and immunohistochemical features. We also discuss possible differential diagnoses, and how they can be excluded. Cytologic and/or small biopsy diagnosis is valuable in these cases as these tumours are typically not amenable to surgical resection. With the correct diagnosis, the patient may instead be a candidate for immune checkpoint inhibitors or experimental therapy targeting SWI/SNF deficiency.