Thoracic SMARCA4-deficient undifferentiated tumour: Diagnostic challenges and potential for misdiagnosis in small tissue samples.

We report a diagnostically challenging case of a SMARCA4-deficient undifferentiated tumour to emphasize its potential to mimic other malignant tumours on histology, especially in small biopsies and where rhabdoid morphology is lacking. A 48-year-old man, who was known for chronic obstructive pulmonary disease and polysubstance use, presented with dyspnoea and an anterior mediastinal mass that had grown rapidly over a seven-month period. The rapid growth and location in the anterior mediastinum raised clinical suspicion for lymphoma or a germ cell tumour.