Malignant granular cell tumour of the mediastinum.

This report describes a case of malignant granular cell tumour arising in the mediastinum, detailing the investigations undertaken to reach this rare diagnosis. A 63-year-old man was referred from the Pacific Islands for investigation of a 8cm mediastinal mass extending into the left pleura and associated with pleural nodules and pleural effusion. Needle aspiration via bronchoscopy yielded insufficient material for cytological interpretation and needle biopsy showed normal respiratory epithelium. CT-guided FNA revealed scattered large polygonal to spindle cells with granular cytoplasm and indistinct borders. The needle core biopsy yielded scanty cells with abundant granular cytoplasm, oval and regular nuclei which were moderately positive for CD68, vimentin and S100 and negative for CKMNF116, CK5/6, CK7, CK20, TTF-1, chromogranin and synaptophysin. In view of the benign morphology, these cells were interpreted to be histiocytes. The incisional biopsy revealed cords and trabeculae of cells identical to the CT samples. These cells were polygonal with abundant granular cytoplasm. Some cells showed large eosinophilic cytoplasmic globules not seen in the FNA sample. The tumour was however, heterogeneous in appearance with some areas exhibiting criteria of malignancy: necrosis, vesicular nuclei with large nucleoli, high nuclear-to-cytoplasmic ratio and nuclei pleomorphism. In addition, p53 expression in 10% of tumour nuclei, a high Ki67 proliferative rate (>10%), the deep seated location and extension of the tumour into adjacent organs favoured a diagnosis of malignancy.