Spindle epithelial tumour with thymus-like element (SETTLE): The predominantly monophasic variant

Aims: To describe two cases of spindle epithelial tumour with thymus- like element (SETTLE) which are composed predominantly of spindle cells. In addition, to highlight some unusual histological features in SETTLE and discuss its separation from histological mimics. Methods and results: The thyroid masses were in a 4-year-old boy and a 25-year-old male. Both patients were euthyroid and were well except for thyromegaly. The specimens were formalin fixed, and immunohistochemistry was performed on this material using a panel of antibodies following microwave antigen retrieval. Morphologically, the dominant pattern was of sheets of spindle cells arranged in several patterns. There was mild pleomorphism and occasional mitoses. There were very small foci of squamous epithelium and occasional ductular structures. The stroma was composed of fibrous tissue and isolated areas of calcification were noted. The spindle cells showed strong and diffuse immunoreactivity with AE1/3, CAM5.2 and vimentin. Conclusion: We describe two cases of SETTLE that are composed mainly of spindle cells and only a very focal ductular component. In addition, calcification was noted within the stroma in one of the cases. These predominantly spindle examples of SETTLE must be separated from synovial sarcoma, which is a more mitotically active, aggressive tumour displaying only patchy immunopositivity with epithelial markers.