An unusual case of benign thyroid tumour consisting of epithelial and nonepithelial components.

An encapsulated tumour consisting of follicular epithelial cells, fat-laden round cells and spindle cells was found in the right lobe of the thyroid gland of a 66-year-old woman. The follicular epithelial cells had oxyphilic cytoplasm in which numerous mitochondria were densely packed. The fat-laden round cells were indistinguishable from mature fat cells by light microscopy. However, they were immunohistochemically negative for S-100 protein and epithelial markers, and lacked a continuous basement membrane. The spindle cells embedded in the collagenous matrix possessed well-developed rough endoplasmic reticulum, most of which contained variably sized, non-membrane-bound lipid droplets. Bundles of thin filaments were occasionally observed in the cytoplasm. Some spindle cells were immunoreactive for PCNA and alpha-smooth muscle actin. There was no striking nuclear atypia of the tumour cells and no capsular or vascular invasion by these components. The tumour can be classified as a benign mixed tumour.