Leiomyosarcoma of the thyroid: immunohistochemical and ultrastructural study.

A primary leiomyosarcoma of the thyroid was seen in a 54 yr old female. The patient had a solitary, cold nodule with no abnormalities of thyroid function tests. Histologically it was composed of characteristic interlacing fascicles of spindle cells which, electron microscopically, showed thin myofilaments with focal dense bodies. Immunohistochemistry showed a strong reaction with vimentin, smooth muscle actin and desmin. No epithelial differentiation was noted immunohistochemically or ultrastructurally. No tumor recurrence was detected 15 mths after surgery. It is postulated that the smooth muscle differentiation had arisen by metaplastic change or de-differentiation with re-differentiation of anaplastic thyroid carcinoma.