Primary smooth muscle tumors of the thyroid gland

BACKGROUND. Primary smooth muscle tumors of the thyroid gland are rare. To date, there are few cases reported of primary thyroid leiomyomas and leiomyosarcomas. METHODS. One leiomyoma and four leiomyosarcomas arising within the thyroid gland were identified in the files of the Endocrine Tumor Registry of the Armed Forces Institute of Pathology. Histologic and immunohistochemical features were reviewed and follow-up obtained. RESULTS. The patients included 2 females, ages 56 and 64 years, and 3 males, ages 45, 68 and 83 years. The patients presented with a mass in the thyroid gland that had increased in size over a number of months. All the tumors originated within a single lobe of the thyroid gland and measured from 1.1 to 9 cm in greatest dimension. Histologically, there was a fascicular pattern of growth comprised of spindle-shaped cells with blunt-ended nuclei. The leiomyoma was encapsulated, cytologically bland, and amitotic; the leiomyosarcomas were invasive with increased cellularity, pleomorphism, a high mitotic rate, necrosis, and hemorrhage. Immunohistochemical staining showed reactivity with vimentin, smooth muscle actin, muscle specific actin, and desmin. The patient with the leiomyoma was alive without evidence of disease 11 years after the initial presentation, with surgical resection as the only treatment. Three of the patients with leiomyosarcomas were dead within 2 years of diagnosis, in spite of aggressive therapeutic intervention. The remaining patient was still alive 10 months after initial presentation with multiple lung metastases. CONCLUSIONS. Smooth muscle tumors of the thyroid gland are distinctive tumors. Leiomyosarcomas can be distinguished from anaplastic carcinoma, although patient outcome is uniformly unfavorable.