A 42-year-old woman presents with a diffuse swelling in front of the neck. Thyroid function tests are within normal limits. A fine-needle biopsy reveals malignant cells. The tumor is excised and examined. The tumour cells are positive for calcitonin on immunohistochemistry. What is the appropriate pathologic diagnosis?

Medullary carcinoma thyroid microscopic features: Composed of polygonal to spindle-shaped cells, which may form nests, trabeculae, and even follicles. Small, more anaplastic cells are present in some tumors and may be the predominant cell type. Amyloid deposits derived from calcitonin polypeptides are present in the stroma in many cases. Calcitonin is readily demonstrable within the cytoplasm of the tumor cells as well as in the stromal amyloid by immunohistochemical methods. Electron microscopy reveals variable numbers of membrane-bound electron-dense granules within the cytoplasm of the neoplastic cell.