40-year old female presented with neck swelling. Gross and histology is shown below. What is your diagnosis?

Ref. Robbins Pathology. 9th edition. Page. 1099 Medullary carcinoma thyroid Gross Single or multiple Typically nonencapsulated Solid, gray / tan / yellow, firm, may be infiltrative Microscopy Round, polygonal or spindle cells in nests, cords or follicles, defined by sharply outlined fibrous bands Tumor cells have granular cytoplasm and uniform round / oval nuclei with punctate chromatin Stroma has amyloid deposits from calcitonin, prominent vascularity with glomeruloid configuration or long cords of vessels, coarse calcifications   IHC – Calcitonin