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