Medullary carcinoma of the thyroid - an unusual case of hyalinizing trabecular adenoma - like variant (encapsulated)

Medullary thyroid carcinoma is a neoplasm occurring in sporadic and familial patterns. A rare variant of medullary thyroid carcinoma shows microscopic features similar to hyalinizing trabecular adenoma of thyroid. Detection of this variant requires a high index of suspicion and immunohistochemical confirmation by calcitonin positivity. We present a 36 years old female patient with a thyroid nodule which, on microscopy, displayed an encapsulated tumor with elongated cells arranged in trabecular pattern separated by hyalinized fibrous septae simulating a hyalinizing trabecular adenoma. Also present were spindle cells arranged in an organoid fashion. Most of the cells showed salt and pepper chromatin pattern. The lesion was negative for amyloid but showed diffuse calcitonin positivity indicative of a tumor of C-cell origin i.e. medullary carcinoma of thyroid – hyalinizing trabecular variant.