A biopsy of a thyroid nodule showed malignant cells in an amyloid stroma. What is the likely diagnosis?

The presence of amyloid stroma in a thyroid nodule strongly indicates medullary carcinoma of the thyroid. Medullary thyroid carcinoma arises from parafollicular C cells (also known as C-cell or parafollicular cells), which produce calcitonin. The deposit of amyloid is a hallmark of this type of cancer and can occur due to the accumulation of calcitonin or other peptides secreted by the tumor.

In medullary carcinoma, the amyloid deposits are distinct and can be demonstrated histologically, often through special staining techniques. This characteristic finding helps differentiate medullary carcinoma from other thyroid malignancies, which typically do not exhibit this amyloid stroma.

Follicular carcinoma and papillary carcinoma do not present with amyloid deposition, while anaplastic carcinoma is marked by an aggressive behavior and poor differentiation rather than the specific presence of amyloid. Therefore, the identification of malignant cells in an amyloid stroma points decisively to medullary carcinoma as the correct diagnosis.