Tumor-in-tumor of the thyroid with basaloid differentiation: a lesion with a solid cell nest neoplastic component?

This study describes an encapsulated thyroid tumor having 3 different concentric appearances in a 70-year-old man. The most peripheral neoplasm showed features of a microfollicular adenoma; the intermediate lesion displayed typical features of the follicular variant of papillary thyroid carcinoma (FVPTC); and the main, central neoplasm, showed a basaloid configuration. At variance with the other neoplasms, the latter component exhibited high mitotic activity. Necrosis, angio-invasion, or capsular invasions were not observed. The centrally located neoplasm was immunoreactive for several cytokeratins, p63, carcinoembryonic antigen, and galectin-3, and negative for thyroid transcription factor 1, thyroglobulin, calcitonin, CD5, and CK20, featuring a phenotype similar to that of thyroid solid cell nests. A N-RAS mutation was found both in the basaloid and in the FVPTC components. The clinicopathological and immunohistochemical data ruled out the alternative possibilities of intrathyroidal metastasis and tumor from ectopic (thymic, parathyroid, or salivary gland) tissues.