Pathology of Endocrine Tumors Update: World Health Organization New Classification 2017—Other Thyroid Tumors

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The data on nonmedullary thyroid tumors in the fourth edition of the World Health Organization classification of endocrine tumors contain significant revisions. The tumors could be remembered as follicular-derived neoplasms, other epithelial tumors, nonepithelial tumors, and secondary tumors. The major modifications are seen in the follicular-derived neoplasms. Some of these changes are based on the data from The Cancer Genome Atlas. A “borderline” tumor group—follicular tumor of uncertain malignant potential, well-differentiated tumor of uncertain malignant potential, and noninvasive follicular thyroid neoplasm with papillary nuclear features—is introduced in the current classification. Papillary carcinoma comprises 15 variants, which include a new histologic variant—hobnail variant. A few variants of papillary carcinoma have their definitions and data updated. Follicular carcinomas are subdivided into 3 groups: minimally invasive (capsule invasion only), encapsulated angioinvasive, and widely invasive. The clinical, pathological, and molecular profiles of Hürthle cell tumors (Hürthle cell adenoma/carcinoma) are different from follicular adenoma/carcinomas, which justify them as separate entities. The classification also adopted the Turin criteria for the histologic diagnosis of poorly differentiated carcinoma. Anaplastic carcinoma and squamous cell carcinoma are the 2 most clinically aggressive entities of the group, and they may be developmentally linked. The other thyroid tumors are uncommon, but cautions are needed to be aware of their presence in some instances. Overall, the new classification incorporated the new knowledge on pathology, clinical behavior, and genetics of the thyroid tumors, which are important for management of patients with these tumors.

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