WHO 2022 Classification of Thyroid Neoplasms

The World Health Organization (WHO) has released the 5th edition of the WHO Classification of Endocrine and Neuroendocrine Tumors, with updates relating to the thyroid gland.

The 5th edition divides thyroid tumors into new categories to provide a clearer understanding of their cell of origin, pathologic features, molecular classification, and biological behavior.

The most common thyroid tumors, follicular-cell–derived tumors, are now divided into 3 categories: benign, low-risk, and malignant neoplasms. Benign tumors include follicular adenoma and its variants, such as those with papillary architecture. Non-invasive follicular thyroid neoplasm with papillary-like nuclear features, thyroid tumors of uncertain malignant potential, and hyalinizing trabecular tumor are among the low-risk follicular-cell–derived neoplasms included.

Malignant follicular-cell–derived neoplasms are now stratified on the basis of their molecular profile and aggressiveness. Papillary thyroid carcinomas (PTCs) have many morphological subtypes and are classified as BRAF-like malignancies. In contrast, invasive encapsulated follicular variant PTC and follicular thyroid carcinoma are classified as RAS-like malignancies.

For papillary microcarcinomas, the new classification requires detailed subtyping like that required for tumors greater than 1 cm. This edition also recommends not designating these carcinomas or cribriform-morular thyroid carcinoma as a PTC subtype. In addition, using the term “Hürthle cell” is discouraged.

Furthermore, the 2022 edition does not refer to oncocytic carcinoma as a distinct entity; in the new classification, this term refers to oncocytic follicular-cell–derived neoplasms that lack characteristic nuclear features of PTC and high-grade features. In the new edition, high-grade follicular-cell–derived cancers encompass both poorly differentiated carcinoma and high-grade differentiated thyroid carcinomas.

Squamous cell carcinoma of the thyroid is now classified as a subtype of anaplastic thyroid carcinoma, the most undifferentiated form. The new classification also introduces a grading system for medullary thyroid carcinomas based on mitotic count, tumor necrosis, and Ki67 labeling index. Finally, several unusual neoplasms have been categorized into new sections on the basis of their cytogenesis.

“The current classification also emphasized the value of biomarkers that may aid diagnosis and provide prognostic information,” authors noted.

—Ellen Kurek

Reference:

Baloch ZW, Asa SL, Barletta JA, et al. Overview of the 2022 WHO Classification of Thyroid Neoplasms. Endocrine Pathol. 2022;33:27-63. doi:10.1007/s12022-022-09707-3