Molecular Pathology of Thyroid Tumors: Essential Points to Comprehend Regarding the Latest WHO Classification

Abstract

In 2022, the new WHO Classification of Endocrine and Neuroendocrine Tumors, Fifth Edition (beta version) (WHO 5th), was published. Large-scale genomic analyses such as The Cancer Genome Atlas (TCGA) have revealed the importance of understanding the molecular genetics of thyroid tumors. Consequently, the WHO 5th was fundamentally revised, resulting in a systematic classification based on the cell of origin of tumors and their clinical risk. This paper outlines the following critical points of the WHO 5th. 1. Genetic mutations in follicular cell-derived neoplasms (FDNs) highlight the role of mutations in the MAP kinase pathway, including RET, RAS, and BRAF, as drivers of carcinogenesis. Differentiated thyroid cancers such as follicular thyroid carcinoma (FTC) and papillary thyroid carcinoma (PTC) have specific genetic alterations that correlate with morphological classifications: RAS-like tumors (RLTs) and BRAF p.V600E-like tumors (BLTs), respectively. 2. The framework for benign lesions has been revised. The WHO 5th introduces a new category: "developmental abnormalities". Benign FDNs comprise "thyroid follicular nodular disease", follicular thyroid adenoma (FTA), FTA with papillary architecture, and oncocytic adenoma (OA). "Hürthle cell adenoma/carcinoma" is renamed oncocytic adenoma/carcinoma of the thyroid (OA/OCA), which can be distinguished from FTA/FTC by its unique genetic background. 3. Low-risk tumors include NIFTP, TT-UMP, and HTT, and they have an extremely low malignant potential or an uncertain malignant potential. 4. PTC histological variants are reclassified as "subtypes" in the WHO 5th. 5. The concept of high-grade carcinomas is introduced, encompassing poorly differentiated thyroid carcinoma (PDTC), differentiated high-grade thyroid carcinoma (DHGTC), and high-grade medullary thyroid carcinoma (MTC). 6. Squamous cell carcinoma is included in anaplastic thyroid carcinoma (ATC) in the WHO 5th due to their shared genetic and prognostic features. 7. Other miscellaneous tumors are categorized as salivary-gland-type carcinomas of the thyroid, thyroid tumors of uncertain histogenesis, thymic tumors within the thyroid, and embryonal thyroid neoplasms. The WHO 5th thus emphasizes the importance of classifying tumors based on both genetic abnormalities and histomorphology. This approach aids in achieving accurate pathological diagnosis and facilitates the early selection of appropriate treatment options, including molecular targeted therapies.

Keywords: WHO classification; genomic alterations; pathological diagnosis; thyroid cancer.

Figure 1

Morphology, genetic abnormalities, and intracellular signaling pathways of differentiated thyroid tumors. The three most important gross characteristics of thyroid tumors are (i) single or multiple nodules, (ii) capsule formation, and (iii) the presence or absence of invasive growth. A lesion with a single nodule and a thick fibrous capsule indicates a neoplastic lesion that has expansively grown over time. Multinodular lesions are more likely to be non-neoplastic. Lesions with irregular margins are suspected of infiltrative growth and may be malignant. The most important histological findings are the histological architecture (follicular, papillary, or solid) and papillary carcinoma (PTC)-like nuclear features (“nuclear score”: nuclear enlargement, glassy chromatin, and irregular nuclear shape [nuclear grooves and pseudo-inclusions]), with one point given for each [Total 0–3 points]). Regarding genetic mutations, well-differentiated tumors such as thyroid follicular nodular disease (or functional nodules) show abnormalities in the TSHR to GNAS/cAMP/PKA pathway. Differentiated tumors can be classified into RAS-like tumors (RLTs) and BRAF p.V600E-like tumors (BLTs). Many of the driver gene mutations in differentiated thyroid cancer contribute to the activation of the MAPK and PI3K/Akt pathways downstream of tyrosine kinase receptors such as RET and NTRKs. RLTs activate the MAPK and PI3K pathways, and CRAF induces negative feedback to inhibit MAPK. As a result, the anti-apoptotic effect via the PI3K pathway is predominant in RLTs. BLTs activate the MAPK pathway, which is highly proliferative. In well-differentiated thyroid tumors such as thyroid follicular nodular disease (or functional nodule), activation of the TSH receptor-mediated cAMP pathway is predominant; this promotes hormonal functions, such as iodine metabolism and the expression of hormone-related genes.

Figure 2

Low-risk neoplasms of the thyroid. (A–C): Non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP). HE staining images of NIFTP at a low (A) and high magnification (B). Papanicolaou staining image of an NIFTP fine-needle aspiration specimen (C). (D–I): Hyalinizing trabecular tumor (HTT). HE staining images of HTT at a low (A) and high magnification (B). PAS staining image of HTT (F). Immunohistochemical staining with the MIB1 antibody revealed cytoplasmic (G) and cell membranous (H) immunoreactivity. Papanicolaou staining image of an HTT fine-needle aspiration specimen (I).

Figure 3

Differentiated high-grade thyroid carcinoma (DHGTC). HE staining images of a DHGTC case (A–C). This case is a tall-cell papillary thyroid carcinoma (PTC) (A) that meets the criteria of DHGTC. It shows tumor necrosis (B) and mitosis (C). Immunohistochemistry of Thyroglobulin (D) and TTF1 (E). The Ki-67 (MIB1) labeling index is about 15% (F).

Figure 4

Algorithm for diagnosis of thyroid tumors (WHO 5th edition). The WHO 5th simplified the diagnosis of thyroid cancer by dividing it into three steps: (i) a consideration of the origin or cellular differentiation of the tumor cells, (ii) an assessment of the gene mutations, and (iii) a detailed examination of various histomorphological features, including capsular/vascular invasion, mitosis, and tumor necrosis. The cell of origin or cellular differentiation can be examined using immunohistological markers such as TTF1, PAX8, and calcitonin. The most frequent follicular cell-derived neoplasms are classified into five categories according to genetic mutations, which are closely related to tumor morphology. Oncocytic tumors and RAS-like tumors (RLTs) are commonly encapsulated and are further classified morphologically by the presence or absence of capsular/vascular invasion. Tumors with “Questionable (q)” invasion are diagnosed as FT-UMP or WDT-UMP. In RLTs, papillary-like nuclear features are also important. BRAF p.V600E-like tumors (BLTs) commonly show infiltrative growth with apparent PTC-like nuclear features and papillary structures. Tumors with predominantly solid growth are likely classified as high-grade carcinomas or anaplastic thyroid carcinomas (ATCs). Mitotic counts and tumor necrosis are critical in the diagnosis of high-grade carcinomas.