Clinicopathologic and Molecular Features of Metastatic Follicular Thyroid Carcinoma in Patients Presenting With a Thyroid Nodule Versus a Distant Metastasis

RAS-Mutant Follicular Thyroid Tumors: A Continuous Challenge for Pathologists

The classification of thyroid nodules, particularly those with a follicular growth pattern, has significantly evolved. These tumors, enriched with RAS or RAS-like mutations, remain challenging for pathologists due to variables such as nuclear atypia, invasion, mitotic activity, and tumor necrosis. This review addresses the histological correlates of benign, low-risk, and malignant RAS-mutant thyroid tumors, as well as some difficult-to-classify follicular nodules with worrisome features. One prototypical RAS-mutant nodule is non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP). The assessment of nuclear characteristics in encapsulated/well-demarcated non-invasive RAS-mutant follicular-patterned tumors helps distinguish between follicular thyroid adenoma (FTA) and NIFTP. Despite this straightforward concept, questions about the degree of nuclear atypia necessary for the diagnosis of NIFTP are common in clinical practice. The nomenclature of follicular nodules lacking clear invasive features with increased mitotic activity, tumor necrosis, and/or high-risk mutations (e.g., TERT promoter or TP53) remains debated. Invasion, particularly angioinvasion, is the current hallmark of malignancy in RAS-mutant follicular-patterned neoplasms, with follicular thyroid carcinoma (FTC) as the model. Assessing the tumor interface is critical, though full capsule evaluation can be challenging. Multiple levels and NRASQ61R-specific immunohistochemistry can aid in identifying invasion. Controversies around vascular invasion persist, with ancillary stains like CD31, ERG, and CD61 aiding in its evaluation. Moreover, the review highlights that invasive encapsulated follicular variant papillary thyroid carcinoma (IEFVPTC) is closely associated with FTC, suggesting the need for better nomenclature. The concept of "high-grade" differentiated carcinomas, applicable to FTC or IEFVPTC with necrosis and/or high mitotic activity, is also discussed.

No Longer Well-Differentiated: Diagnostic Criteria and Clinical Importance of Poorly Differentiated/High-Grade Thyroid Carcinoma

Poorly differentiated thyroid carcinoma (PDTC) and differentiated high-grade thyroid carcinoma (DHGTC) are uncommon thyroid malignancies, recently (re)codified into distinct entities with overlapping clinical significance. Recognizing them may be challenging for the general practitioner and subspecialty pathologist alike. This article will describe the required features to diagnose PDTC and DHGTC, differential diagnostic considerations, molecular findings, and clinical implications. It is intended to be a general synopsis of the most critical elements of PDTC and DHGTC as well as a summary of points in approaching these challenging cases.

High-Grade Non-Anaplastic Thyroid Carcinomas of Follicular Cell Origin: A Review of Poorly Differentiated and High-Grade Differentiated Carcinomas

Poorly differentiated thyroid carcinoma (PDTC) and high-grade differentiated thyroid carcinoma (HGDTC) are considered high-grade follicular-derived thyroid carcinomas, with prognoses intermediate between well-differentiated and anaplastic thyroid carcinoma. Both share the presence of invasion, thyroid follicular-cell origin, and tumor necrosis or increased mitoses (≥ 3 mitoses per 2 mm² in PDTC and ≥ 5 mitoses per 2 mm² in HGDTC), without anaplastic dedifferentiation. PDTC must possess solid, trabecular, or insular growth and lack classic papillary-like nuclei; HGDTC can be of any architectural or nuclear morphology (follicular-like, papillary-like, oncocytic). Transformation may be accompanied by acquisition of high-risk mutations (such as TP53 or TERT promoter) on top of RAS-like or BRAF p.V600E-like (including NTRK-fusion) initial driver mutations. These carcinomas most frequently affect adults and often present with metastases (20-50%) or wide local invasion. As PDTC and HGDTC may be radioactive iodine resistant, post-surgical therapy may consist of external beam radiotherapy or targeted, mutation-dependent chemotherapy, such as tyrosine kinase inhibitors. Ten-year disease specific survival is as low as 50%. Awareness of high-grade features in the diagnostic setting is important for patient prognosis and triage of tissue for molecular analysis in order to guide relevant clinical management and therapy.

Overview of the 2022 WHO Classification of Thyroid Neoplasms

This review summarizes the changes in the 5th edition of the WHO Classification of Endocrine and Neuroendocrine Tumors that relate to the thyroid gland. The new classification has divided thyroid tumors into several new categories that allow for a clearer understanding of the cell of origin, pathologic features (cytopathology and histopathology), molecular classification, and biological behavior. Follicular cell-derived tumors constitute the majority of thyroid neoplasms. In this new classification, they are divided into benign, low-risk, and malignant neoplasms. Benign tumors include not only follicular adenoma but also variants of adenoma that are of diagnostic and clinical significance, including the ones with papillary architecture, which are often hyperfunctional and oncocytic adenomas. For the first time, there is a detailed account of the multifocal hyperplastic/neoplastic lesions that commonly occur in the clinical setting of multinodular goiter; the term thyroid follicular nodular disease (FND) achieved consensus as the best to describe this enigmatic entity. Low-risk follicular cell-derived neoplasms include non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP), thyroid tumors of uncertain malignant potential, and hyalinizing trabecular tumor. Malignant follicular cell-derived neoplasms are stratified based on molecular profiles and aggressiveness. Papillary thyroid carcinomas (PTCs), with many morphological subtypes, represent the BRAF-like malignancies, whereas invasive encapsulated follicular variant PTC and follicular thyroid carcinoma represent the RAS-like malignancies. This new classification requires detailed subtyping of papillary microcarcinomas similar to their counterparts that exceed 1.0 cm and recommends not designating them as a subtype of PTC. The criteria of the tall cell subtype of PTC have been revisited. Cribriform-morular thyroid carcinoma is no longer classified as a subtype of PTC. The term "Hürthle cell" is discouraged, since it is a misnomer. Oncocytic carcinoma is discussed as a distinct entity with the clear recognition that it refers to oncocytic follicular cell-derived neoplasms (composed of > 75% oncocytic cells) that lack characteristic nuclear features of PTC (those would be oncocytic PTCs) and high-grade features (necrosis and ≥ 5 mitoses per 2 mm²). High-grade follicular cell-derived malignancies now include both the traditional poorly differentiated carcinoma as well as high-grade differentiated thyroid carcinomas, since both are characterized by increased mitotic activity and tumor necrosis without anaplastic histology and clinically behave in a similar manner. Anaplastic thyroid carcinoma remains the most undifferentiated form; squamous cell carcinoma of the thyroid is now considered as a subtype of anaplastic carcinoma. Medullary thyroid carcinomas derived from thyroid C cells retain their distinct section, and there is a separate section for mixed tumors composed of both C cells and any follicular cell-derived malignancy. A grading system for medullary thyroid carcinomas is also introduced based on mitotic count, tumor necrosis, and Ki67 labeling index. A number of unusual neoplasms that occur in the thyroid have been placed into new sections based on their cytogenesis. Mucoepidermoid carcinoma and secretory carcinoma of the salivary gland type are now included in one section classified as "salivary gland-type carcinomas of the thyroid." Thymomas, thymic carcinomas and spindle epithelial tumor with thymus-like elements are classified as "thymic tumors within the thyroid." There remain several tumors whose cell lineage is unclear, and they are listed as such; these include sclerosing mucoepidermoid carcinoma with eosinophilia and cribriform-morular thyroid carcinoma. Another important addition is thyroblastoma, an unusual embryonal tumor associated with DICER1 mutations. As in all the WHO books in the 5th edition, mesenchymal and stromal tumors, hematolymphoid neoplasms, germ cell tumors, and metastatic malignancies are discussed separately. The current classification also emphasizes the value of biomarkers that may aid diagnosis and provide prognostic information.

Papillary thyroid carcinoma tall cell variant shares accumulation of mitochondria, mitochondrial DNA mutations, and loss of oxidative phosphorylation complex I integrity with oncocytic tumors

Papillary thyroid carcinoma tall cell variant (PTC-TCV), a form of PTC regarded as an aggressive subtype, shares several morphologic features with oncocytic tumors. Pathogenic homoplasmic/highly heteroplasmic somatic mitochondrial DNA (mtDNA) mutations, usually affecting oxidative phosphorylation (OXPHOS) complex I subunits, are hallmarks of oncocytic cells. To clarify the relationship between PTC-TCV and oncocytic thyroid tumors, 17 PTC-TCV and 16 PTC non-TCV controls (cPTC) were subjected to: (1) transmission electron microscopy (TEM) to assess mitochondria accumulation, (2) next-generation sequencing to analyze mtDNA and nuclear genes frequently mutated in thyroid carcinoma, and (3) immunohistochemistry (IHC) for prohibitin and complex I subunit NDUFS4 to evaluate OXPHOS integrity. TEM showed replacement of cytoplasm by mitochondria in PTC-TCV but not in cPTC cells. All 17 PTC-TCV had at least one mtDNA mutation, totaling 21 mutations; 3/16 cPTC (19%) had mtDNA mutations (p < 0.001). PTC-TCV mtDNA mutations were homoplasmic/highly heteroplasmic, 16/21 (76%) mapping within mtDNA-encoded complex I subunits. MtDNA mutations in cPTC were homoplasmic in 2 cases and at low heteroplasmy in the third case, 2/3 mapping to mtDNA-encoded complex I subunits; 2/3 cPTC with mtDNA mutations had small tall cell subpopulations. PTC-TCV showed strong prohibitin expression and cPTC low-level expression, consistent with massive and limited mitochondrial content, respectively. All 17 PTC-TCV showed NDUFS4 loss (partial or complete) and 3 of 16 cPTC (19%) had (partial) NDUFS4 loss, consistent with lack of complex I integrity in PTC-TCV (p < 0.001). IHC loss of NDUFS4 was associated with mtDNA mutations (p < 0.001). Four BRAF V600E mutated PTCs had loss of NDUSF4 expression limited to neoplastic cell subpopulations with tall cell features, indicating that PTCs first acquire BRAF V600E and then mtDNA mutations. Similar to oncocytic thyroid tumors, PTC-TCV is characterized by mtDNA mutations, massive accumulation of mitochondria, and loss of OXPHOS integrity. IHC loss of NDUFS-4 can be used as a surrogate marker for OXPHOS disruption and to reliably diagnose PTC-TCV.

Follicular Thyroid Neoplasms: Comparison of Clinicopathologic and Molecular Features of Atypical Adenomas and Follicular Thyroid Carcinomas

In follicular thyroid neoplasms without invasion, a diagnosis of atypical adenoma (AA) (follicular tumor of uncertain malignant potential) may be rendered if atypical features (indefinite capsular/vascular invasion, necrosis, solid growth, increased mitoses) are present. This study compares clinical, histologic, and molecular features of patients with AAs (n=31), nonmetastatic follicular thyroid carcinoma (nmFTC) (n=18), and metastatic follicular thyroid carcinoma (mFTC) (n=38). Patients with mFTC were older. Mitotic activity in areas of solid growth was greatest in mFTC (P=0.05). Oncocytic tumors tended to show solid growth (P=0.04). The presence or frequency of capsular and/or vascular invasion was not different between nmFTC and mFTC. TERT promoter mutations were higher in patients with mFTC (50%) than nmFTC (25%) and AA (10%) (P=0.02). TERT promoter mutation was associated with necrosis (P=0.01) and solid growth plus increased mitoses (P=0.03). Necrosis and TERT promoter mutations were identified in all groups, most frequently in mFTC. The combination of solid growth with increased mitoses, necrosis, and TERT promoter mutation was only seen in follicular carcinomas. Poorly differentiated features, vascular invasion, and TERT promoter mutation correlated with metastasis in FTC. Given the low frequency of necrosis and TERT promoter mutation in AAs, close clinical follow-up is recommended in patients with these findings, especially if additional atypical features (such as solid growth plus mitoses) are present.

TERT Promoter Mutation Spatial Heterogeneity in a Metastatic Follicular Thyroid Carcinoma: Implications for Clinical Work-Up

Follicular thyroid carcinoma (FTC) is not routinely diagnosed by a preoperative fine needle aspiration biopsy (FNAB), and the final diagnosis relies on histopathological criteria visible upon microscopic examination of the excised tumor. Several markers have been proposed as helpful in the identification of follicular thyroid tumors with malignant potential and worse prognosis, of which the specific point mutations C250T and C228T in the Telomerase Reverse Transcriptase (TERT) promoter region seem to be particularly promising. We describe a patient presenting with a large pelvic mass, in which a core needle biopsy was consistent with follicular-patterned thyroid tissue positive for a Q61R NRAS mutation and the C228T TERT promoter mutation. Upon clinical investigation, a 60-mm lesion was detected in the right thyroid lobe. The ensuing FNAB was consistent with a follicular thyroid tumor, Bethesda IV, positive for the same NRAS mutation and both the C228T and C250T TERT promoter mutations. A total thyroidectomy was performed, and a widely invasive FTC was diagnosed. Tumor tissue samples from various parts of the primary lesion were investigated for TERT promoter mutations, displaying C228T in three samples and C250T in one. Interestingly, the C228T mutations showed a coupling to areas with high Ki-67 proliferation indexes. Our data indicate that TERT promoter mutations can exhibit spatial heterogeneity in FTCs, with implications for clinical management as well as providing insights into the molecular biology underlying the tumoral etiology.

An Evaluation of CD61 Immunohistochemistry in Identification of Vascular Invasion in Follicular Thyroid Neoplasms

The identification of vascular invasion in follicular thyroid neoplasms is essential for categorizing lesions as benign (follicular adenomas) or malignant (follicular thyroid carcinomas). Among the histologic criteria diagnostic of true vascular invasion is tumor-cell associated thrombosis, including fibrin deposition and platelet clumping. This study aims to evaluate whether an immunohistochemical stain for the platelet-associated protein CD61 could assist in identifying tumor-associated thromboses and thereby confirm vascular invasion in follicular thyroid neoplasms. Histologic review and CD61 immunostaining of 19 atypical follicular adenomas, 13 non-metastatic follicular thyroid carcinomas, and 11 metastatic follicular thyroid carcinomas was performed. Linear arrays or clustered groups of CD61-expressing intravascular platelets were present in 51% of cases overall, including 54% of follicular thyroid carcinomas and 47% of follicular adenomas, mostly within intracapsular or peritumoral vessels. In three follicular thyroid carcinomas (all with distant metastases), CD61-expressing platelets were present in association with intravascular tumor cells. This finding was not present in adenomas. CD61 staining alone did not distinguish between atypical follicular adenomas, non-metastatic carcinomas, and metastatic carcinomas. When present in association with intravascular tumor cells, however, CD61-expressing platelets may serve as a marker for vascular invasion and aid in the diagnosis of follicular thyroid carcinoma.