Clear Cell Variant of a Follicular Thyroid Tumor With Uncertain Malignant Potential: A Case Report

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.

Clear cell change in follicular adenoma of the thyroid. A diagnostic challenge

Clear cells in thyroid neoplasms can take two main forms: balloon-shaped and signet-ring cells. Balloon-shaped cell change in follicular adenoma is rare. A review of the literature has revealed only 20 previously published cases. We report herein a new case in the right thyroid lobe of a 45-year-old man. The clinicopathological data of the 21 cases including our paper have revealed that the ages of the patients ranged from 22-70 years, with a mean of 41.6 years. There was a clear predominance in women (M:F, 1:6). The most frequent location was in either of both lobes (81.8%), rarely affecting the isthmus. One case was observed in an ectopic thyroid in the submandibular region. The size ranged from 0.7-5.5 cm (mean 2.9 cm). The type of surgical intervention where this data was reported it was lobectomy for 10 (55.5%) cases, thyroidectomy for six (33.3%) cases, and simple excision for two (11.1%) cases. In one patient, the lobectomy was accompanied by cervical lymph node dissection. No cases recurred or extended outside the thyroid. The main differential diagnoses include intrathyroidal clear cell tumor of parathyroid origin, clear cell carcinoma of follicular, oncocytic, papillary, medullary or undifferentiated (anaplastic) origin, paraganglioma, metastatic clear cell carcinoma, especially of renal origin, metastatic balloon cell melanoma, and clear large-cell lymphoma. A thyroid lesion showing clear cell change constitutes a diagnostic challenge in cytological and biopsy diagnosis. Careful observation of the routine techniques along with the aid of an adequate immunohistochemical panel is essential to reach a correct biopsy diagnosis.

Signet ring cell variant of follicular thyroid carcinoma: Report of two cases with focus on morphological, expressional and genetic characteristics

Background

Follicular thyroid carcinoma (FTC) is a neoplasm that presents with a micro-follicular growth pattern and a neutrally stained cytoplasm. Seldom, FTCs display unusual morphological characteristics – but given the rarity of these histological subtypes, little is known regarding the underlying genetics and the coupling to patient outcome.

Case presentation

We present two extremely rare cases of minimally invasive FTC with signet ring cell morphology (SRC-FTC) and describe the cytological, microscopic, immunohistochemical and molecular features for both tumors. Both were male patients, age 71 and 51 respectively. The preoperative cytology for both cases could not pinpoint a clear-cut signet ring cell morphology, but a tendency towards nuclear marginalization was seen. The tumors were 38 mm and 22 mm respectively and displayed evident signet ring cell features in subsets of tumor cells as well as degenerative stromal changes. The tumor cells were positive for TTF1, PAX8 and thyroglobulin, and the proliferation indexes were 4% and 1,9% respectively. Both tumors displayed capsular invasion, but not lymphovascular invasion. The tumors were sequenced for mutations in the TERT promoter and 22 additional cancer-related genes, interestingly; one patient was shown to carry a deleterious intronic variant in PTEN, a tumor suppressor gene coupled to thyroid tumorigenesis and Cowden syndrome. Both patients are alive and well awaiting postoperative radioiodine treatment.

Conclusions

The SRC-FTCs described herein were small, TERT promoter wildtype tumors exhibiting low proliferation, thereby suggesting that these exceedingly rare lesions probably carry a favorable prognosis – although the scarce availability regarding descriptions of this tumor entity nevertheless might justify careful clinical monitoring and mandate investigations in larger case series.

Clinical Routine TERT Promoter Mutational Screening of Follicular Thyroid Tumors of Uncertain Malignant Potential (FT-UMPs): A Useful Predictor of Metastatic Disease

Mutations of the Telomerase reverse transcriptase (TERT) gene promoter are recurrently found in follicular thyroid carcinoma (FTC) and follicular tumors of uncertain malignant potential (FT-UMP), but nearly never in follicular thyroid adenoma (FTA). We, therefore, believe these mutations could signify malignant potential. At our department, postoperative TERT promoter mutational testing of FT-UMPs was implemented in 2014, with a positive mutation screening leading to vigilant follow-up and sometimes adjuvant treatment. To date, we screened 51 FT-UMPs and compared outcomes to 40 minimally invasive FTCs (miFTCs) with known TERT genotypes. Eight FT-UMPs (16%) displayed TERT promoter mutations, of which four cases underwent a completion lobectomy at the discretion of the patient, and a single patient also opted in for radioiodine (RAI) treatment. Three mutation-positive patients developed distant metastases, registered in one patient receiving a completion lobectomy and in two patients with no additional treatment. Three out of four patients who received additional surgery, including the RAI-treated patient, are still without metastatic disease. We conclude that FT-UMPs with TERT promoter mutations harbor malignant potential and exhibit at least similar recurrence rates to TERT-promoter-mutated miFTCs. Mutational screening should constitute a cornerstone analysis in the histopathological work-up of FT-UMPs.

Clear Cell Variant of Papillary Thyroid Carcinoma With Associated Anaplastic Thyroid Carcinoma: Description of an Extraordinary Case

Clear cell change is a rare observation in thyroid cancer, resulting from aberrant cytoplasmic accumulation of lipids, glycogen, or thyroglobulin in the tumor cells. The phenomenon is most common for follicular thyroid neoplasia, with no definite coupling to patient outcome. The clear cell variant of papillary thyroid carcinoma (ccPTC) is even more infrequent-making conclusions regarding prognosis difficult. Single reports describe distant metastases of ccPTCs as well as co-occurrence with anaplastic thyroid carcinoma (ATC). In this report, a case of a therapy-resistant ccPTC dedifferentiating into an ATC is characterized from morphological and immunohistochemical standpoints. The patient was a 79-year-old female presenting with a 45-mm nodule in her right thyroid lobe. A first round of cytology raised the suspicion of PTC, but a repeated biopsy verified an ATC diagnosis. Neoadjuvant doxorubicin and external irradiation therapy was administered, and the patient developed lung metastases concomitantly. A palliative lobectomy was performed, and the final diagnosis was a ccPTC with focal dedifferentiation into an ATC. Intriguingly, the ccPTC component was viable and dominated the lesion. The clear cell morphology stemmed from an accumulation of glycogen, while the anaplastic component was devoid of evident clear cell changes. The case is one of exceedingly few descriptions of a ccPTC that dedifferentiates to an ATC, suggesting that this PTC subtype is not without potential for development of a highly lethal tumor component. Moreover, the partial lack of response to neoadjuvant therapy suggests a possible underlying resistance to aggressive treatment modalities in this particular case.