The molecular signatures of many thyroid tumors have been uncovered. These discoveries have translated into clinical practice and are changing diagnostic and tumor classification paradigms. Here, the findings of recent studies are presented with special emphasis on how molecular insights are impacting the understating of RAS mutant thyroid nodules, Hürthel cell neoplasms, and unusual thyroid tumors, such as hyalinizing trabecular tumor, secretory carcinoma of the thyroid, and sclerosing mucoepidermoid carcinoma with eosinophilia. In addition, the utility of detecting actionable molecular alterations by immunohistochemistry in advanced and aggressive thyroid cancer is also discussed.

BACKGROUND: Hyalinizing trabecular tumor (HTT) is an uncommon follicular cell-derived thyroid tumor classified as a low-risk neoplasm by the World Health Organization Classification of Tumors of Endocrine Organs, 5th edition. The PAX8-GLIS3 gene fusion is reportedly a pathognomonic genetic alteration of HTT.
METHODS: A 43-year-old Japanese female was incidentally discovered to have an 8-mm, well-defined, hypoechoic mass in the left lobe of the thyroid gland by ultrasound examination. Contrast-enhanced computed tomography scan revealed a solid mass exhibiting slight homogeneous enhancement in the lower pole of the thyroid gland. The mass was diagnosed as atypia of undetermined significance by fine-needle aspiration cytology. The patient underwent left hemithyroidectomy with routine central compartment dissection. Histologic findings revealed tumor cells with elongated nuclei and intranuclear pseudoinclusions arranged with trabeculae architecture or small nests in hyalinized stroma. Weak membranous and cytoplasmic staining was found by MIB1 (Ki-67) immunostaining. The final diagnosis was HTT of the thyroid gland. Next-generation sequencing genetic analysis of a surgical specimen revealed no pathologic mutations, including BRAF, H/K/NRAS, or RET-PTC fusions. The PAX8-GLIS3 fusion was detected by RT-PCR.
CONCLUSIONS: A rare case of HTT was demonstrated through imaging, cytologic, histologic and molecular investigations. PAX8-GLIS3 fusion detected by RT-PCR and Sanger sequencing was confirmed to be a genetic hallmark of HTT.

UNASSIGNED: The hyalinizing trabecular tumor (HTT) is a rare benign neoplasm of the thyroid gland. This neoplasm has overlapping cytological features with Papillary Thyroid Carcinoma, Medullary Carcinoma and Follicular Neoplasm with Nuclear Features of Papillary Carcinoma. This can lead to misdiagnosis of malignancy in fine needle aspiration (FNA) cytology specimens with unnecessary total thyroidectomy. The aim of this study is to determine if there are some cytological features that could help us to suspect HTT on FNA specimens and avoid radical surgery.
UNASSIGNED: With this purpose we have collected 6 cases diagnosed of HTT in Hospital Clínico San Carlos of Madrid (Spain) in the last 10 years and reviewed the cytological specimens.
UNASSIGNED: We conclude that the presence of hyaline material in FNA specimens of HTT is a constant feature being a diagnostic clue. We must be cautious not to confuse it with dense colloid or amyloid material, the latter seen in Medullary Carcinoma. Papillary architecture and fibrovascular cores are not present in a HTT. Special stains as ki-67, calcitonin and Congo Red staining could help us in achieving the correct diagnosis.
UNASSIGNED: We feel the cytopathologists must be aware of the distinguishing features of this lesion, mainly the typical hyaline material to achieve a proper diagnosis and be able to reduce unnecessary aggressive management of these patients.

暂无摘要。

BACKGROUND: Only one thyroid follicular cell-derived tumor with a purely trabecular growth pattern has previously been described. This report aims to describe the histological, immunohistochemical, and molecular findings of our second case, propose a novel thyroid tumor, and discuss its diagnostic pitfalls.
METHODS: A 68-year-old female presented with an encapsulated thyroid tumor composed of thin and long trabeculae. No papillary, follicular, solid, or insular patterns are observed. The tumor cells were elongated or fusiform and arranged perpendicular to the trabecular axis. No nuclear findings of papillary thyroid carcinoma and increased basement membrane material were found. Immunohistochemically, the tumor cells were positive for paired-box gene 8, thyroid transcription factor-1, and negative for thyroglobulin, calcitonin, and chromogranin A. Inter- and intra-trabecular accumulation of type IV collagen-positive materials was not demonstrated. None of PAX8/GLIS1 and PAX8/GLIS3 and mutations in BRAF, HRAS, KRAS, NRAS, TERT promoter, CTNNB1, PTEN, and RET were detected.
CONCLUSIONS: We report our case as a novel disease entity called non-hyalinizing trabecular thyroid adenoma, which has the diagnostic pitfalls of hyalinizing trabecular tumor and medullary thyroid carcinoma.

A hyalinizing trabecular tumor (HTT) of the thyroid gland is a very rare type of tumor. It is usually diagnosed incidentally during the examination for thyroid gland diseases that need thyroidectomy. Here we report a case of HTT in a 60-year-old male patient who presented with anterior neck swelling and underwent total thyroidectomy for a Bethesda category V nodule. The final histologic diagnosis of the left lobe was consistent with a hyalinized trabecular adenoma of the thyroid gland, or paraganglioma-like adenoma. We discuss the clinical picture and diagnostic approach, including the role of fine needle aspiration biopsy, and the pathologic features of HTT, with particular reference to the possible differential diagnosis.

BACKGROUND: Hyalinizing trabecular tumor is a rare follicular cell-derived thyroid neoplasm that is considered to be a borderline tumor with malignant potential rather than a benign tumor. The detection of RET/PTC rearrangements and nuclear cytologic features suggests a relationship between hyalinizing trabecular tumor and papillary thyroid carcinoma. Some recent observations of pathogenic genetic alterations in hyalinizing trabecular tumor have indicated that hyalinizing trabecular tumor is not related to papillary thyroid carcinoma, and should be considered an independent entity. Here we present a case of papillary thyroid carcinoma with hyalinizing trabecular tumor-like features and discuss its interesting aspects and diagnostic issues from a histopathological perspective.
METHODS: A 19-year-old Japanese woman with an enlarged thyroid gland was admitted to our hospital. Based on fine-needle aspiration cytology, the enlarged nodule was suspected to be a follicular lesion or follicular tumor. A nodular lesion approximately 3 cm in diameter was detected in the left lobe of the thyroid gland. Histological analysis revealed that the tumor cells were mainly arranged in follicles. Solid nests with occasional trabecular arrangements and papillary structures were intermingled, and the tumor cells showed ground-glass nuclei and occasional nuclear grooving. Petaloid and block-like periodic-acid-Schiff and periodic-acid-methenamine-positive basement membrane components were observed in the interstitium of the solid portions of the tumor. Incomplete membranous immunoreactivity of MIB-1 (Ki-67 (cell prolferation marker)) was also observed in the cells within the solid areas. Moreover, this tumor displayed extracapsular invasion and metastasis to the perithyroidal lymph nodes, suggesting that it may be a malignant tumor. However, BRAFV600E mutation, RET/PTC rearrangements, and PAX8/GLIS 1 and PAX8/GLIS 3 rearrangements were not detected.
CONCLUSIONS: We diagnosed the tumor as a papillary thyroid carcinoma with characteristic features of hyalinizing trabecular tumor. Importantly, this case may indicate a possible relationship between papillary thyroid carcinoma and hyalinizing trabecular tumor, although specific genetic alterations could not be detected. We also discuss the preoperative diagnostic difficulties with fine-needle aspiration cytology and the unusual pathological findings in this case.

Objective To investigate the fine needle aspiration cytology and differential diagnosis of hyalinizing trabecular tumor (HTT) of the thyroid.Methods The fine needle aspiration smears of four HTT cases with histopathological controls were analyzed,which were then combined with the histopathological changes and immunophenotypes for diagnosis.The key points of cytological diagnosis and the differential diagnosis of HTT were then summarized.Results The fine needle aspiration cytology showed that the tumor cells were scattered,presenting as partially cohesive clusters or clusters with trabecular manifestations.The tumor cells were polygonal or spindle,with medium or rich cytoplasm.The nuclei were oval or short spindle,with fine granular chromatin,visible small nucleoli,common nuclear grooves and nuclear pseudoinclusions,and irregular outline,which demonstrated the nucleus characteristics of papillary thyroid carcinoma.The interstitium showed transparent basement membrane-like material deposition,loose tumor cell clusters,trabecular or syncytial structure,radially distributed tumor cells around the hyaline-like material,rich eosinophilic or dichromophile cytoplasm,elongated nuclei,no papillary structure or fibrovascular axis,and no psammoma bodies.Histopathology showed tumor cells arranged in beam and organoid,transparent basement membrane-like material deposition between trabecular beams,and polygonal or spindle cells containing fine granular eosinophilic cytoplasm and round or oval nuclei with common nuclear grooves and nuclear pseudoinclusions.Conclusion Combining the ultrasound results with the arrangement,interstitial components,and cytological characteristics of tumor cells,we suggest that Ki-67(MIB-1)staining can be employed to assist diagnosis and improve the diagnostic accuracy of HTT or intraoperative freezing can be adopted for further diagnosis.

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.

Hyalinizing trabecular tumors are a follicular origin neoplasm of the thyroid that usually present as an asymptomatic, well circumscribed, solitary mass. However, diagnosis of a hyalinizing trabecular tumor may be challenging especially on fine needle aspiration cytology and requires careful examination of the specimen to rule out potential mimickers such as papillary thyroid carcinoma, medullary thyroid carcinoma, paraganglioma, other follicular patterned neoplasms, intrathyroidal parathyroid tissue, and metastatic disease. We will review the cytologic, histologic and molecular features of hyalinizing trabecular tumors that aid in distinction from these mimickers with overlapping morphologic features and help ensure proper diagnosis for appropriate management.