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.

OBJECTIVE: To explore the clinicopathological features, diagnosis and differential diagnosis of hyalinizing trabecular tumor (HTT) of the thyroid.
METHODS: The four HTT specimens were collected including demographics, clinical information, relevant images, the extent of thyroidectomy, the follow-up and representative pathological data of tumors were available for analysis. In addition, the immumohistochemical staining related to the tumor as well as the BRAF and N-ras mutation analysis were analysed.
RESULTS: The mean age of four patients was 47 years old and the mean size of the tumor was 2.8 cm. Most of the patients were asymptomatic, while detecting incidentally by using neck ultrasound test. Ultrasound imaging of all cases showed demarcated substantial hypoechoic nodules in ipsilateral thyroid lobe. Computed Tomography (CT) showed a clear low density shadow in the affected thyroid lobe. Tumors of three cases were located at the left, but the other one was located at the right thyroid gland with a complete fibrous capsule. The cytological features resembled papillary thyroid carcinoma (PTC). The histological test indicated that the tumors had characteristic of trabecular growth pattern with hyalinizing material. The tumor cells were in shape of polygonal, oval or high columnar with an acidophilic or clear cytoplasm. The nuclei were oval with inconspicuous small nucleoli, prominent grooves and pseudoinclusion body in cell nucleus. Mitosis and psammoma bodies were rare to be observed. Cytoplasmic \"yellow bodies\" were frequently observed. The hyaline material was prominent, with positive periodic acid-Schiff (PAS) and negative Congo red staining. Immunohistochemically, tumor cells were positive for thyroglobulin (Tg), thyroid transcription factor-1 (TTF-1), CD56 and negative for calcitonin, cytokeratin 19 (CK19), HBME-1, S-100 and synaptophysin (SyN). Chromogranin A (CgA) and galectin-3 were expressed weakly in some cases. Staining with the MIB-1 antibody showed membranous/cytoplasmic immunoreactivity. Whereas, another clone of Ki-67 (SP6) showed a common nuclear pattern with an index of <1%. None of the four cases exhibited the BRAF V600E protein reactivity. Gene mutation analysis demonstrated no BRAF and N-ras mutation. There was no evidence of local recurrence or metastasis after 6 to 36 months of follow-up.
CONCLUSIONS: HTT is an uncommon thyroid tumor with very low malignant potential. It has no particular clinical features, so it\'s often misdiagnosed in fine needle aspiration cytology (FNAC)/Ultrasonography-guided fine needle aspiration cytology (US-FNAC) and frozen section (FS). Its final diagnosis mainly relies on typical histopathological features and characteristic expression pattern of MIB-1 immunohistochemical staining.

BACKGROUND: Fine-needle aspiration cytology (FNAC) is very accurate in detecting papillary thyroid carcinomas (PTCs). According to the Bethesda system for reporting thyroid cytopathology, the risk for malignancy is 97-99% when FNAC is used to diagnose PTC; the malignancy risk is 60-75% when FNAC results in suspected PTCs. The presence of hyalinizing trabecular tumor (HTT) of the thyroid can cause misdiagnosis because its cytological features mimic PTCs. However, the use of frozen section analysis can assist in the recognition of unique architecture features of HTT, and thus may help prevent the undertaking of an unnecessarily aggressive operation.
METHODS: We retrospectively reviewed all patients diagnosed with HTT by permanent histopathology from February 2009 to October 2013. After acquired agreement of the patients, we analyzed all data and reviewed another nine cases of HTT reported in the English-language medical literature to examine the efficacy of frozen section.
RESULTS: There were six patients included in our research (5 women and 1 man), with an average age of 48.8 years. Using frozen section, four patients were diagnosed with HTT and two patients were misinterpreted as PTC. Consequently, four patients had lobectomy and two patients had total thyroidectomy, with no surgical complications. Of the nine cases of HTT reviewed from the English literature, the use of frozen section showed three HTT cases, three PTC cases, two medullary thyroid carcinoma cases, and one deferral case. Overall, the use of frozen section as a diagnostic method prevented additional surgical resection in eight patients (53%).
CONCLUSIONS: Frozen section can sometimes but not always be used to diagnose HTT. When HTT is diagnosed by its trabecular pattern through the use of frozen section, it may prevent total thyroidectomy.