[Clinicopathological and genetic features of hyalinizing trabecular tumor of the thyroid].

Objective: To analyze the cytological, histological, immunohistochemical, and molecular pathological features of hyalinizing trabecular tumor (HTT). Methods: Clinical and pathological data of the HTT cases diagnosed at Shanghai Sixth People's Hospital affiliated to Shanghai Jiao Tong University School of Medicine between 2020 and 2024 were collected and analyzed. HE staining, special staining, immunohistochemical staining, and next-generation sequencing were performed on all cases. Results: Among the 10 HTT patients, 4 were male and 6 were female. The age at onset ranged from 29 to 85 years, with a median age of 49 (35,61) years. The maximum tumor diameter ranged from 0.3 to 5.3 cm. Cytologically, the smears were hypercellular and showed tumor cells arranged in nested clusters with visible basement membrane-like material. The nuclei were oval with finely granular chromatin, and nuclear pseudoinclusions were readily identifiable. Histologically, the tumors were well demarcated. The tumor cells were arranged in a paraganglioma-like pattern, exhibiting typical nuclear features of papillary thyroid carcinoma and psammoma bodies. Yellow bodies were observed in the cytoplasm. The stroma was rich in hyalinized material, which was periodic acid-Schiff stain (PAS)-positive. Immunohistochemically, the tumor cells showed diffuse expression of TTF-1 and focal expression of thyroglobulin. Aberrant immunoreaction with Ki-67 was present in the cytoplasm and membrane of the tumor cells. Molecular testing was performed on 8 cases. The PAX8-GLIS3 gene fusion was detected in 7 cases. Among these fusion-positive cases, 4 exhibited additional genetic abnormalities: one concurrent TSHR point mutation (p.D617H); one concurrent HRAS point mutation (p.Q61R); one concurrent LRP1B point mutation (p.S1752L), SUGCT point mutation (p.K137), and TERT point mutation (p.P785L); one concurrent MTOR mutation (7528+27A>T) and FLT3 mutation (p.E77K). The key initiating factors for thyroid carcinoma, including the BRAF V600E mutation and RET rearrangements, were absent in all cases tested. Conclusions: Cellular pleomorphism, yellow bodies and basement membrane-like material constitute important cytological and histological features for the differential diagnosis of HTT. Immunophenotypically, thyroglobulin may show focal expression, while Ki-67 is typically localized in the tumor cell membrane and cytoplasm. This study also demonstrates that PAX8-GLIS3 fusion is a characteristic molecular abnormality in HTT, although cases with wild type of GLIS gene may also present. Although rare, HTT may harbor point mutations in HRAS and TSHR, and other uncommon genetic alterations.