Total thyroidectomy can still remain the method of choice in some Bethesda III cases.

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The latest WHO classification of tumours of endocrine organs defines new units of borderline thyroid tumours (BTT). The aim of our study was to evaluate ultrasonographic and cytological features, mutation profile and surgery treatment in rare thyroid tumours. An analysis of 8 BTT out of 487 patients, who underwent thyroid surgery between June 2016 and June 2020. The definitive diagnosis was made postoperatively by extensive histopathological examination. Molecular genetic analysis of genes associated with thyroid oncology (BRAF, HRAS, KRAS, NRAS, TERT, TP53, fused genes) were performed from one FNAB, and 7 formalin-fixed paraffin-embedded (FFPE) samples. BTT were found in a total of 8 patients (1.6%), with a predominance of men with respect to other operated patients. FNAB samples were classified in the Bethesda system as Bethesda I, Bethesda II and Bethesda III in one, four and three cases, respectively. Hemithyroidectomy and total thyroidectomy were performed equally in four patients. The histopathological diagnosis revealed non-invasive encapsulated follicular neoplasm with papillary-like nuclear features (NIFTP) in three patients, follicular tumour of uncertain malignant potential (FT-UMP) in three patients, well differentiated tumour of uncertain malignant potential (WDT-UMP) in one patient, and hyalinizing trabecular tumour (HTT) in one case. In NIFTP cases mutation in HRAS gene in one patient together with probable pathogenic variant in TP53 gene and in NRAS gene in two patients were detected. In HTT patient PAX8/GLIS3 fusion gene was detected. The surgical treatment of BTT is necessarily individual influenced by preoperative clinical, ultrasonographic, cytological and molecular genetic findings, and the presence of other comorbidities.