Endocrine Abstracts

Introduction: Although the thyroid isthmus seems like a structure that connects bilateral lobes, it is an undiscovered area that needs to be explored. Currently, the data is evolving that the increase in the risk of malignancy is higher in the isthmic nodules, and extrathyroidal extensions and lymph node metastases are more common in isthmic-derived malignant thyroid nodules. Therefore, we aimed to compare the malignancy rate of isthmic and lobar nodules, the ultrasonographic features of isthmic and lobar nodules, and presence of lymph node metastases, distant metastases, and extrathyroidal invasions in malignant isthmic nodules.

Methods: In this retrospective study, we enrolled patients between the ages of 18-80 years, who had thyroid nodule/nodules cytology and/or pathology results from January 2009 to November 2020. 9504 nodules were selected for the analysis of US findings, cytopathology results, and malignancy rates.

Results: A median age of 56 (18-80) years with a Female to male ratio of [7618 (80.2%)/1886(19.8%)] were included in the study. 962 of the nodules were at isthmic localization; whereas 8542 nodules were at lobar localization. 803 nodules were surgically removed and resulted as malignant from histopathological evaluation. The median age of the patients with malignant nodules was 54 (18-80). Of the 803 malignant nodules, 704 nodules were (87.7%) PTC, 46 nodules (5.7%) were FTC, 35 nodules were (4.4%) MTC, 14 nodules 1.7% were Hurtle cell carcinoma, and 4 nodules (0.5%) were anaplastic thyroid carcinoma. 108 of the malignant nodules (13.4%) were located in the isthmus, whereas the majority of the malignant nodules (n:695, 86.6%) were located at the lobar parts (right or left) of the thyroid. When the metastasis patterns of isthmic and lobar thyroid cancers were examined, no significant relationship was found between isthmic and lobar cancers in terms of capsule invasion (P=0.437), muscle invasion (P=0.294), and lymph node metastasis (P=0.476). A significant correlation was found between nodule localization (isthmus-upper-middle and lower lobes) and malignancy (P<0.0001). In our logistic regression analysis, isthmic nodule localization was evaluated as an independent risk factor for malignancy (than lobar nodules)(OR: 1.35 95% CI, P=0.008). Nodules in the isthmus region were found to have a higher risk of malignancy compared to the lower lobes (OR: 1.88 95% CI, P=0.005).

Conclusions: We recommend, nodule localization has to be considered an additional risk factor when performing a Fine Needle Aspiration Biopsy (FNAB).