Active surveillance of highly suspicious thyroid nodules in the isthmus compared with non-isthmus locations: a...

To compare progression outcomes during active surveillance between highly suspicious thyroid nodules located in the isthmus and those in non-isthmic regions, and to explore the feasibility of active surveillance for selected isthmic lesions.

This multicenter retrospective study included 449 solitary thyroid nodules that were re-evaluated by blinded image review and classified as American College of Radiology Thyroid Imaging Reporting and Data System (ACR TI-RADS) category 5. Nodules were anatomically categorized as isthmic (n = 33) or non-isthmic (n = 416). Cytological results were available for 144 of 449 nodules (32.1%). Progression was evaluated according to size-based criteria (≥3 mm increase in maximum diameter, ≥3 mm increase in one, two, or all three orthogonal diameters, and ≥50% increase in volume), the occurrence of new suspicious lesions, lymph node metastasis, and distant metastasis, as well as a composite endpoint encompassing the latter three events.

The median follow-up duration was 47 months, with no significant difference between the two groups. Non-isthmic nodules had significantly larger volumes at baseline and at final follow-up, whereas maximum diameter did not differ significantly between groups (P>0.05). A ≥50% increase in volume occurred in 30.30% of the isthmus group and 24.52% of the non-isthmus group (P = 0.53). A composite endpoint occurred in 3.03% of the isthmus group and 4.09% of the non-isthmus group (P = 1.00). Observed differences in progression indicators were not statistically significant (P>0.05). No distant metastases or thyroid cancer-related deaths occurred during follow-up.

In this multicenter retrospective cohort, ACR TI-RADS 5 thyroid nodules located in the isthmus were not observed to have higher progression rates than non-isthmic nodules during AS. These findings support the potential feasibility of active surveillance for selected isthmic nodules under careful monitoring. However, the small isthmus sample size and low number of progression events preclude definitive conclusions regarding equivalence between the two groups.