Thyroid Nodules with Benign Findings at Cytologic Examination: Results of Long- term Follow- up with US

Original Research n Ultrasonography 1Purpose: To investigate the natural history of thyroid nodules found to be benign at initial fine-needle aspiration biopsy (FNAB) to determine the percentage of nodules that increased in volume by more than 50% as being an indicator of malignancy. Materials and Methods: This retrospective observational cohort study was approved by the institutional review board, and the need to obtain informed consent was waived. The study included 854 FNABconfirmed benign thyroid nodules. Suspicious ultrasonographic (US) features included marked hypoechogenicity, irregular or microlobulated margin, microcalcification, and taller-than-wide shape. Univariate and multivariate generalized linear mixed models were used to assess the association with nodule growth greater than 50% in volume. Results: For the 854 nodules, the initial mean diameter was 19.92 mm (range, 3.10-60.00 mm), and the initial mean volume was 3.19 cm(3) (range, 0.01-4.64 cm(3)). The majority (682 [ 79.9%] of 854) of thyroid nodules with benign cytologic results at initial FNAB did not grow more than 50% in volume during 4 years of mean follow-up (range, 7-101 months). More than 4 years of follow-up time versus less than 2 years, younger age, a cystic component of less than 25%, and nodule size 1 cm or larger versus less than 1 cm were independently associated with growth. There was only one malignant nodule (0.6%) among 172 thyroid nodules with a volume increase of 50% or greater during the entire follow-up time. Ten malignant nodules (overall malignancy rate: 1.2%) were detected among the 854 total nodules, and eight of these 10 nodules showed suspicious features at US. Conclusion: Repeat FNAB for nodules showing more than 50% growth in volume is unlikely to result in a diagnosis of malignancy. A positive FNAB result for malignancy is significantly more likely in the presence of suspicious US features.