Sonographically Estimated Risks of Malignancy for Thyroid Nodules Computed with Five Standard Classification Systems: Changes over Time and Their Relation to Malignancy

Background: Over 50% of newly diagnosed thyroid nodules are either cytologically benign or presumed to be benign on the basis of low-suspicion sonographic findings. The strategies used for their long-term surveillance are based mainly on the estimated residual risk of malignancy calculated with various ultrasonographic classification systems (e.g., Thyroid Image Reporting and Data Systems [TIRADS]). We conducted a longitudinal study to evaluate the temporal stability of the initial risk estimates computed with five widely used systems and to determine whether risk class increases during follow-up are indeed predictive of malignancy. Methods: We re-analyzed data prospectively collected at a single academic referral center on 232 patients (age: 54.1 +/- 13.7 years) with 432 asymptomatic, sonographically or cytologically benign thyroid nodules at baseline (T0) and 122 new nodules that were present five years later (T5). At both time points, the sonographically estimated risk of malignancy was calculated as recommended by the American Association of Clinical Endocrinologists/American College of Endocrinology/Associazione Medici Endocrinologi, the American College of Radiologists' TIRADS, the American Thyroid Association's 2015 practice guidelines, the European Thyroid Association's TIRADS (EU-TIRADS), and the TIRADS of the Korean Society of Thyroid Radiology (K-TIRADS). Results: For 57 to 127 (13.2-29.4%) of the original nodules, depending on the system used, the estimated malignancy risk increased over the 5-year interval. Of the nodules whose baseline risk had not warranted cytological assessment, very few (6.3-8.3%) met the criteria for cytology at the 5-year evaluation. Biopsy was indicated for only 4 to 8 (3.3-6.6%) of the new nodules based on T5 risk estimates. Despite these changes, none of the 232 patients was ever diagnosed with a cancer. Conclusions: Ultrasound-based risk classes of presumably benign thyroid nodules remain fairly stable over time, and changes warranting biopsy are rare indeed. The appearance of new nodules is a frequent event, but very few (<5%) are classified as high risk, and only the 3-7% meet the criteria for cytological assessment. Collectively, these findings support the view that patients with presumably benign thyroid nodules can be safely followed with less intensive protocols.