"Onion Skin-liked Sign" in Thyroid Ultrasonography: A Characteristic Feature of Benign Thyroid Nodules

Background: Some ultrasonographic (US) signs overlap between benign and malignant nodules. The purpose of this study was to raise a special US sign of benign thyroid nodules, termed the "onion skin-liked sign."Methods: Twenty-seven patients with 27 nodules who shrank naturally and the "onion skin-liked sign" appeared on the final US images were enrolled in the study. The ultrasound characters and risk stratifications at the start and end of observation were compared. Then, thirty goiters with fibrosis and thirty papillary thyroid carcinomas (PTC) were randomly selected from the database of our hospital, matched the sizes of 27 nodules at the end point of observation. The differences of "onion skin-liked sign" between the two groups were analyzed.Results: The average duration of follow-up of 27 nodules was 24.0 ± 12.2 months (range, 12-65 months). At the end of the follow-up, the size of the nodules decreased on average by 1.26 ± 0.82 cm (range, 0.3-3.4 cm) and calcification was found in 21 nodules, compared with only 2 nodules with calcification at the start of the follow-up. In addition, only negligible or no blood flow signal could be detected at the periphery of all the nodules and 100% (27/27) were high suspicion at the end of observation. In matched groups, all PTC showed high suspicion of malignancy, 18/30 (60%) goiters with fibrosis were high suspicion and 11/30 (37%) were intermediate suspicion. Twenty-two patients in the group of nodular fibrosis presented "onion skin-liked sign," which was not shown in any patient of PTC group. The sensitivity, specificity, positive predictive value, and negative predictive value of "onion skin-liked sign" in predicting nodular goiter with fibrosis were 73.3%, 100%, 100%, and 78.9%, respectively.Conclusions: The "onion skin-liked sign" was a characteristic US feature of benign thyroid nodules detected in the follow-up of thyroid nodules. It is useful to differentiate PTCs and nodular goiters with fibrosis.