Interobserver Agreement Among Thyroid Ultrasound Operators in Defining Thyroid Nodules as Subcapsular

Background: Ultrasound (US) examination is the pivotal test to assess the risk of cancer and the indication for fine needle aspiration cytology (FNAC) in thyroid nodules (TNs). The subcapsular location of a TN may strengthen the indication for FNAC as, in TN resulting malignant at cytology may favor surgery rather than active surveillance. However, the definition of a subcapsular TN remains unclear. This study aimed to evaluate the interobserver agreement (IOA) among thyroid US experts in classifying TNs as subcapsular or not. Methods: Twelve raters received an electronic link to a file containing static US images and were asked to assess 60 TNs for subcapsular location, blinded to all other TN characteristics. The overall IOA was calculated, and the TN US features were subsequently analyzed to evaluate their potential influence on interobserver variability. The raters had high or very high thyroid US experience. Two experienced operators preliminarily selected the case series and analyzed the findings. All cases were derived from patients undergoing surgery and histological diagnosis. The IOA was calculated according to Fleiss' kappa, ranging from 0.0 (no agreement) to 1.0 (perfect agreement). Results: The overall IOA was fair (κ = 0.34), with a slightly better result in the subgroup of raters with higher experience (κ = 0.39). A higher IOA value (κ = 0.38) was observed in TNs of medium size. Following multiple sub-analyses, the highest κ value (0.46) was observed in the subgroup of TNs that were categorized as EU-TIRADS 5 and were smaller than 2 cm. Conclusions: The overall IOA among US experts when assessing TNs as subcapsular was unsatisfactory. A clear and standardized definition of subcapsular position is needed to improve clinical decision-making. Future guidelines should address this issue to ensure consistent assessment and management of subcapsular TNs.