External validation of a predictive model for post-treatment persistent disease by 131I whole-body scintigraphy in patients with differentiated thyroid cancer

Purpose: We performed an external validation of a predictive model for persistent/metastatic disease in patients with differentiated thyroid cancer (DTC) at post-treatment 131I whole-body scintigraphy (WBS). Methods: Our study population included 836 patients (median age 44 years, 78% women) with DTC referred from 1994 to 2021 at our center. Age, sex, histology, T stage, N stage, American Thyroid Association risk classes, thyroid-stimulating hormone, radioactive iodine (RAI) activity, and thyroglobulin (Tg) levels were considered potential predictors of post-treatment WBS results. For the external validation, N stage and Tg levels were put into the decision tree (DT) model using its same Tg cut-off values. Results: Ninety-nine patients (12%) had positive post-treatment WBS. The area under receiver operating characteristic (ROC) curve for predicting WBS findings through the external validation was 0.60 (95% confidence interval, CI, 0.56-0.64), and positive and negative predictive values were 58% (95% CI, 41-74%) and 90% (95% CI, 88-92%). We also developed an internal model including the independent predictors of WBS findings (i.e., Tg levels, T stage, N stage, and RAI activity). For this model the area under ROC curve was 0.75 (95% CI, 0.69-0.81), and positive and negative predictive values were 90% (95% CI, 68-99% and 88-92%). Conclusions: The external validation of the proposed DT model has a limited value for predicting post-treatment 131I-WBS findings in our patients. The internal model including also T stage and RAI activity demonstrates higher predictive value.