Background: The goal of radioactive iodine (RAI) in differentiated thyroid cancer (DTC) is to treat metastasis and reduce recurrence risk. International guidelines provide broad risk stratification to aid treatment decisions, but a more nuanced approach to individualize care is warranted. We developed a predictive risk model for DTC. Methods: We performed a retrospective multivariable analysis of 899 patients who received RAI after thyroidectomy at a quaternary center in Australia between 2008 and 2016. Collected data included age, gender, histology, stimulated thyroglobulin (sTg), and 8th American Joint Committee Cancer (AJCC) staging. The ATA Modified Initial Risk (ATA) was calculated retrospectively. Recurrence was defined as clinically significant progression requiring either surgical intervention or administration of a second activity of RAI. Synchronous metastasis was defined as distant metastasis (i.e., outside of the neck) that was present at the time of diagnosis on structural imaging or initial post-iodine treatment scan. The features significantly associated with synchronous metastasis or recurrence were employed in the generation of a predictive risk model. A separate cohort of 393 patients who received RAI in 2017-2021 was used for validation. Results: On multivariate analysis, sTg ≥10 μg/L, extrathyroidal extension (ETE) and lymph node involvement predicted recurrence. Independent of ATA, patients with sTg ≥10 μg/L had a shorter disease-free survival (DFS) than those with sTg <10 μg/L (p < 0.001). The ETE stratified by four histological categories was significantly associated with worse DFS (p < 0.001). In a subset of patients, the presence of thyroglobulin antibody (TgAb) did not influence recurrence in patients with sTg <10 μg/L. On multivariate analysis, widespread ETE, sTg ≥10 μg/L, multifocal papillary thyroid cancer and follicular thyroid cancer were positively associated with synchronous metastasis. A predictive risk model was developed to estimate synchronous metastasis/recurrence risk and validated successfully in the second cohort. Conclusions: Our novel predictive risk model modifies and extends ATA stratification by including sTg and ETE, which we found to be independent predictors of both recurrence and synchronous metastasis in DTC.
Keywords: ATA; differentiated thyroid cancer; predictive risk model; predictors of recurrence.