BACKGROUND: This study evaluates whether age modifies the association between tumor size and the risk of incomplete response to therapy (IRT) in patients with differentiated thyroid cancer (DTC). METHODS: A retrospective cohort study that included patients with DTC who underwent thyroidectomy from 2018 to 2022. Clinical outcomes were based on the 2015 ATA response to therapy classification, with the study outcome defined as biochemical or structural IRT. The cutoffs for age and tumor size were 55 years and 2 cm, respectively. We tested the interaction between age and tumor size on the risk of IRT using logistic regression models adjusted for aggressive histological factors. RESULTS: There were 232 patients included, with a mean age of 41.3 years and a median follow-up of 4.0 years. There were 39 patients (17%) who were 55 years and older, and 118 (51%) patients who had atumor size of >2 cm. There were 54 (23%) patients with IRT. Age modified the association between tumor size and IRT (P value for interaction=0.03), such that the IRT OR (95% CI) for age >= 55 years in patients with tumor size <= 2 cm was 0.56 (0.06, 4.87). While in patients >= 55 years of age with tumor size >2 cm, it was 7.54 (2.25, 25.31). Tumor size >2 cm in patients <55 years had OR=1.53 (0.65, 3.60). CONCLUSIONS: In tumors <= 2cm in size, age did not affect the risk of IRT, but when the size is >2cm, advanced age is associated with a higher risk of IRT.
