SUN-345 Low Dose Radioiodine Ablation In Intermediate And High Risk Differentiated Thyroid Cancer

Abstract Disclosure: M.E. Koome: None. M.J. Bolland: None. Optimal radioiodine (RI) treatment following surgery for intermediate and high risk differentiated thyroid cancer (DTC) is uncertain. Randomised controlled trial data are lacking, and previous retrospective studies report conflicting results. Some studies show worse outcomes with lower initial RI doses and others show no difference. This is a retrospective audit of RI prescribing and outcomes for those receiving inpatient RI for DTC at a single centre where low dose ablation has been routinely prescribed in intermediate/high risk DTC for many years. All patients receiving inpatient RI for DTC from 2007 to 2023 were reviewed. Patients were only excluded if there was insufficient information available in the medical record. Data on demographics, cancer characteristics, RI prescribing, and disease specific outcomes were collected.145 patients were included. The median follow-up was 6 years (IQR 3-10). Most had high (67%) or intermediate (17%) risk cancer, 58% had nodal involvement, and 26% distant metastases at diagnosis. Patients were divided into two groups based on the first RI dose: low dose (≤1.0 GBq, 79%) and high dose (>1.0 GBq, 21%). RI dose was at the discretion of the prescriber. At diagnosis, the high dose group had significantly larger primary cancers (median 48 vs 35mm), higher AJCC (8th ed) stage (26% vs 61% stage I), more distant metastases (45% vs 21%), more poorly differentiated histology (19% vs 6%), and were more likely to have neck radiotherapy prior to RI (48% vs 3%). There was no difference in cumulative total RI dose between groups. After adjustment for AJCC stage, disease specific mortality (DSM) was significantly higher in the high dose group: HR 2.34, 95%CI 1.02 – 5.37. There was no difference between groups in disease recurrence, progression, or response to treatment at final follow-up. DSM in the low dose group was broadly comparable to other published cohorts.This cohort of high/intermediate risk DTC patients had a high proportion receiving low dose RI ablation at odds with current ATA guidelines. The high dose group had worse DSM reflecting their poorer prognostic features. These data suggest that intermediate/high risk DTC may still be appropriately treated with lower dose RI ablation (1GBq) followed by higher therapeutic RI doses in the absence of prior external beam radiotherapy, or certain poor prognostic features such as distant metastases or poorly differentiated histology. Presentation: Sunday, July 13, 2025