Locoregional Control and Toxicity of Neck Radiation with Field Reduction in Advanced Differentiated Thyroid Carcinoma: A Retrospective Cohort Study

Background: External beam radiotherapy (RT) is indicated for high-risk, gross residual, unresected, or recurrent differentiated thyroid cancer (DTC) when further surgery is not recommended. Although RT improves time to locoregional control (LRC), head and neck toxicities remain a significant concern. Whether field reduction can lessen toxicity while maintaining tumor control in advanced DTC remains undefined. Methods: In this retrospective cohort study, detailed RT plans were available for 284 of 327 consecutive nonanaplastic, nonmedullary DTC patients treated from 1999 to 2023 at a single tertiary center. Treatment fields were classified as: comprehensive (bilateral central compartment and bilateral necks), ipsilateral (central compartment plus ipsilateral neck), or central (central compartment only). Acute toxicities were defined as the worst grade within 90 days of RT start (CTCAE) and late toxicities thereafter (RTOG). Time to locoregional recurrence (LRR) was assessed using cumulative incidence functions, treating death without LRR as a competing risk. Overall median follow-up was 8.9 years (interquartile range 5.3–12.7 years). Results: Among 284 patients, 103 (36%) received comprehensive, 116 (41%) ipsilateral, and 65 (23%) central fields. Group differences in acute grade ≥ 2 toxicities were observed in xerostomia (36% comprehensive, 17% ipsilateral, 16% central, p = 0.002), dysphagia (58% comprehensive, 48% ipsilateral, 36% central, p = 0.02), voice changes (39% comprehensive, 22% ipsilateral, 28% central, p = 0.02) and fatigue (41% comprehensive, 28% ipsilateral, 22% central, p = 0.02). Also differing was late grade ≥ 2 dysphagia (23% comprehensive, 6% ipsilateral, 8% central, p = 0.0004). Percutaneous endoscopic gastrostomy tube placement was higher in comprehensive versus central ( p = 0.0003) and ipsilateral ( p = 0.019) fields. In-field failure rates were 11/103 (10.7%) for comprehensive, 11/116 (9.5%) for ipsilateral, and 5/65 (7.7%) for central fields, with no difference in LRC across groups ( p = 0.88). Six patients with central RT developed out-of-field lateral neck recurrences, all successfully salvaged with surgery. Conclusions: Progressive RT field reduction in appropriately selected DTC patients was associated with significantly lower rates of acute and late toxicities without compromising LRC. These findings support individualized field selection and highlight the potential role of RT field de-escalation in this population. Prospective multicenter studies are needed to confirm these findings.