Definitive Radiotherapy for Oligometastatic and Oligoprogressive Thyroid Cancer: A Potential Strategy for Systemic Therapy Deferral.

[BACKGROUND] Definitive radiotherapy (dRT) has been shown to be an effective option for patients with oligometastatic and oligoprogressive cancers; however, this approach has not been well-studied in metastatic thyroid cancer.

[METHODS] This retrospective cohort included 119 patients with oligometastatic (34%) and oligoprogressive (66%) metastatic thyroid cancer treated from 2005 to 2024 with 207 dRT courses for 344 sites (50% thoracic, 37% bone, 7.5% brain, 4% abdominopelvic, and 1.5% neck/skull base). Histologies included 61% papillary, 15% poorly differentiated, 13% follicular, and 10% oncocytic, and 114 (96%) patients had radioiodine-refractory disease prior to dRT. Each course involved 1 to 5 sites, with prescriptions intended for definitive control (median BED10, 72 Gy), and palliative RT was excluded. Somatic mutation testing for oncologic drivers was performed in 103 (87%) patients.

[RESULTS] Each patient had an average of 3 sites (range, 1-23) treated over 2 courses (range, 1-9). Follow-up from first dRT was a median 2.5 years, with overall survival at 3 and 5 years of 81.5% and 70%, respectively. Actuarial local control per site was 91% at 3 years. Median progression-free survival (PFS) after first course was 17 months (95% CI, 10-24 months), with poorly differentiated histology associated with worse outcomes (hazard ratio [HR], 2.20; 95% CI, 1.24-3.90; P=.007), BRAF mutation with improved PFS (HR, 0.59; 95% CI, 0.37-0.95; P=.029), and no significant findings with respect to systemic therapy. At initial dRT, 92 (77%) patients were not on systemic therapy; and after first dRT, freedom from systemic therapy escalation was a median 4.1 years (95% CI, 1.7-6.5 years), with 2- and 5-year continued deferral rates of 73% and 46%, respectively. Grade 3 toxicities were noted for 1.5% of courses, with no grade 4-5 events observed.

[CONCLUSIONS] This study underscores the potential of dRT as a feasible strategy for deferring systemic therapy escalation in patients with oligometastatic and oligoprogressive metastatic thyroid cancer, demonstrating that sequential dRT courses impart excellent local control and are safe to deliver repeatedly for multiple distant sites. Further studies are warranted to validate these findings and elucidate the full benefit of dRT as part of a multidisciplinary approach for metastatic thyroid cancer.