3483 - Stereotactic Radiation for Recurrent and Metastatic Thyroid Cancer: Site-Dependent Local Control in the Modern Systemic Therapy Era

Purpose/Objective(s):

Recurrent and metastatic thyroid cancer has traditionally been considered relatively radioresistant. We evaluated dosimetric parameters and oncologic outcomes following stereotactic radiation therapy (SBRT/SRS), comparing skeletal and non-skeletal disease sites.

Materials/Methods:

We retrospectively reviewed 36 patients with 75 treated lesions (45 skeletal, 30 non-skeletal) treated between 2012–2025. Skeletal lesions were commonly treated with 16–20 Gy ×1 or 7–9 Gy ×3; non-skeletal lesions more often received 7–9 Gy ×5. BED10 was calculated using a/ß = 10 Gy. Dosimetric parameters include PTV D90, V95, and conformity index (CI). Local control (LC) was estimated using Kaplan–Meier methods and compared by log-rank testing.

Results:

Median follow-up was 3.2 years (range, 0.3–13 years). Histologies included papillary (58%), follicular (22%), Hurthle cell (2%), medullary (10%), poorly differentiated (4%), and squamous carcinoma (4%). Plans demonstrated high quality (mean D90=100%, mean V95=97%, and mean CI=0.98). Median BED10 was lower for skeletal (50.4Gy range 45-55Gy) versus non-skeletal lesions (72.0Gy range 60–85 Gy; p<0.001). One- and 2-year LC for skeletal lesions were 90% (95% CI, 82–97%) and 86% (95% CI, 76–94%), compared with 78% (95% CI, 66–90%) and 70% (95% CI, 55–85%) for non-skeletal disease (p=0.03). Local failure was more common in poorly differentiated and squamous histologies (p=0.01). Treatment was well tolerated; Most common radiotherapy related toxicities were Grade 2 dysphagia in 2 patients (6%) and Grade 2 dysphonia in 2 patients (6%), with no grade 3–5 events, radiation myelopathy, or treatment-related fractures.

Conclusion:

Stereotactic radiation achieves durable LC in recurrent/metastatic thyroid cancer, with superior control in skeletal metastases despite lower BED. Treatment was safe with low high-grade toxicity, supporting a site-adapted stereotactic strategy in the era of modern systemic therapy.