Proton Beam Therapy for Differentiated Thyroid Cancer: Initial Report of the Mayo Clinic Experience

Abstract

External beam radiation therapy (EBRT) is utilized in a subset of high-risk patients with differentiated thyroid cancer (DTC). Proton beam therapy is an attractive modality given the potential enhanced biological dose with improved sparing of normal structures. We review our multi-site, single-institution experience of intensity-modulated proton therapy (IMPT) for DTC. Eight patients with DTC who underwent IMPT from 2016 to 2018 were reviewed at our institution. Patient, tumor, and treatment characteristics were recorded. Overall survival (OS), freedom-from distant metastasis (FFDM), and freedom-from local-regional recurrence (FFLR) were recorded. Survival rates were calculated using Kaplan-Meier estimates. Acute and chronic toxicities were recorded based on CTCAE v4.03. Median follow-up is 15.7 (range, 5.4-27.5) months for all patients. Median age at treatment with IMPT is 62.6 (range, 50.1-75.9) years and the majority are female (63%). Histologies included Hurthle cell (37.5%) and papillary carcinoma (62.5%); 1 patient had tall cell variant. The median adjuvant (50%) or definitive (50%) IMPT dose is 70 (range, 30-70) Gy. One patient received 30 Gy in 5 fractions. Concurrent chemotherapy was not administered for any patient in this cohort. At 2 years, all patients are alive, FFDM is 85.7% (95% CI, 45-99%), and FFLR is 71.4% (95% CI, 33-94%). One patient underwent tracheostomy for acute grade 4 laryngeal edema. There were no other grade≥3 acute or chronic treatment-related toxicities. IMPT is feasible and provides promising local control for recurrent DTC. Larger studies are needed to determine the role of IMPT for DTC.