ESTRO 2025 - Abstract Book

S1107

Clinical – Head & neck

ESTRO 2025

3988

Digital Poster Outcomes in anaplastic thyroid carcinoma after spatially fractionated radiotherapy

Elizabeth L. McKone 1 , Paul J. Dizona 2 , Jared Hobson 1 , Eric J. Moore 3 , Mabel Ryder 4 , Michael P. Grams 5 , Daniel J. Ma 1 1 Radiation Oncology, Mayo Clinic, Rochester, MN, USA. 2 Clinical Trials and Biostatistics, Mayo Clinic, Rochester, MN, USA. 3 Otorhinolaryngology - Head and Neck Surgery, Mayo Clinic, Rochester, MN, USA. 4 Endocrinology, Mayo Clinic, Rochester, MN, USA. 5 Radiation Oncology Medical Physics, Mayo Clinic, Rochester, MN, USA Purpose/Objective: Spatially fractionated radiotherapy (SFRT) is a technique for delivering an intentionally heterogenous target dose distribution with alternating high and low doses throughout. Our institution piloted SFRT for local treatment in select patients with anaplastic thyroid carcinoma (ATC). Material/Methods: Adults with ATC treated with SFRT were retrospectively reviewed. Overall survival (OS) and progression-free survival (PFS) were assessed using Kaplan Meier methods. Hazard ratios for locoregional failure (LRF) and OS were generated using cox proportional hazards models. Results: Fifteen patients (8 female; 15 white; 1 Hispanic/Latino; median age 67 years; ECOG 0-2) diagnosed between 10/2019 and 4/2024 were included. Tumors were T3 (n=3) or T4 (n=12), with 12 N+ and 10 M1 (AJCC 8th). Single fraction SFRT prescriptions of 1800 (n=3) or 2000 cGy (n=12) were delivered using VMAT lattice (n=14) or brass grid (n=1) with median Dmax=2533 cGy (range:1861-2881), median Dmin=22 cGy (range:7.2-244), and median mean dose=459 cGy (range:377-887). Ten received sequential EBRT to a median of 4000 cGy/10 daily fractions (range:2000-5800cGy/5-20 fractions) without breaks, except in 1 split course. Five had primary tumor surgery. Eleven received chemotherapy. Fourteen received immunotherapy and/or targeted therapy (n=7 BRAFV600E mutations). With a 7.6 month median follow up, respective median OS and PFS were 7.8 and 2.6 months. Five remain alive a median of 11 months (range:3-33) post-SFRT. Eight distant and 9 LRFs occurred in a median of 3.1 and 5.1 months, respectively. Compared to SFRT alone, SFRT+EBRT had a non-significant protective effect for OS and PFS (OS HR 0.32, 95% CI 0.09-1.2, p=0.092; PFS HR 0.46, 95% CI 0.15-1.5, p=0.20) and median time to LRF of 5.2 versus 2.2 months. Radiographic tumor response (≥25% tumor volume reduction) in a median of 26 days post-SFRT (range:2 94) was associated with a non-significant protective effect for LRF (n=5/14 responders; HR= 0.59, p=0.47). HRs for OS and PFS failures without chemotherapy were 0.66 (95% CI 0.12-3.6, p=0.60) and 0.67 (95% CI 0.12-3.6, p=0.60), respectively. With surgery, median OS was 15 months, and PFS failure HR=1.5 (95% CI 0.45-5.2, p=0.50). For grade 3+ toxicity at least possibly attributable to RT (CTCAE v5), there were 2 acute (anorexia; dysphagia) and 2 late events (tracheostomy hemorrhage after 7 EBRT courses; death in the setting of tracheal cartilage necrosis and superimposed infection after SFRT+EBRT). Conclusion: Multimodality treatment including SFRT±EBRT for ATC allowed fewer fractions with satisfactory toxicity and similar survival outcomes compared to RTOG 0912 and other modern series.

Keywords: ATC, SFRT, thyroid

References: Bronk, J.K., et al., Patterns of loco-regional progression and patient outcomes after definitive-dose radiation therapy for anaplastic thyroid cancer. Radiother Oncol, 2024. 202: p. 110602. Prasongsook, N., et al., Survival in Response to Multimodal Therapy in Anaplastic Thyroid Cancer. J Clin Endocrinol Metab, 2017. 102(12): p. 4506-4514.