The Role of Stereotactic Radiotherapy in Deferring the Need for Systemic Therapies in Patients with Progressive Oligometastatic Iodine-Refractory Thyroid Cancer

Differentiated thyroid cancer (DTC) is usually associated with a good prognosis, although development of metastases in iodine-refractory patients adversely affects quality of life and survival. The advent of tyrosine-kinase inhibitors drugs (TKI) allowed for a significant improvement in patients' outcome: however, in case of oligometastatic disease, a locoregional ablative approach such as Stereotactic Radiation Therapy (SBRT) may be proposed. The aim of our study is to assess if SBRT in oligometastatic DTC patients could effectively control tumor progression and possibly defer the need of systemic therapies.We retrospectively analyzed patients with differentiated oligometastatic iodine refractory thyroid cancer treated with SBRT in our Radiation Oncology Unit from 2011 to 2022. We collected demographics and treatment-related characteristics. Local Control (LC), Progression Free Survival (PFS), Overall Survival (OS) and the need for systemic therapy were evaluated. Patients with anaplastic histology, incomplete treatment or without follow-up information were excluded.Twenty-two patients were included in our analysis, accounting for 63 tumors: 26 were located in bones (41,2%), 21 in lymph nodes (33,3%), 7 in the brain (11,1%), 6 in the lungs (9,5%) and 3 visceral (4,7%). SBRT was delivered in 1-8 fractions, with a median dose of 30Gy (range 24-60Gy). Median follow-up was 36,8 months (range 6,1-130,9 months). After SBRT we observed a complete response in 29 lesions (46,0%), partial response in 20 (31,7%), stable disease in 13 (20,6%) and only 1 progressive lesion (1,6%). We observed 10 local recurrences (15,8%) with an actuarial LC of 92,4% and 76,1% at 24 and 48 months respectively, while PFS was 62,8% and 52,3% at 12 and 24 months respectively. The OS at 24 and 48 months was 89,2% and 82,3% respectively. Patients with oligoprogressive disease were treated with further SBRT, while patients with polymetastatic progression of disease or oligoprogression not suitable for ablative treatment received systemic therapy. Eleven patients (50%) needed to start TKI treatment (6 Lenvatinib, 3 Sunitinib and 2 Sorafenib) for progressive disease: median time to first systemic treatment from SRT was 18,5 months (range 0,5-67 months). At the time of this analysis, 11 patients (50%) were still without systemic therapy, showing a good disease control after a median follow-up of 26,2 months (range 5,7-83). Freedom from systemic therapy rate from the time of SBRT was 85,6% and 57,6% at 12 and 24 months respectively.In our experience, SBRT yields satisfying local control rates in oligometastatic Iodine-refractory DTC, potentially allowing for a deferral of systemic therapies, and should always be considered in these patients.