Treatment at relapse for synovial sarcoma of children and adolescents: A multi‐institutional European retrospective analysis

Purpose: Though the prognosis for pediatric patients with localised synovial sarcoma (SS) is generally good, the chances of being cured after relapse are limited. This study describes a retrospective multi-institutional series of relapsing SS patients treated at six selected European referral centers for pediatric sarcoma. Patients and methods: The study included 41 patients <21 years with relapsing SS, treated between 2002 and 2022. The analysis included patient's characteristics at first diagnosis, first-line treatments, clinical findings at relapse, and second-line treatment modalities. Results: The first relapse occurred within 3-132 months (median 18 months) after first diagnosis and was local in 34%, metastatic in 54%, and both in 12%. Treatment at first relapse included surgery in 56% of cases, radiotherapy in 34%, and systemic therapy in 88%. In all, 36 patients received second-line medical treatment, that was chemotherapy in 32 cases (with 10 different regimens) and targeted therapy in four. No patient was included in an early-phase clinical trial as second-line therapy-line therapy. Overall response rate was 42%. Median event-free survival (EFS) was 12 months, postrelapse 5-year EFS was 15.8%. Median overall survival (OS) was 30 months, postrelapse 5-year OS was 22.2%. At the Cox's multivariable regression analysis, OS was significantly associated with time and type of relapse. Conclusion: Pediatric patients with relapsed SS have a poor prognosis and generally receive an individualized approach, due to the lack of a uniform standardized approach. New comprehensive strategies are needed to improve the knowledge on the biologic landscape of SS and develop tailored prospective clinical trials.