The management of pediatric thoracic synovial sarcoma remains a matter of debate in clinical oncology, especially as regard to the local control of the disease. Surgery remains the gold standard, while the role and timing of radiotherapy is still controversial. We report a 14-year-old male, who has not received proper treatment at the time of diagnosis and initial management. Intensity-modulated irradiation was performed only at relapse, as a salvage treatment and, at 10-month follow-up, the young patient was free from relapse, without significant acute and subacute toxicity. We discuss the role and timing of radiotherapy in thoracic synovial sarcoma, a disease in which the need to increase local control should be placed in the foreground.