We present a rare cause of superior vena cava syndrome (SVC) in a previously healthy male aged 31 years. Malignancy was suspected due to unintentional weight loss and childhood exposure to radioactive fallout from a nuclear facility accident. A very large anterior mediastinal mass was identified and demonstrated to be an extragonadal seminoma. Extragonadal germ cell tumours are rare tumours with a high potential for cardiovascular, pulmonary and vascular sequelae. Studies have documented an increased risk of developing seminoma in patients with radioactive exposure. Chemotherapy was initiated, during which the patient experienced progressive and new symptoms, found to be due to extensive thromboembolic disease, which responded well to anticoagulation. Seventy-two months after completing chemotherapy, without need for surgical management, he remains free of the disease.