Summary

A 45-year-old man presented with a large para-aortic retroperitoneal tumour, a smaller second mediastinal tumour and elevated lactate dehydrogenase (LDH). Biopsy established a diagnosis of extragonadal seminoma. Treatment with cisplatin and etoposide resulted in complete resolution of the mediastinal mass, reduction of the size of the retroperitoneal mass and normalisation of LDH. Postchemotherapy positron emission tomography (PET) scan showed a small residual focus of uptake in the retroperitoneal mass and an unexpected focus in the left side of the neck. This was initially thought to represent residual active disease, but an ultrasound (US) scan and US-guided core biopsy of a cervical lymph node demonstrated metastatic papillary thyroid cancer rather than seminoma. A small (1 cm) primary papillary tumour in the thyroid was identified subsequently. The patient received consolidation radiotherapy to the retroperitoneum and underwent total thyroidectomy and neck dissection followed by radio-iodine treatment. He is currently in complete remission from both cancers.