A malignant retroperitoneal mass--a rare presentation of recurrent thymoma.

A 60-year-old man presented with a suspected retroperitoneal mass, after primarily resected thymoma (type B1/B2, Masaoke stage 1). A germ cell tumour was excluded and a diagnostic biopsy was performed. The mass appeared to be a local recurrence of the primary thymoma, for example, a droplet metastasis, progressed to type B3.