18F-FDG PET/CT Showing Rare Mediastinal Growing Teratoma Syndrome Following Chemotherapy.

Growing teratoma syndrome (GTS) is a condition in which poorly differentiated cells in a mixed-germ cell tumor (GCT) regress after chemotherapy, and the number of well-differentiated components increases. A 60-year-old man had an 8.0 cm mediastinal tumor with strong 18F-fluorodeoxyglucose (FDG) uptake [maximum standardized uptake value (SUVmax): 9.2], which was diagnosed as a GCT. After chemotherapy, serum alpha fetoprotein, beta-human chorionic gonadotropin, and tumor 18F-FDG uptake decreased (SUVmax: 3.9), but the tumor volume increased. The tumor was completely resected, and pathology confirmed the diagnosis of GTS. 18F-FDG positron emission tomography after chemotherapy reflects the proliferation of highly differentiated tumor components with poor 18F-FDG uptake.