Primary retroperitoneal teratoma in an adult is rare, as is the occurrence of a malignant tumor within a mature teratoma, known as malignant transformation. A 24-year-old woman was admitted to our hospital for investigation of an abdominal mass. Computed tomography and magnetic resonance imaging revealed a multilocular mass in the right upper abdomen. The tumor consisted of fat, soft tissue, and bone, with a slightly enhanced solid component. The tumor was diagnosed preoperatively as a retroperitoneal mature teratoma with an immature component, and excised. Histologically, it was composed mainly of mature fat, soft tissue, and bone, accompanied by a solid component of prostate-like tissue. In addition, a latent carcinoid tumor was recognized in the middle of the tumor. The tumor was finally diagnosed as a primary carcinoid tumor within the retroperitoneal mature teratoma. The patient has been followed-up for 24 months since her operation without any evidence of recurrence. We report this case to highlight the possibility of malignant transformation in adult retroperitoneal teratoma, even when the preoperative diagnosis is benign mature teratoma.
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