Thymoma is a rare malignancy of unknown etiology. The author examined patterns in thymoma incidence in the US general population using data from Surveillance, Epidemiology, and End Results (SEER) cancer registries. Prior studies concerning the risk of additional malignancies in thymoma patients were reviewed. Based on cancer registry data, the overall incidence of thymoma in the US is 0.13 per 100,000 person-years. Thymoma is exceedingly uncommon in children and young adults, rises in incidence in middle age, and peaks in the seventh decade of life. Thymoma incidence is especially high among Asians and Pacific Islanders in the US. While several studies based at single treatment centers have suggested that thymoma patients have a broadly increased risk for other malignancies, follow up data from US cancer registries support a more limited spectrum of cancer risk. In particular, thymoma patients have a subsequently elevated risk for developing B-cell non-Hodgkin's lymphoma. Based on limited data, thymoma patients may also have an elevated risk for developing soft tissue sarcomas. Thymoma is a rare malignancy. The excess risk for non-Hodgkin's lymphoma is consistent with an effect of immune disturbance arising from the thymoma or its treatment. While descriptive epidemiologic data may yield clues to the etiology of thymoma, large multi-center case-control studies will be required to formally evaluate environmental and genetic risk factors.
