Testis cancer or germ cell tumor (GCT) of the testis is most often seen among males from birth to age 50. It is thought of as a young man's disease with most cases occurring from age 15-35 although other peaks are seen during infancy and the 6th decade of life. Approximately 8000 new cases of testis cancer are diagnosed in the US per year.

The incidence rate of new cancers appears to be increasing worldwide among Caucasian males but not among other racial groups. The ratio of African-American to Caucasian patients is about 1:5. Geographic variation in the occurrence of testis cancer has been noted with higher rates observed in Germany, Switzerland, Scandinavia, and New Zealand. Lowest rates are seen in Asia and Africa.

Risk Factors

Certain risk factors have been identified which may increase an individual's likelihood of developing testis cancer. Cryptorchidism (or undescended testis) can increase the risk of developing cancer in the affected testis as well as the opposite testis, even if it is normally descended. The degree to which surgical correction of cryptorchidism (called "orchidopexy") can decrease this risk has not been determined although there appears to be a protective effect when surgery is performed prior to puberty. Individuals who have had testis cancer in one testis have about a 5% chance of developing cancer in the opposite testis. All individuals who have a history of cryptorchidism or testis cancer require lifelong surveillance by examination.

Evidence for other risk factors has been lacking. Although patients often report a history of preceding testicular trauma, this event usually leads to self-examination and incidental discovery of a mass that is not associated with the trauma. HIV patients have a greater risk of developing other tumors such as lymphoma and sarcoma that can affect the testes, however, these patients' risk of GCT appears to be the same as seen in the overall population. Patients with Klinefelter's syndrome are at greater risk of forming GCT outside of the testes, notably inside the chest (mediastinum). Exposure of pregnant women to diethylstilbesterol (DES) appears to increase the incidence of testis cancer in their male children, however the effect of DES on adult men does not appear to increase the risk of developing testis cancer. Some have proposed that increased scrotal temperature, which may occur in those with a sedentary lifestyle, may be a putative factor in the formation of testicular neoplasia. Investigations thus far are inconclusive.