Testicular Seminoma is a slow-growing form of testicular cancer usually found in men in their 30s and 40s. The cancer is usually just in the testes, but it can spread to the lymph nodes. Seminomas are very sensitive to radiation therapy. Testicular cancer is cancer that starts in the testicles, the male reproductive glands located in the scrotum. Causes, incidence, and risk factors The exact cause of testicular cancer is unknown. There is no link between vasectomy and testicular cancer. Factors that may increase a man's risk for testicular cancer include: Abnormal testicle development History of testicular cancer History of undescended testicle Klinefelter syndrome Other possible causes include exposure to certain chemicals and HIV infection. A family history of testicular cancer may also increase risk. Testicular cancer is the most common form of cancer in men between the ages of 15 and 35. It can occur in older men, and rarely, in younger boys.