Seminoma

Seminoma is a cancer of the testicles which arise from the cells that line the tubules that carry semen. They are generally found as a lump or mass on one of the testicles. They usually develop on young men aged 15-35. Although they are malignant, they are generally easy to treat and have a good prognosis. 
A common treatment is removal of the affected testicle. As a rule, this does not affect fertility or testosterone production as the remaining testicle increases production of testosterone. However, radiation therapy and chemotherapy are also commonly used and are usually successful. 
Seminoma at Wikipedia