Cancer, testicles: Cancer of the male sex organ (testicle) that normally produces the hormone testosterone. One of the most common cancers in young men. Most testicular cancers are found by men themselves as a lump in the testicle. The risk of cancer of the testicles is increased in males whose testicles did not move down normally into the scrotum (holding sac for the testicles) during development if the problem is not corrected in early childhood. This condition is referred to as undescended testicles. When a growth in the testicle is detected, cancer is confirmed after surgical removal of the affected testicle (orchiectomy) and examination of the tissue under a microscope. Testicular cancer is almost always curable if it is found early.