Testicular cancer is a cancer that forms in tissues of one or both testicles. It is the most common cancer in men 20 to 35 years old. Almost all testicular cancers start in the germ cells. The two main types of testicular germ cell tumors are seminomas and nonseminomas. These 2 types grow and spread differently and are treated differently. Nonseminomas tend to grow and spread more quickly than seminomas. Seminomas are more sensitive to radiation.
A testicular tumor that contains both seminoma and nonseminoma cells is treated as a nonseminoma.