Germ cell tumors: Most testicular cancers begin in a germ cell that produces sperm. There are two types of germ cell tumors (GCTs): seminomas and non-seminomas. Many testicular cancers have both seminoma and non-seminoma cells and are called mixed germ cell tumors.

Seminomas tend to grow slower than non-seminomas. It can increase blood levels of a protein called human chorionic gonadotropin (HCG). There are two subtypes:

• Classical (typical): More than 95% of seminomas are classical. These are mostly found in men between the ages of 25 and 45.
• Spermatocytic: This uncommon kind of seminoma is more common in older men. (About 65 years old is the average.) Spermatocytic tumors grow more slowly than classical seminomas and are less prone to spread to other body regions.

Non-seminomas begin in the cells which form sperm or eggs. It commonly occurs in men between their late teens and early 30s. Our main types of non-seminoma tumors include:

• Embryonal carcinoma: This type of cancer usually begins in the testicles, which are located in the scrotum. It tends to grow and spread quickly outside the testicle. It can raise blood levels of alpha-fetoprotein (AFP) as well as human chorionic gonadotropin (HCG).
• Yolk sac carcinoma: This rare type of cancer begins in germ cells that form sperms or eggs. It tends to grow and spread to other body parts if not treated properly. It always secretes alpha-fetoprotein (AFP), and it is the most commonly occurring tumor in children.
• Choriocarcinoma: This is a rare and aggressive form of testicular cancer in adults. It often spreads quickly to other body parts, like the bones, lungs, and brain. It increases blood levels of HCG (human chorionic gonadotropin).
• Teratomas: Under a microscope, these tumors look like the three layers of tissue in a growing embryo: the endoderm (innermost layer), mesoderm (middle layer), and ectoderm (outer layer). Teratomas do not increase the levels of AFP (alpha-fetoprotein) or HCG (human chorionic gonadotropin).

Stromal tumors can develop from the supportive tissues around the germ cells in the testicle, and these tumors are otherwise known as gonadal stromal tumors. The main types of stromal tumors include:

• Leydig cell tumors: These tumors start in Leydig cells that normally make male sex hormones (testosterone). It occurs in both children and adults. They are usually not benign.
• Sertoli cell tumors: These tumors start in normal Sertoli cells, which support and nourish the sperm-making germ cells. They are usually benign.

Secondary testicular tumors are cancers that begin in another organ and spread to the testicles. Lymphoma is the most common secondary testicular cancer, which is common in men older than 50. The testicles may also be affected by prostate, melanoma, kidney, and lung cancer.