The testicles (also called the testes; a single testicle is called a testis) are a part of the male reproductive system. These 2 organs, each normally somewhat smaller than a golf ball in adult males, are contained within a sac of skin called the scrotum, which hangs beneath the base of the penis.

The testicles manufacture the male hormone testosterone. They also produce sperm. Sperm cells are carried from the testicle through the vas deferens to the seminal vesicles, where they are mixed with fluid produced by the prostate gland. During ejaculation, sperm cells, seminal vesicle fluid, and prostatic fluid enter the urethra, the tube in the center of the penis through which both urine and semen are passed.

The testicles have several types of cells, each of which may develop into one or more types of cancer. It is important to distinguish these types of cancers from one another because they differ in the ways they are treated and in their prognosis (the course of the disease and the outlook for survival).

Germ Cell Tumors

More than 90% of cancers of the testicle develop in special cells known as germ cells. These are the cells that produce sperm. Two main types of germ cell tumors (GCTs) occur in men: seminomas and nonseminomas. Seminomas and nonseminomas cells look very different when seen under a microscope.

In the past, these 2 types occurred about equally. Now, seminomas may be slightly more common. Some cancers contain both nonseminoma and seminoma cells. These are treated as nonseminomas because they grow and spread like nonseminomas.

Seminomas

Seminomas develop from the sperm-producing germ cells of the testicle. The 2 main subtypes of these tumors are classical (or typical) seminomas and spermatocytic seminomas. Doctors can tell them apart by how they look under the microscope. More than 95% of seminomas are classical. These usually occur in men when they are between their late 30s and early 50s.

Spermatocytic seminoma tends to occur in older men. The average age of men diagnosed with spermatocytic seminoma is about 55, which is 10 to 15 years older than the average age of men with classical seminomas. Spermatocytic tumors tend to grow more slowly and are less likely to spread to other parts of the body than classical seminomas.

Nonseminomas

This type of germ cell tumor usually occurs in men between their late teens and early 40s. There are 4 main types of nonseminoma tumors: embryonal carcinoma, yolk sac carcinoma, choriocarcinoma, and teratoma. Most tumors are mixed with at least 2 different types, but this does not change treatment. All nonseminoma germ cell cancers are treated the same way.

• Embryonal carcinoma: This type of nonseminoma germ cell cancer is present in about 40% of testicular tumors. Pure embryonal carcinomas occur only 3% to 4% of the time. When seen under a microscope, these tumors can look like tissues of very early embryos. This type of nonseminoma tends to grow rapidly and spread outside the testicle. 
• Yolk sac carcinomas: These are so named because their cells look like the yolk sac of an early human embryo. Other names for this cancer include endodermal sinus tumors, infantile embryonal carcinoma, or orchidoblastoma. Yolk sac carcinoma is the most common form of testicular cancer in children. When they occur in children, these tumors usually are treated successfully. When yolk sac tumors develop in adults, however, they are of more concern, especially if they are "pure" (that is, the tumor does not contain other types of nonseminoma cells). Yolk sac carcinomas respond very well to chemotherapy, even if they have spread. This type of tumor releases a protein into the bloodstream known as alpha-fetoprotein (AFP). The presence of AFP helps confirm the diagnosis and is used to track the patient’s response to treatment.
• Choriocarcinomas: This is a very rare and aggressive type of testicular cancer that occurs in adults. Such cancers are likely to spread rapidly to distant organs of the body, including the lungs, bone, and brain. Pure choriocarcinoma does not often occur in the testicles. More often, choriocarcinoma cells are present with other types of nonseminoma cells in a mixed germ cell tumor. This type of tumor produces a protein, human chorionic gonadotropin (HCG), which can be used to confirm diagnosis and to track the patient’s response to treatment.. 
• Teratomas: Teratomas are germ cell tumors with areas that, when seen under the microscope, look like each of the 3 layers of a developing embryo: the endoderm (innermost layer), mesoderm (middle layer), and ectoderm (outer layer). The 3 main types of these tumors are the mature teratoma, immature teratoma, and teratoma with malignant transformation.

Mature teratomas are tumors formed by cells similar to cells of adult tissues. They rarely spread to nearby tissues and distant parts of the body. They can usually be cured with surgery.

Sometimes deposits of mature teratoma are found after chemotherapy to treat a nonseminomatous mixed germ cell tumor is finished. These may be the part of a tumor that was left behind after chemotherapy has killed the other components of the tumors. Some experts believe that chemotherapy can change other types of nonseminoma into teratoma.

Immature teratomas are less well-developed cancers with cells that look like those of an early embryo. Unlike mature teratomas, this type is more likely to grow into surrounding tissues (invade) and to spread outside the testicle (metastasize). Also, this type can sometimes recur (come back) years after treatment.

Teratoma with malignant transformation is a very rare cancer. These cancers have some areas that look like mature teratomas but have other areas where the cells have become a type of cancer that develops outside of the testicle, in tissues such as muscles, glands of the lungs or intestines, or the brain.

Carcinoma in situ

Testicular germ cell cancers may begin as a noninvasive form of the disease called carcinoma in situ (CIS) or intratubular germ cell neoplasia. Carcinoma-in-situ may not always progress to cancer. Researchers have estimated that it can take about 5 years for CIS to progress to the invasive form of germ cell cancer. It is hard to find carcinoma-in-situ before it develops into cancer because it generally causes no symptoms and often does not form a lump that you or the doctor can feel. The only way to diagnose testicular carcinoma in situ is to have a biopsy. Some cases are found incidentally (by accident) in men who have a testicular biopsy for some other reason, such as infertility. Experts don't agree about the best treatment for CIS. Since carcinoma-in-situ doesn't always become an invasive cancer, many doctors in this country consider observation (watchful waiting) to be the best treatment option.

When a testicular tumor like CIS becomes invasive, its cells are no longer just in the seminiferous tubules (where sperm cells are formed) but have grown into other structures of the testicle. These cancer cells can then spread through either the blood circulation or the lymph nodes (small, bean-shaped collections of white blood cells that fight infection) and lymphatic channels (fluid-filled vessels that connect the series of lymph nodes) to other parts of the body.

Stromal Tumors

Tumors can also develop in the supportive and hormone-producing tissues, or stroma, of the testicles. These tumors are known as gonadal stromal tumors. They make up less than 4% of adult testicle tumors but up to 20% of childhood testicular tumors. The 2 main types are Leydig cell tumors and Sertoli cell tumors.

• Leydig cell tumors: These tumors develop from the Leydig cells in the testicle that normally produce male sex hormones (androgens like testosterone). Leydig cell tumors develop in both adults (75% of cases) and children (25% of cases). They often produce androgens but sometimes produce estrogens (female sex hormones). Most Leydig cell tumors do not spread beyond the testicle and are cured with surgery. Sometimes, however, these tumors do spread to other parts of the body. If they do metastasize, Leydig cell tumors have a poor prognosis because they do not respond well to chemotherapy or radiation therapy. 
• Sertoli cell tumors: These tumors develop from normal Sertoli cells, which support and nourish the sperm-producing germ cells. Like the Leydig cell tumors, they are usually benign; however, if they spread, they tend to be resistant to chemotherapy and radiation therapy.

Secondary Testicular Tumors

Secondary testicular tumors are those that start in another organ and then spread to the testicle. Lymphoma is the most common secondary testicular cancer. Testicular lymphoma is more common than primary testicular tumors in men older than 50. Their prognosis depends on the type and stage of lymphoma. The usual treatment is surgical removal, followed by radiation and/or chemotherapy. In children with acute leukemia, the leukemia cells can sometimes form a tumor in the testicle.

Cancers of the prostate, lung, skin (melanoma), kidney, and other organs also can spread to the testicles. The prognosis for these cancers is usually poor because these cancers generally have spread widely to other organs as well. Treatment depends on the specific type of cancer.