DEAR DOCTOR K: I'm in my 30s. A friend of mine was recently diagnosed with testicular cancer. What should I know about this cancer? Should I be screened for it?

DEAR READER: Testicular cancer is the uncontrolled growth of abnormal cells in one or both testicles (testes). Nearly all testicular cancers start in germ cells. These are the cells that make sperm.

The testicles are located in the scrotum, behind the penis. In addition to sperm, they produce testosterone and other male hormones. (I've put an illustration of the male reproductive system on my website, Ask DoctorK.com.)

Most types of cancer affect older folks. But testicular cancer strikes young men, usually between the ages of 20 and 35 years.

The biggest risk factor for the disease is an undescended testicle. Before birth, a boy's testicles develop in the lower abdomen. They travel down into the scrotum during the seventh month of fetal development. A small percentage of boys are born with one testicle that failed to descend into the scrotum. Many descend on their own during infancy. If not, doctors can perform surgery to fix this problem. This surgery is usually done before age 2.

The brothers or sons of men with testicular cancer are at much higher risk for developing it themselves. I strongly advise any first-degree male relative of a patient with testicular cancer to: (1) examine themselves every several months; (2) have their doctor examine them at least every year.

The main symptom of testicular cancer is a gradual enlargement of the testicle. It loses its spongy texture and becomes abnormally firm. Some men notice a lump or mass in the testicle, which is usually painless. Some men experience a dull ache in the groin or lower abdomen.

Most of the time, men detect the mass or enlargement on their own. Or their sexual partner notices it. Unless you have a first-degree relative with testicular cancer, regular self-examination is not recommended. Neither is screening by doctors.

Nevertheless, I and many doctors regard examination of the testicles to be an appropriate part of a complete physical examination. At the same time, I have never discovered testicular cancer during a routine examination. The cases I've seen have been noticed first by the patients.

Once testicular cancer develops, it is likely to spread within or beyond the testicles if it is not treated.

Treatment depends on a person's overall health, the type of testicular cancer and its stage. (The stage is a measure of how far the cancer has spread.) If the cancer is caught early, surgery to remove the testicle may be enough. In other cases, additional treatment with radiation therapy or chemotherapy may be needed.

Today, testicular cancer is one of the most curable cancers. But treatment can cause side effects. Fertility is a common concern, because radiation therapy and chemotherapy can damage sperm. For this reason, men who undergo intensive therapy can store their sperm in a sperm bank before the treatment begins. In this way, they can father children in the future. Fortunately, nearly all survivors keep normal sexual function.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.