It can be hard to detect cancer in a lot of organs because they are inside the body, hidden from view. Testicles are another story because there are signs that show up on the outside. Men can become familiar with symptoms to stop testicular cancer from kicking them where it hurts.

The two testicles hang below the penis within the skin sac called the scrotum and make both sperm and male hormones. In adults they are usually almost the size of a golf ball and are connected to the penis with tubes. The American Cancer Society says cancer can form in any of the several different kinds of cells within the testicles, but in more than 90 percent of cases the cells affected are the germ cells, the ones responsible for making sperm.

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Early-stage testicular cancer could come with pain and discomfort in a testicle, a lump or swelling, or an aching in the lower abdomen, the U.S. Department of Health and Human Services says. Other symptoms that the Mayo Clinic adds are a feeling of heaviness in the scrotum or a collection of fluid there. Some men may also feel their breast tissue swell or become tender. If pain or swelling continues for more than two weeks in particular, men should see a doctor.

That organization says that testicular cancer, which often affects just one testicle, is rare compared to other forms of the disease, but it is the most common cancer for American males who are between 15 and 35 years old. And the American Cancer Society adds that the average age at diagnosis is 33: “This is largely a disease of young and middle-aged men.” It estimates the chances of developing the disease at about one in 263, although the rate has been increasing in the U.S.

Some are more at risk than others. Among the factors that increase a person’s risk of testicular cancer, the Mayo Clinic lists an undescended or abnormally developed testicle; family history of the disease; being a teenager or a younger man; and race — white men are more likely to develop testicular cancer than black men.

Survival rates are good because it is often treated successfully. The American Cancer Society puts a man’s lifetime risk of dying of the cancer at just one in 5,000.

See also:

The Evolution of Cancer

Men vs. Women: Surgeons Who Listen to Music