A devastating side effect of cancer treatment is finally getting attention from doctors and lawmakers, but many patients still aren't aware of the risk.

The problem is called lymphedema, a painful and debilitating swelling of an arm or leg that can occur months or even years after cancer treatment. In patients whose lymphatic systems have been damaged by radiation or the removal of lymph nodes, even minor overuse of a limb can trigger extreme swelling. Breast-cancer patients, who may have lost a dozen or more lymph nodes as part of their diagnosis and treatment, are particularly susceptible. But patients treated for prostate cancer, gynecological cancers, head and neck cancer, testicular cancer, bladder cancer, colon cancer and melanoma are also at risk.

If not treated swiftly, the condition can be irreversible. But the problem is, many patients don't even know about it. Doctors often fail to warn about lymphedema, either because they themselves aren't aware of it or because they don't want to scare patients. Some physicians even mistakenly prescribe diuretics to reduce swelling — a treatment that can actually exacerbate the disease.

Ignorance can have serious consequences. Simple acts like carrying a heavy bag, cutting a finger or working too long at the computer can trigger the swelling. The body's normal reaction to exertion or trauma is to increase the rate fluid flows through the affected area. The lymphatic system normally helps transport body fluids, but in someone with a compromised system, that can quickly lead to overload, causing buildup in an arm or leg, depending on where the lymph nodes were removed. The limb can swell far out of proportion to the rest of the body.

While data are scarce, studies so far show from 20 percent to 45 percent of patients who are at risk for lymphedema will eventually develop it. Treatment can last for a few weeks, or for the rest of a patient's life, and involves constant care, including physical-therapy treatments and the daily wearing of compression bandages.

"Patients usually are not educated in advance so they're not aware of some of the risk factors and the fact that they could take precautions," says Pam Massey, director of rehabilitation services at University of Texas M.D. Anderson Cancer Center in Houston. At-risk patients often are advised to avoid heavy lifting, among other precautions, and learn to recognize the symptoms so they can seek treatment quickly.