More and more men, at an earlier and earlier age, are going to be confronted with a decision on how to be treated for prostate cancer.

The impression of several physicians at last week's National Institutes of Health consensus conference on the treatment of prostate cancer is that they are seeing more men in their forties and fifties with prostate cancer and even an occassional patient in his thirties.

In December, the National Cancer Institute reported that prostate cancer seems to be on the rise since the mid-1970s. No one knows why, but environmental factors, including perhaps the use of anabolic steroids by athletes, may be involved.

This year, 90,000 Americans will develop cancer of that uniquely male gland. More than 26,000 will die.

Prostate cancer is different from, and not associated with, benign prostate hypertrophy, an enlargement of the prostate gland with age that sometimes interferes with urination. Former president Richard Nixon, 74, had surgery for this condition last week, and in January, President Reagan had similar surgery, his second.

Despite its impact, prostate cancer is a disease men tend to know little about. The problem arises in part because prostate cancer is usually found in older men -- 80 percent of the victims are older than 65 -- and it often is not lethal. "Many more men die with prostate cancer that of it," said Dr. Willet F. Whitmore Jr., a surgeon at Memorial Sloan-Kettering Cancer Center in New York.

That may change. Many people believe that prostate cancer "is a disease of old men that doesn't kill people," said Dr. Mitchell C. Benson of Columbia-Presbyterian Medical Center in New York. "Nothing could be further from the truth."

Prostate cancer already is the second leading cause of cancer death in men. And if lung cancer rates come down and men live longer, prostate cancer could become the leading cancer killer.

For a small percentage of patients, 15 percent or so, there is a choice about how to be treated: prostate surgery or radiation therapy.

Surgery, especially for early stages of the disease, has clearly been shown to give a patient 15 years of disease-free existence, but it usually causes sexual impotence because it destroyes the nerves that control erection.

Radiation therapy is the alternative. A beam of high-energy X-rays passes through the body to destroy the cancer cells within the prostate. This treatment usually maintains sexual potency, but there is a chance that cancer cells can survive in the prostate and cause the disease to flare up again later.

The NIH consensus panel concluded that both surgery and radiation therapy provide essentially equivalent 10-year survival rates, but that the comparison for longer disease-free survival was less clear. Patients receiving radiation have not been followed as long as surgery patients. The panel recommended a large, long-term study to "evaluate both disease control and quality of life." Dr. Vincent T. DeVita Jr, NCI director, would like to see a long-term study done, especially if it tests additional therapies, such as hormone treatments.

Now that surgery and radiation therapy can be considered equivalent, the issue becomes the quality of life.

"Younger men are concerned about sexuality," said Shannon B. McGowan, a psychotherapist at the John Muir Community Hospital and Cancer Center in Point Richmond, Calif. There was wide agreement among the scientists that men in their sixties should be considered sexually young.

Whether treatment leaves a man impotent depends almost entirely on the stage of the disease and the therapy chosen. To some degree, radiation therapy always had the edge. While traditional prostate surgery nearly always causes impotency, radiation therapy doesn't.

"We are getting fewer complications than we did 10 or 15 years ago," said Dr. Malcolm A. Bagshaw, chairman of therapeutic radiology at Stanford University. More than 85 percent of the patients receiving radiation therapy for prostate cancer remained potent 15 months after treatment. More than half were potent seven years after treatment.

Surgeons, however, have begun to fight back. More and more urologic surgeons have been learning a relatively new surgical approach developed by Dr. Patrick Walsh at Johns Hopkins Hospital that avoids damage to the nerves that control erections. When used on 206 potent men with prostate cancer, 72 percent remained potent.

Although a few hundred urologists have attended seminars to learn Walsh's technique, it is not clear how widely available the approach has become. It also is not yet clear whether the chance of cancer recurrence is higher with the Walsh technique, since some tissue previously removed during prostate surgery remains within the body and could harbor cancer cells.

These options are available only during the early stages of the disease, when the cancer is confined to the prostate itself. Once the cancer grows beyond the confines of the prostate, surgery is not curative. Some 75 percent will die in five years once it spreads to the bone, said Dr. Richard G. Middleton, a surgeon at the University of Utah Medical Center in Salt Lake City. At some research centers, patients with widespread disease die in two years.

Radiation treatments to the lymph nodes and other parts of the body, such as the bone, become the major treatment after the cancer has spread. But by then, it has a much lower success rate. Drugs used to poison other cancers fail to kill prostate cancer because it is slow growing.

Slow growth, however, means that not every prostate cancer detected during a screening exam will produce a life-threatening disease. A large number of prostate tumors are detected during an autopsy after the man died of some other disease.

The problem is knowing which prostate cancers will grow aggressively and need to be treated, and which will not and can be safely left alone. Tests to make that determination are under development.