When a tiny shadow turned up on Nancy Reagan's mammogram, nearly everyone understood that it raised the specter of breast cancer. A person on the street could tick off the next steps as well as any doctor: biopsy, a choice of surgeries, examination of the lymph nodes, and possibly radiation and follow-up chemotherapy.

There was even a public debate over whether, in deciding on a mastectomy, Mrs. Reagan made the right choice.

Such a debate, however, would never have occurred if the patient had been not the first lady but the president, and the disease had been not breast cancer but prostate cancer. The general public knows almost nothing about prostate cancer.

Yet in many ways "prostate cancer is very similar to breast cancer in women," said Dr. John Lynch, a urologist at Georgetown University School of Medicine.

According to the American Cancer Society, 96,000 cases of prostate cancer and 130,000 cases of breast cancer will be diagnosed this year. Both diseases are lethal: Prostate cancer will kill about 26,000 men this year; breast cancer will kill 41,000 women and 300 men.

Breast cancer tends to strike a little earlier in life. The median age of women is 60.3. The median age for men when prostate cancer strikes is 72.3. The median age of death is 63.2 for breast cancer and 76.5 for prostate cancer.

Both can be cured if caught early enough: 100 percent five-year survival for non-invasive breast cancer and 83 percent for prostate cancer, the cancer society reported. Once they spread, however, both cancers are more difficult to control. Overall survival rates for all stages of their respective cancers are 90 percent for women and 70 percent for men.

If the two cancers are relatively comparable, however, the attention paid to them is not. Breast cancer gets far more attention from the public, the research community and a medical community undecided about which is the best way to treat the disease of the prostate. The question is why.

"Men can't see it, for one thing," said Dr. William Fair, a urologist and cancer specialist at Memorial Sloan-Kettering Cancer Center in New York City. The breast is visible; it has cosmetic importance. The prostate is a walnut-sized gland tucked deep in the pelvis around the neck of the bladder. Its primary function is to produce semen. It cannot be seen or easily felt. A screening exam is more unpleasant than the mammogram exam for the breast.

And there hasn't been a Betty Ford or a Happy Rockefeller or a Nancy Reagan to stand up and take prostate cancer out of the closet for a careful, and very public, inspection.

"We still wouldn't be talking about breast cancer if it wasn't for Betty Ford," said Rose Kushner, a lay advocate for changes in cancer therapies, especially breast cancer, and former member of the National Cancer Institute's national advisory board. "Men need a celebrity to stand up and say he has prostate cancer and then talk about it."

So far, that has not happened, even though Supreme Court Justice Harry A. Blackmun and retired justice Lewis F. Powell Jr. have had prostate cancer, and other public figures -- including President Reagan and Richard Nixon and Supreme Court Justices Thurgood Marshall and William J. Brennan Jr. -- have had non-life-threatening but uncomfortable prostate enlargements. Yet many men barely know they have a prostate, let alone where it is or what it does. And almost none know how to figure out when something goes wrong.

"Perhaps the public relations for prostate cancer has not been as good as it has been for breast cancer," offered Dr. Willet F. Whitmore Jr., a Sloan-Kettering urologist, during a National Institutes of Health consensus conference on prostate cancer during the summer.

There may be a more fundamental cultural problem, Kushner said: "Nobody pays attention to men's health," and men themselves "don't want to talk about it."

"Women pay more attention to their health than men do," agreed Georgetown's Lynch. "I see a lot of men who say they came in because their wife wanted them to have something examined or checked."

But it is something men need to start thinking about, especially as they get older. Only about 1 percent of prostate cancers turn up in men younger than 50, according to published studies. The lifetime probability of developing prostate cancer is 5.2 percent for a white man and 9.6 percent for a black man. The probability of dying from prostate cancer is 1.4 percent for white men and 3 percent for black men.

But the statistics also show that the number of cases riser steeply after age 50. About 30 percent of men older than 50 have prostate cancer, said Dr. Thomas Stamey, chairman of urology at Stanford University School of Medicine. "That would be a public health epidemic that would make AIDS look like an ice cream party if it were not for the fact that only one in 100 of the 30 percent of men over 50 will ever by bothered by their prostate cancer. Only one in 300 will actually die of it."

And the incidence of prostate cancer has been on the rise. From 1937 to 1977, the prostate cancer rate rose 53.3 percent for white men and 151.6 percent for black men. By the year 2010, the increase in the number of older Americans will drive up the number of breast cancers by 48 percent and prostate cancers by 40 percent, said Curtis Mettlin, an epidemiologist at Roswell Park Memorial Institute in Buffalo, N.Y.

Although prostate cancer clearly is associated with increasing age, the specific cause remains a mystery. It may be a disease of the fifties and sixties because of the male climacteric -- the male equivalent of menopause -- a change in hormones, especially the ratio of the male androgens to female estrogens, said Dr. William Gardner, chairman of pathology at the University of Southern Alabama. "This hormonal status may be what ultimately initiates prostate cancer." It also has been linked to genes, the environment and diet.

There is a growing perception among some physicians, but no solid data, that more and more younger men, men in their forties and even a rare man in his thirties, have been turning up with prostate cancer. "That may be because we are looking for it in the younger patients," Fair said. "You only find what you look for."

Once it is found, disease in men so young must be treated. Even if the cancer is cured, treatment can have a significant impact on the quality of life. Until recently, surgically removing the diseased prostate left nearly every man impotent and a small percentage of them incontinent. The trade-off was deemed acceptable: It was better to be alive and impotent than to preserve potency at the risk of death.

"The idea that impotence is not really a problem because it attacks elderly men goes back to when 65 was old," Kushner said. "Sixty-five is not old anymore. I don't want my husband to be elderly at 65. {South Carolina Republican Senator} Strom Thurmond sired four children in his seventies.

"We women have a vested interest in keeping our men potent," Kushner said. "My husband has a prostate, and I want him to keep it." Tough Choices Men Face

"It was the most shocking news I ever had," said 49-year-old David Brock of Asheville, N.C., recalling his reaction when told two years ago that he had prostate cancer. "It really knocks you on your butt."

At age 47, Brock had never married but had two regular female companions. Sex was an issue. "I was concerned that I needed to have surgery at all," he said. "I knew what that meant. If you take out the prostate, you are impotent. It was like a one-two punch."

When researchers ask prostate cancer patients what they think are important considerations in their treatment, "survival is right up there," said Dr. Harry Herr, also of Sloan-Kettering. "But there are a lot of things that are a very close second: sexual function, urinary and bowel function, pain-free existence. All are very close seconds and thirds."

There are two main treatment options for prostate cancer that has not spread: surgery or radiation. If the cancer has spread, hormone therapy may help, but no effective course of chemotherapy has yet been devised.

The surgical approach removes the entire prostate. If the tumor has not spread, the patient is cured.

Men, however, tended to shy away from surgery. Until recently, the surgeon always destroyed the nerves that control erection during the procedure, leaving the patient impotent. A small percentage of patients also lost control of their urine flow and a few lost bowel control.

"Surgeons tend to emphasize the survival and minimize the trade-offs," Herr said. "We have tried to balance the decision of what patients get with survival versus the price they are going to pay for that survival."

In the last few years, however, it has become possible to dramatically lessen the consequences of surgery. Dr. Patrick Walsh, head of the urology at Johns Hopkins Medical Institutions in Baltimore, discovered the location of the nerves that control erection and devised a new surgical procedure to remove the prostate while leaving those nerves intact.

"People forget that there is a lot of anatomy yet to be learned," Walsh said. He made his critical anatomical discoveries in 1981, in the lab of a retired chief of urology in the Netherlands who was studying stillborn male infants because the nerves were easier to see.

"At that time, I realized I had seen these nerves before," Walsh said. "You only see what you look for and you only look for what you know." Once the nerve fibers were identified, Walsh figured out how to protect them.

Among the nearly 500 men Walsh has operated on since April, 1982 -- mostly young men with their disease detected early so it was still confined to the prostate -- 75 percent are potent one year after surgery. More than 90 percent are potent at one year if their cancer is in the earliest stages and the surgery does not have to be too extensive.

But the Walsh technique is not available everywhere. "In a lot of places, surgeons still don't use it," said Dr. Andrew Dorr, coordinator of prostate studies at the National Cancer Institute. "but surgeons are learning it."

"Before Walsh," said Georgetown's Lynch, "men were choosing not to have the prostatectomy because of the impotency problem and the fear of incontinence."

Statistics bear that out. A study for the American College of Surgeons on the treatment of prostate cancer found that only 16.4 percent of all patients with the disease had surgery with or without other therapies in 1974. That number dropped to 9 percent in 1983.

Radiation therapy, meanwhile, grew in popularity. It was used to treat 12 percent of the patients in 1974 but climbed to 26.8 percent nine years later.

The rest of the patients either had their cancer discovered while undergoing transurethral resection, a procedure used to treat enlargement of the prostate in which a catheter is placed into the penis and through the urethral wall to scrape out excessive tissue, or were treated with hormones for prostate cancer that already had spread to other parts of the body.

There are two kinds of radiation therapy: Either a beam of radiation blasts through the body to kill the tumor, or seeds of radioactive iodine are implanted into the gland. Both forms of radiation preserve sexual function two thirds of the time.

As in surgery, however, "radiation can cause impotency," said Sloan-Kettering's Fair, himself a surgeon. "With the nerve-sparing techniques, two thirds of the patients will retain their potency. With external beam therapy, the potency rate is about the same. That has minimized the difference {in terms of side effects} between the two."

And there is a great deal of debate about whether radiation kills all the cancer cells or whether some cells survive.

That may not matter, because in terms of survival for patients with a tumor that has not spread beyond the prostate, surgery and radiation therapy give equal results for 10-year survival, a National Institutes of Health consensus conference on prostate cancer concluded this summer.

This finding has changed the script for prostate cancer patients, said Dr. Vincent T. DeVita, director of the National Cancer Institute. "A patient can now go to any doctor and say I understand that there is no difference {between the two therapies}." The patient can then quiz his doctor on why one approach would be better for him than the other. In the past, that often was not possible because physicians had strong feelings about which they thought was better. The consensus panel, DeVita said, made it clear there is no difference in survival.

There is a third group: patients who should not be treated at all with surgery or radiation either because they are so old or because the cancer already has spread.

"No treatment at all basically depends on the age of the patient and the extent of the disease," said Sloan-Kettering's Fair. "For a man over 75 or so, if he has a small cancer of the prostate, the likelihood that he will die of prostate cancer is small. It would be inappropriate to put him through a big surgery to cure the prostate cancer when the most likely thing that is going to kill him is a heart attack."

While the scientists debate techniques and statistics, humans with prostate cancer have to make decisions.

"The most frustrating thing was finding good advice," said Brock, a former civilian worker with the Department of Defense, now retired. He went for a second opinion. The diagnosis was confirmed and surgery recommended.

That's when the all-too-frequent random event occurred. A friend gave Brock a copy of a Washingtonian magazine with an article listing the region's "best doctors." Patrick Walsh was down for prostate surgery.

Brock called the reporter, John Pekkanen.

"Would you go to Walsh?" Brock asked.

"Yes, he's the best doctor for this in the area," Pekkanen replied.

"That pretty well cinched it for me," Brock said. In October 1985, five months after Brock's cancer was found, Walsh opened Brock's pelvis and took out his prostate.

The Prostate Operation

The scalpel slices from the pubic bone to the belly button. A vise-like device spreads open the 8-inch cut and a huge curved metal spoon-like device is attached to hold the intestines back out of the way. There, deep within the pelvic cavity, the bladder and the prostate glisten beneath the operating room lamps.

Today, it's patient number 483, Julie Lipper, a 58-year-old New Jersey importer of brass machine parts from Germany. He's awake, though groggy. A "spinal," the kind of anesthesia frequently used for women undergoing a cesarean birth, numbs Lipper from the chest down.

"I still remember bits and pieces of it," Lipper recalled later. "I didn't feel anything as far as pain was concerned. But I could feel them pulling and tugging. I could tell they were working on me, but no pain whatsoever. I remember hearing Dr. Walsh talking. He seemed to talk incessantly. I have been told that he tends to jabber away."

Walsh does talk a lot. Head down. Hands moving crisply with grace and economy. With the wrist flicks of a conductor, he directs the work in the crowded space of the pelvis as instruments shuttle across the fields of sterile blue and bright red. Things seem to move quickly; surgery lasts 2 hours, 15 minutes.

Today there is a large audience in the operating room. Besides a reporter, there is a Scandinavian surgeon who will take the procedure back to his country, a surgical resident and a medical student. Walsh provides a running commentary on each step.

First, the four lymph nodes that filter fluids draining from the pelvis are removed and sent to the pathology lab for inspection. If they contain cancer cells, the seeds of Lipper's death already have been sown.

Blood vessels are marked to control bleeding, and the prostate is freed from nearby tissues.

The news from the lab is good: no spread to the lymph nodes. But the news here in the O.R. is troubling: The tumor has engulfed most of the right side of Lipper's prostate, so the nerves on the right side will have to be removed to ensure that all the cancer is taken out.

An erection is triggered by a pair of nerves that control the blood flow to the spongy tissue inside the penis. "You only need to preserve the nerves on one side to preserve potency," Walsh said.

He leaves the left nerve bundle intact, but the purplish prostate gland comes out. Walsh holds up the gland for gloved hands to feel the hard tumor filling half the volume. A few months longer and the cancer might have escaped.

The scene in the family waiting room is familiar. Yes, he's fine. The surgery went well. The cancer is out. He'll recover quickly. Hugs, tears from wife and son. Some small talk. Walsh leaves smiling, racing off to dictate the notes of the operation.

Lipper knows about the pain of first getting out of bed the day after abdominal surgery and walking down the hall with intravenous bottle and catheter bag in tow, but the surgery has been too recent to know whether his sexual function was preserved. It takes many months to a year before the ability to have an erection returns.

But Brock, who had his surgery two years ago, said he is able to have erections again. Walsh says he is a typical case.

"I recovered from the operation quite well," Brock said. For the first four to five months, although he had no erections, he had "semi-erections."

"Walsh kept encouraging me and telling me it would come back. After six or seven months, I had a full erection. Now, I can have an erection and I can have normal sex. You do have orgasms. You still have the good feeling, but it is not as great as it was before."

Without a prostate gland, there is no ejaculate. There is also no sperm because the conduit from the testes must be cut to remove the prostate.

Brock remains philosophical. "It is better than having nothing. Some of the guys who have this operation walk away and have nothing."

Lipper is hopeful. So is his wife. "Yeah, potency was part of the consideration," Lipper said. "We have had a good life and a good, active sexual life. If there is a way to continue that, then that is the way to go."

"It was the quality of life that you hope to enjoy for a long time," agreed his wife Zeda.

Walsh says his goals are to cure the patient, preserve potency and preserve bladder control. "If you can get all three," Lipper asked, "why not? If I had a choice to make, I would get rid of the cancer."

Controversies and Consequences

Just as his patients have had to struggle with their decision, Walsh has had to struggle with controversy about his surgical approach.

"Some said these patients can't be potent," Walsh said. "Or, if they are, the whole prostate can't be removed." Other critics fear that because surrounding tissue is left behind, some cancer may be left behind, too.

After five years, there has been no sign of that, Walsh said.

Walsh isn't the only one in the fray. Radiation therapists and surgeons have been fighting with each other for years about whose treatment of prostate cancer gives the best results.

"If there ever was a field with conflict, it is prostate cancer," said Dr. Gerald P. Murphy, a urologist at the State University of New York at Buffalo.

After two days of arguing over the available scientific studies, the NIH consensus panel on therapy for early prostate cancer could only conclude that both treatments provide essentially equal survival for a decade.

Physicians in the community tend to recommend what they know: When the cancer is confined to the prostate, surgeons operate and radiation therapists treat with radiation.

"There always will be a pluralism in American medicine," Murphy said. "The censensus conference showed that there isn't a consensus. That probably is an accurate reflection of the experts at this time."

The absence of a consensus and the low profile of prostate cancer itself have other consequences for both the public and the medical community.

The public, for example, called the National Cancer Institute's information network only 23,819 times for prostate cancer information from January 1983 to December 1986. There were 150,813 calls for breast cancer information during the same period.

Just as ignorance prevents men from getting the physical exams needed to diagnose the disease, it also blocks the groundswell of political pressure needed to push for the kinds of funding for research advancements and changes in treatment that occurred in the treatment of breast cancer in the last decade.

"There are half a dozen organizations devoted to women's breast cancer issues," Kushner said. "You men are doing nothing" comparable for prostate cancer. "Men are really so far behind us, and your budget {for prostate cancer research} is smaller. I feel that women should be sensitized to the fact that this is something that we must do because men won't do it."

There is only one grass-roots support group for prostate cancer patients; it began accepting members three months ago: Patient Advocates for Advanced Cancer Treatments, in Grand Rapids, Mich. "This is a patient-oriented organization," said founder Lloyd J. Ney, himself a prostate cancer patient. "We feel as patients that we have the right to be heard and we have the right to participate in what is going on in the field of prostate cancer."

The lack of political clout clearly shows up in the bottom line. In fiscal year 1986, the latest year for which statistics are available, the National Cancer Institute, the main source of cancer research funding, spent 5 1/2 times as much for breast cancer research than it did for prostate cancer research: $51.6 million versus $9.2 million.

That difference shows up very directly in the amount of research performed as measured by the number of publications in the medical literature. For example, the National Library of Medicine's computerized data base of the medical literature, Medline, recorded 848 English language publications on prostate cancer between January 1985 and July 1987. For breast cancer, Medline records 3 1/2 times as many studies, more than 3,000 scientific reports in all.

"Prostate cancer is just understudied compared to breast cancer," said NCI's Dorr. "But it is a big disease with high incidence that could, with the increasing age of the population, become an epidemic."

And it is in prostate cancer that medicine can best be seen as an art, said Buffalo's Murphy. "We really do not have a natural history of the disease, we have a therapeutic history. We have questions of what the therapy should be and when it should be used. We have differences of opinion. We have new technology. And we don't have all the answers. But out of all of this, we have hope." PROSTATE CANCER: THE THERAPY OPTIONS

Normal Anatomy

The prostate gland plays a central role in the male reproductive system. Located at the base of the bladder, it produces most of the material contained in semen. During an annual exam recommended for men over 40, the prostate can be felt through the rectum wall by a physician, and enlargement or a tumor can be detected. Ultrasound and new blood tests also can help detect prostate cancer.

The Surgical Option

Traditionally, surgeons simply cut out the prostate gland and the surrounding structures, including the vein complex above the prostate, and usually damaged the neurovascular bundle, a collection of nerves that control erection. More recently, a surgical techique has been developed to remove the prostate while preserving these nerves, thereby preserving potency.

The Radiation Therapy Option

Prostate cancer can also be treated with either several sessions of focused radiation or with seeds of radioactive iodine implanted into the prostate itself. Radiation therapy has been judged to be as effective as surgery in prolonging life and generally is able to preserve potency. It does not, however, always kill all of the cancer cells.

How the Cancer Spreads

Prostate cancer tends to grow slowly. When the tumor reaches more than 4 centimeters, however, the chances of its escaping the prostate and spreading to other parts of the body is high. Once it spreads, the cancer can seldom be cured. Hormone treatments have been used to help preserve life and lessen the pain of prostate cells growing in the bones.