President Reagan's malignant colon polyp focuses national attention on a problem that many people -- and even some doctors -- deem too indelicate to discuss, much less to address diagnostically.

Yet at the moment, the only way to make any kind of significant inroads into the alarming death rate from colon cancer -- an estimated 51,600 will die from it this year -- is to diagnose it early. Once it has spread beyond the polyp in which it starts, nothing beyond surgery has been found to stem its progress, although promising experiments are under way. It responds poorly to radiation and almost not at all to chemotherapy. Yet found early, still within the polyp itself, it can be almost 100 percent cured by simply excising the growth.

"It is fairly well accepted that colon cancer begins in almost all circumstances as a benign polyp," says Dr. Arnold Levy, vice president for education of the area chapter of the American Digestive Diseases Society. "Except for a couple of relatively rare conditions -- inherited intestinal polyposis and ulcerative colitis -- as far as the general population is concerned, we're talking polyps" as the major source of colon cancer.

"We don't know what causes them, although there are all kinds of theories. But the feeling is, if you identify a polyp, it must be removed so that it does not have the opportunity to turn into something cancerous."

Colon cancer, combined with rectal cancer, will strike an estimated 138,000 people this year, afflicting almost equally blacks and whites, males and females.

Because it is rare among underdeveloped groups who live principally on low fat, high fiber diets -- consisting primarily of whole grains, fresh fruits and vegetables and little meat -- it has been hypothesized that lowering fat and raising fiber in diets can help prevent colorectal cancers. However, this has not been fully demonstrated.

The incidence of this cancer rises with age. Says Levy, "it begins to pick up at age 40 and really takes off after 50." According to the American Cancer Society, more than 94 percent of colorectal cancers are found in people over 50.

Men and women over 40 should be considered at moderate risk, and even without symptoms, everyone over 40 should undergo certain medical procedures every year.

People at any age with suspicious symptoms should see their physician at once, and may undergo one or more of a series of diagnostic techniques that, within the past decade, have been proving invaluable for detecting polyps and other intestinal conditions, often before cancer has a chance to spread or even develop.

These are the procedures:

* A digital examination. This is performed by a physician using a gloved finger and can reach 4 to 5 percent of polyps or cancers. Should be performed annually on everyone over 40.

* A routine sigmoidoscopy. The rigid sigmoidoscope is a a lighted tube about 25 centimeters (about 12 inches) long that is inserted into the colon. It can reach approximately 25 to 30 percent of cancers or polyps. Many internists and most gastroenterologists now use a flexible, fiber optic sigmoidoscope. It is as thin and flexible as cooked spaghetti, and permits a physician to reach 50 centimeters -- about 20 inches -- of the colon with better visual inspection. It is more comfortable for patients who have especially twisted colons. Sigmoidoscopy should be done regularly, although not necessarily annually, especially after age 50.

* A Hemoccult test. This is a card and three applicators, usually obtained from a physician. The card has spaces for a fecal sample every day for three consecutive days. It is returned to the physician or to a laboratory, where it is tested for traces of blood so minute they cannot be seen with the naked eye. The presence of blood -- or a "positive" reading from this test -- means that something is bleeding somewhere along the entire digestive system. It could be caused by bleeding gums, by eating red meat or other foods that can give a "false positive" reading, or by a stomach ulcer.

But it also would show bleeding in the colon, so a positive reading should tell the physician and patient to look further. "After all," says Levy, "wherever the bleeding is coming from, it shouldn't be there." Moreover, a Hemoccult positive reading followed by a negative test should not preclude a further investigation. Polyps or cancers often bleed only intermittently, and therefore might not turn up on any given day When results are negative, the test should be done at least annually.

(Tests that can be developed and read at home are available, but physicians are concerned that a mistake or misinterpretation by the patient could lead to a false negative reading and therefore a false sense of security.)

* The barium enema or air-contrast barium enema. In this procedure the colon is cleaned out with laxatives. Then, into the rectum, a small tube is inserted, through which the colon is filled with barium, a white material opaque to X-rays. Multiple X-ray pictures are taken in multiple di- rections. They shows only shadows where a polyp or some sort of growth is preventing the barium from filling the space. It is about 80 percent accurate in diagnosing polyps and cancers larger than five to 10 millimeters (about a quarter inch) -- very small, indeed. The air-contrast barium enema is, says Levy, "an excellent test in the hands of a good radiologist," because it lines the colon and discloses other conditions, such as colitis. This procedure is useful when the Hemoccult test is positive or when symptoms persist, and the other tests were negative.

* Colonoscopy. A relatively new, sophisticated and flexible fiber-optic instrument that permits the specialist to examine literally every square inch of the colon. "The visual optics are as good as any fine camera," Levy says, "and one can both see and photograph, take biopsy samples or actually perform a polyptectomy." That is the surgical procedure done with a wire lasso or snare that is inserted through the instrument. It excises and cauterizes accessible polyps at the same time. Should be performed whenever a polyp is found or when symptoms persist and other procedures are negative. The presence of one polyp, even if it turns out to be a type that rarely becomes cancerous, should suggest a search for more. As a rule of thumb, most gastroenterologists agree, if one polyp is there, another one or more is likely to be lurking nearby.

In addition to having the digital and Hemoccult test, and regular -- but less frequent -- sigmoidoscopic examinations, the following symptoms require prompt medical evaluation:

* Rectal bleeding. Of any amount or any color -- bright or dark. It may be a hemorrhoid or diverticulosis, a disorder involving pouching of the colon lining -- especially common in older people -- or it may mean something more ominous.

* Change in bowel habits. Anything, basically, different from normal for you in frequency, consistency or size. The most common problems are:

Diarrhea. Occasionally polyps secrete a lot of fluid, resulting in diarrhea. Bleeding may also produce diarrhea. Polyps or growths on the right side of the colon, where liquids enter directly from the small intestine, may cause no interference with normal bowel function, one reason why they can go undetected until they are quite large.

Constipation. If a polyp or growth is on the left side of the colon where solid foods pass, it can produce an obstruction. Constipation that continues after the diet is augmented with fiber and roughage, or where stools appear thin and narrow, should be brought to a physician's attention.

* Pain. There is sometimes cramping and abdominal pain, usually dull, that may persist. If it occurs along with change in bowel function, it is especially ominous.

More Information:

Write to the American Digestive Diseases Society Inc., 7720 Wisconsin Ave., Bethesda Md. 20814. Phone: 652-9293.