The growth removed from President Reagan's bowels may have been developing there for at least two years and perhaps as long as four or more years, one prominent medical specialist said yesterday.

The growth was about two inches across, said Dr. Arnold Levy, a Maryland gastroenterologist who is vice president for education of the American Digestive Disease Society here. Its large size for a growth of that type suggests it has been growing for some time, he said.

The intestinal growth is called a villous adenoma and is known as a "mixed" polyp, containing characteristics of two of the three types of tumors common to the intestinal tract.

The part of the growth called the villus is a finger-like projection from the intestinal wall, and is the most likely part of the mass to turn cancerous if any part does.

The adenoma or glandular polyp is somewhat less likely to become malignant, but still more so than the most common intestinal tumor, the inflammatory fibroid polyp, which "almost never" becomes malignant, Levy said.

According to a statistical analyis of all types of intestinal polyps, the size of the growth can determine a percentage chance of whether or not cancer is present, Levy said: "If a polyp is less than one centimeter in diameter, the chance of cancer is less than 1.5 percent. If the polyp is one to 1.4 cm, the chance of cancer is 2.4 percent. If it is greater than 1.5 cm, the chance of cancer is 9.6 percent or more, depending on the size."

The percentage continues to rise at larger sizes, and doctors estimated yesterday that up to 50 percent or more of tumors of the type and size of Reagan's are cancerous. However, in Reagan's case, other indicators so far show no malignancy.

Colon cancer (combined with rectal cancer) is second only to lung cancer as the most common malignancy in this country. It has a high mortality rate -- about 50 percent overall. Some 96,000 cases of colon cancer will be diagnosed this year, said Levy, and 60,000 deaths will occur from the disease.

"The key to controlling colon or rectal cancer is early detection," Levy said, "because once it has spread beyond the stalk of the polyp, it responds poorly to either chemotherapy or radiation."

On the other hand, if action is taken early, the patient is simply cured.

Levy described the villous adenoma as "looking rather like a head of cauliflower." Like other polyps, it may be "sessile" -- broad-based, like a small hump -- or "pedunculated" -- on a stem or stalk like a mushroom.

The decision by the president's surgeons to take a portion of the intestine on either side of the growth was made because his adenoma was broad-based, not susceptible to being nipped off by the colonoscopic snare inserted during Friday's operation.

Although a preliminary biopsy of Reagan's polyp found no trace of cancer, "you can't really tell until you get it under the microscope," Levy said.

A growth like the villous adenoma can sometimes be troublesome even if it is benign, he said.

"If it is on the left side of the intestine, where solid foods pass, it can obstruct the colon. When it is on the right side, in the cecum, as the president's was, it is less bothersome because that is where the liquids pass. That is why, sometimes, cancers in that area are detected later, because they don't usually present symptoms."

However, the presence of one polyp does suggest the presence of more, both at the time the polyp is discovered and in the future. "People who have polyps tend to be polyp formers," Levy said. "If you identify one, there is at least a 10 percent chance that there is another in the colon at that moment. And even if there is not, there probably will be a strong chance another will turn up later on."

If a final biopsy should indicate the presence of cancer, the specialists will determine its "stage" which, in turn, dictates treatment and suggests a prognosis.

If the cancer is limited to the polyp, the patient may be cured with its removal.

If it is limited to the bowel wall, five-year survival is estimated at about 80 percent. If it has penetrated the wall, the percentage drops to 75 percent, and if it is detected in the lymph nodes, five-year survival drops to about 58 percent.