Medical experts interviewed yesterday about President Reagan's cancer agreed with his doctors' assessment that there is a better than 50-50 chance that his tumor is permanently cured, but added that the cancer's penetration into the muscle layer of the colon is a serious complication raising concern about his future health.

Some doctors interviewed expressed confidence that the president will finish his term without further evidence of cancer. "He has an excellent probability that he will do very well during the remainder of his term in office," said Dr. Donald A. O'Kieffe, a Washington gastroenterologist affiliated with George Washington University. "He is not likely to be disabled within the next three years."

Others had a gloomier view of the situation. "It's not such a great report," said one National Institutes of Health surgeon familiar with the case. "There's a 50 percent chance that the president is going to do well, but unfortunately a 50 percent chance that he is going to do poorly."

Several doctors said again yesterday that Reagan's prognosis might have been better if the tumor had been discovered and removed 14 months ago, when the first polyp was discovered in his colon. Doctors had earlier questioned why the president's physicians had waited so long to give him a thorough bowel examination that would have detected the growth, which must have been in his colon for several years.

Based on the pathological grading of Reagan's tumor, which has invaded the muscular middle layer of the large intestine's wall, the president has about a 50 to 75 percent chance of surviving the next five years with no recurrence of his cancer.

This would be considered almost equivalent to a permanent cure, since only 5 percent of cancers recur after five disease-free years.

There was a difference of opinion among those interviewed on how soon the cancer might recur, if it does do so. Eighty percent of recurrences show up within the first three years following surgery, according to Dr. Paul Sugarbaker, chief of the colorectal surgery branch of the National Cancer Institute. This indicates that if Reagan's tumor does return, it may well do so while he is still in office.

Dr. John L. Cameron, chief of surgery at Johns Hopkins University Hospital and chief of the medical school's department of surgery, said that in the cases in which the cancer recurs in another location in the body there is an "excellent chance" that this might not happen for four years or more.

Experts said the most disturbing aspect of the pathologists' findings was that the tumor had breached the "basement" membrane -- the barrier that divides the inner layer of the large intestine -- from the underlying layer containing muscle, connective tissue, blood vessels and lymph channels.

"It clearly has begun some phases of invasion," said a National Cancer Institute pathologist. He added that when a cancer penetrates below the basement membrane into the muscular part of the bowel wall, "it enters the realm where blood vessels and lymphatics are, and can spread further."

He said the fact that pathologists who examined slices of the growth saw no evidence of cancer cells within blood vessels or lymph channels was a good sign, but emphasized that the spread of tumor cells by those routes might still have occurred.

The pathologists' findings reflect only what they could see on selected cross-sections of the tumor that had been preserved and stained right after surgery, a process somewhat analogous to looking at still photographs of a moving animal.

Individual cells making up the tumor were described as "moderately well differentiated" by Dr. Steven Rosenberg, chief of surgery at the National Cancer Institute, at yesterday's briefing.

The National Cancer Institute pathologist interviewed said this, too, was a favorable sign. "If tumor cells look all regular and the same, and do not look bizarre, that's less aggressive . . . , whereas if they greatly vary in size and shape, . . . there's a poorer prognosis." The term "moderately well differentiated" means that the cells in Reagan's tumor were probably fairly uniform.

The estimate of a 50 to 75 percent chance of a cure for the president comes from studies based on the Dukes classification of colon cancer, a system devised in 1931 for grading malignancies according to how far they have spread beyond the innermost lining of the colon to involve other tissues.

The Dukes classification has proven to be the most reliable way of predicting recurrence of colon cancer.

The president's tumor is graded Dukes B -- more dangerous than a growth confined to the inner layer of the large intestine, but not as ominous as one that is found to have spread to lymph nodes outside the colon at the time of surgery.

Questions have been raised about whether or not President Reagan should have undergone more extensive tests of his colon after earlier polyps were detected. His first polyp, found to be non-cancerous, was found in May 1984. Later signs of blood in the stool and a second benign colon polyp were detected by March of this year.

From May last year to this past Friday, Reagan underwent tests that explored only the first two feet or so of his intestine. Some doctors believe that he should have received a barium enema X-ray or a colonoscopy, an examination of the entire six-foot colon, after the earlier polyps were discovered. Last Friday, the large tumor was found deep inside the intestines at the point where the large intestine meets the small intestine.

Yesterday, experts said the cancer would almost certainly have been seen earlier if the more complete tests had been done. But none could say whether the complete testing would have caught the cancer before it entered the wall or the muscle of the president's bowel.

"Sure, now that they know it is a cancer, it would have been a helluva lot better to have picked it up 14 months ago," said Dr. John L. Cameron, chief of surgery at Johns Hopkins University Hospital.

He said that it was also possible that if it had been detected four months ago the cancer might have been less severe. Cameron said there was "no defense" for not doing the more extensive exploration of the colon at least in March.

Sugarbaker said that colon cancers have been found to approximately double in size every two years -- a slow rate of growth, compared to many other malignancies. He said this meant that Reagan's tumor may have been 50 percent larger at the time of discovery than it would have been if found in May 1984 at the time his first polyp was discovered.

All of the doctors interviewed said there was no way to estimate how long ago the growth spread from the innermost layer of the colon into the muscle layer, converting itself from a highly curable tumor into one with a 25 to 50 percent chance of recurrence.

O'Kieffe, Cameron and other experts agreed with the judgment of Reagan's physician that no further therapy would be needed now, beyond the surgery he underwent Saturday.

"There is no adjuvant additional therapy known to be effective," said Cameron.

"If he was a 34- or 44-year-old man, we might suggest something . . . maybe a combination of radiotherapy and chemotherapy," said Sugarbaker. "But in a 74-year-old man it would just sap his energy . . . . On him or on any other 74-year-old, it just wouldn't make sense."

Rosenberg said yesterday that although no further treatment is planned, a variety of expert opinions would be sought by Reagan's doctors to make sure that all possible options were considered for the president.

One new treatment for colon cancer, discounted by Rosenberg as experimental and not applicable to Reagan's case, is the use of monoclonal antibodies, proteins that find and tag tumor cells so they can be destroyed by the body's own immune system.

Those interviewed said that the schedule of follow-up tests planned for the president is fairly standard for patients who have had tumors like his removed surgically. Those tests will include blood tests every two months, a repeat colonoscopy examination -- a look at the inside of the colon with a fiber-optic viewing instrument -- in six months, and periodic scans or X-rays of the liver and lungs to search for cancer spread.

Sugarbaker said the most important blood test to check will be the CEA, or carcinoembryonic antigen, a protein sometimes released into the blood by colon cancer cells. An elevated CEA level in the blood would probably be the earliest warning that cancer had appeared in the liver, the commonest site of recurrence, he said.

He said patients with this type of tumor should also have a CAT scan of the abdomen every six months -- a cross-sectional X-ray that shows the liver, spleen, bowel and other abdominal structures -- and yearly colonoscopy and chest X-rays. They should also be interviewed every few months by a doctor to detect any new symptoms.

Follow-up examinations will be important in Reagan's case not only to check for return of the cancer removed Saturday but also to look for new polyps that might in time develop into other cancers. Reagan's risk of forming additional polyps may be "as high as 30 percent," according to Dr. Kenneth A. Forde, chief of surgical endoscopy at Columbia-Presbyterian Hospital in New York.