President Reagan's intestinal tumor, a large growth with a greater than 50 percent chance of malignancy, might have been found and removed 14 months ago if his entire colon had been examined when the first small growth was detected, medical experts said yesterday.

The doctors who removed the tumor yesterday defended the treatment Reagan received, saying that he was given standard care for the two small polyps found on his two most recent physical examinations and that there was no reason to check the entire large intestine for additional abnormalities.

Dr. Edward Cattau of Bethesda Naval Hospital, who found and treated the first polyp in May of last year, said that the small polyps found then and last March were not the type that would indicate cancerous growth elsewhere in the bowel. Therefore, the examination that found the tumor was not done until Friday.

Some experts in the diagnosis and treatment of intestinal-tract growth -- but not involved in the president's care -- questioned the timing of his treatment. In particular, they were puzzled at the delays in conducting some common, but crucial, follow-up tests to X-ray and visualize the entire colon to make sure other potentially dangerous growths were not hidden.

"There's no question in my mind that the sequence of events was not in his best medical interest," said Dr. Donald A. O'Kieffe, a Washington gastroenterologist and expert in colonoscopy who is affiliated with George Washington University. "Nothing was gained by waiting . . . . There is no real defense for the timing."

"The standard of practice suggests that a patient be given a barium enema" after a polyp is found to make sure no other polyps are present, O'Kieffe said. (Barium in the intestine heightens contrast in X-rays, sometimes enabling physicians to identify a polyp.)

"If that was negative, they would have made a later assessment to make sure nothing significant was missed. But the standard of practice in the community would have directed a more aggressive approach," O'Kieffe said.

He said that even if the barium enema had been negative, he would have advised another definitive test, a colonoscopy, within six months or so, to make virtually certain there were no polyps. Reagan underwent a colonoscopy Friday.

Cattau, who attended Reagan, said that "there certainly was a lot of consideration given to doing a colonoscopy then," but that doctors decided against it "because of the histological findings," which indicated a type of growth not expected to develop into cancer.

He said the standard of care recommended for people Reagan's age, 74, and without known risk factors for colon cancer includes yearly examination of the stool for blood and a sigmoidoscopy (examination of the tract) every three to five years. "Only if one of those two tests is positive is there an indication to look at the total colon," he said.

"The standard of care for you, for me, and for the president of the United States is just as I have explained it to you," Cattau said.

Dr. Steven Rosenberg, the chief of surgery at the National Cancer Institute who was called in to help with Reagan's operation yesterday, endorsed Cattau's treatment plan. Standard practices were followed in the president's case, and there were no indications for more frequent follow-up than he had, Rosenberg said.

Other experts in the field of gastrointestinal cancer surgery questioned that assessment yesterday.

"I would guess any kind of polyp would indicate a look at the colon," said Dr. John S. Najarian, chief of surgery at the University of Minnesota. "He's at the age of peak incidence for colon cancer, which is between 70 and 80 years."

Najarian said that when Reagan's first polyp was found, in May 1984, "you'd think that might have been a good time" to do tests -- a barium enema and colonoscopy -- to search the remainder of the six-foot-long intestine for other growths.

The small polyp discovered during a routine physical examination last March was biopsied at the time and was termed an "inflammatory pseudopolyp," a type not considered to have the potential of becoming malignant.

Both procedures were done via a flexible sigmoidoscope, which examines the portion of the large intestine extending about two feet from the rectum as far as the left side of the transverse colon.

The tumor found Friday was four feet farther along the intestine. But until yesterday, no tests were done to check the right half of Reagan's large intestine for similar growths.

Outside doctors said yesterday that regardless of what was done with the polyp found last year, their level of concern would have been heightened by the March findings of another polyp as well as hidden blood in the stool. Follow-up stool blood tests were reported last week as negative by White House spokesman Larry Speakes.

Nonetheless, the outside experts said they might have acted more vigorously after the March examination.

"I personally am puzzled," said Dr. Marshall Bedine, of the Johns Hopkins University Medical School. In terms of growth, he said, "a year could have made a fantastic difference. Four months still could have made a difference."

Bedine said that in general, he felt it would have been routine to use a barium-enema X-ray and perhaps a colonoscopy after the first polyp was discovered. "These are very routine procedures," he said.

Dr. Bergein Overholt, a Knoxville, Tenn., gastrointestinal expert and past president of the American Society for Gastrointestinal Endoscopy, said that he did not want to "second-guess advisers, both medical and political, of the president. Time may not have allowed him to have a colonoscopy."

Overholt, a developer of the colonoscope, said that the decision whether to use the instrument would have depended on what kind of polyp was found, since some types may be considered totally innocuous, but he said that "in the ideal situation, colonoscopy should have been done much earlier."

"But the circumstances surrounding the president are so complex, I would not criticize the care he received . . . . If it had been you, you'd have had it done a lot earlier," he said.

However, some doctors not connected with the case have expressed the opinion that even a single positive stool test should have prompted immediate further evaluation of the colon in March, rather than delaying colonoscopy until now.

Other experts interviewed said it was difficult to assess whether tests such as barium enema or colonoscopy should have been done sooner without more information on the exact cells present in the two known polyps. But all said that the growth removed yesterday, because of its size, undoubtedly had been present for years and would have been discovered last year if the tests had been done.

During the postoperative press briefing, Dr. Dale Oller, chief surgeon on the case, was asked, "Do you wish you had done a colonoscopy 14 months ago? . . . In hindsight do you wish you that you had?"

"That's easy to answer," Oller replied. "Of course."

"Do you think you should have?" a reporter asked.

"I think we used modern medical state of the art in order to do what we did," Oller said.