Amid growing indications that President Reagan received advice that delayed the diagnosis and treatment of his cancer, a number of doctors not connected with his case have advocated a new system to assure that presidents receive the best health care possible.

The president's safety is guarded by the Secret Service, so why should he not also have the most vigilant protection from threats to his health, they say.

These doctors recommend scrapping the tradition of allowing presidents to choose their own doctors.

Instead, the outside doctors say, the president's doctor should be a top specialist with up-to-date clinical experience in internal medicine who could act as a primary physician, able to call upon a panel of the best outside specialists.

"No matter how you feel about Ronald Reagan being president, he is the president. His health is important to the nation and he deserves the finest medical care and the best-informed advice this nation can provide," said a doctor at the National Institutes of Health who, because of White House instructions to NIH doctors not to talk to the press, asked not to be identified.

"What we've got now is no system at all. The president can pick an old crony who isn't up on the latest knowledge and doesn't have either the training to do the job or the contacts to find the best consultants," the NIH doctor said. "And, when he needs a hospital, why does he go to a military hospital and get whoever's on a two-year assignment there?

"Somebody ought to call for a national commission to look into this and recommend a better setup," he said. "I would, but I'm not supposed to be talking about this."

Although many have argued that candidates for president should be able to assure voters that they are in good health, the question of the quality of medical care after the election has seldom been raised except when presidents fall ill or are injured.

During Reagan's bout with cancer, many doctors expressed surprise that Reagan did not receive a full examination of his colon until 14 months after the first hint that he might be vulnerable, the discovery of a polyp in his lower intestine. A second polyp was found four months ago along with blood in the stool, but White House doctors again did not insist on an immediate full colonoscopy.

"Why wasn't Steve Rosenberg called in as soon as they found blood in the stool," one doctor asked, referring to the National Cancer Institute surgeon who was brought in on Reagan's case after the tumor was found. Rosenberg is widely acknowledged as one of the nation's top experts on colon cancer.

White House spokesman Larry Speakes said that if either the president or Nancy Reagan had known polyps and stool blood were warning signs of cancer, they would have wanted a complete colon exam immediately.

"The president was not getting what we would consider optimal medical advice," said Dr. Victor Gilbertsen, director of the University of Minnesota's cancer detection center. "It does seem he needs some way of getting better advice."

Dr. Donald Ritt, who diagnosed cancer in the president's brother, J. Neil (Moon) Reagan, just two weeks before Reagan's colonoscopy, said the president is "not being well served by the system as it is."

"The clear thing that comes to my mind is that when you're dealing with the president of the United States, you don't apply the same standard of care that would be recommended for the population at large," Ritt said. "It's very nice to get up on a high plane and say everybody should get the same care but, let's face it, the life of the president is important to the whole country and the doctors ought to be looking at him much more frequently and intensively than they would look at you and me."

Dr. Edward Cattau, Reagan's intestinal specialist, has defended the decision not to do a colonoscopy sooner by saying he was following accepted standards developed by the American Cancer Society.

When a reporter challenged Cattau, saying, "We're not dealing with you or me. We're dealing with the president of the United States," he replied: "The answer is the same. The standard of care for you, for me, for the president of the United States is just as I have explained it to you."

While egalitarians might applaud Cattau's stance, many doctors who spoke with The Washington Post take the position that the president should get a degree of medical expertise and diligence beyond the standards developed for the country as a whole.

Doctors do not like to make much of it, but the fact is that ordinary standards of care are formulated according to a cost-benefit analysis. Diagnostic procedures that have only a small chance of revealing disease are not done routinely if they are deemed expensive.

Some doctors also challenged the tradition of presidents going to military hospitals. Bethesda Naval Hospital recently has been the target of a number of malpractice suits, most prominently against Dr. Donal M. Billig, who was ousted in April as chief of heart and chest surgery. In 1983 the hospital nearly lost its accreditation because of irregularities in its record-keeping.

Sen. Edward Zorinsky (D-Neb.), for example, said he had a colon polyp removed in 1980 at the hospital, which treats many members of Congress, but the doctors could not tell him whether it was benign or malignant.

"I was worried about it," Zorinsky said, "so I called them a couple of times and they kept telling me the lab work was not completed." After two weeks the hospital wrote Zorinsky to say, "Although the biopsy specimen was unfortunately lost, our consultant felt there was absolutely no chance that there was a malignancy."

Several doctors who spoke to The Post said there was no need to go to the naval hospital when Washington has several excellent teaching hospitals attached to its medical schools.