So dread is the very thought of cancer that many people were no doubt stunned and saddened yesterday at the first word that cancer had been found in the intestinal tumor removed from President Reagan on Saturday. Yet his doctors immediately went on to report that all of the malignancy had been removed and that Mr. Reagan has an excellent chance to recover quickly and completely, to return to his former level of activity and to live a good and long life. All of us surely wish that that will be so.

In speaking of Mr. Reagan's medical prospects, the doctors referred to percentages: they said, for instance, that there is "greater than a 50 percent chance" of a complete cure. On the calculator that each of us carries in his head, that can produce the doleful conclusion that the president hs as much as a 50 percent chance of more sickness. But that sort of calculation leaves out the fact that Mr. Reagan is already at an age where he is, at least theoretically, vulnerable to assorted illnesses. Perhaps the correct conclusion is that he is in his seventies but is also basically healthy. Those are the two things the American people knew about his physical condition when they reelected him less than a year ago. In this respect, not much has changed.

What all of us now know about Mr. Reagan's cancer comes chiefly from the accounts given by his doctors in the last few days. They are medical men, but the implications of their analyses are profoundly political -- in the way they affect public confidence. Once again, it appears, the president has been well served in his choice of doctors. We speak not so much of their medical skills, which it is for others to judge, as of the evident clarity and candor they have displayed in their appearances before journalists. At a time when a 74-year-old president has had a brush with cancer, nothing is more useful and necessary than the public's conviction that 1)the doctors are competent and intend to practice a vigilant watch on their patient and 2)they are telling the public everything that is on the public's mind to ask.

Ronald Reagan has been beating the odds and prevailing over 50-percent chances for as long as we can remember. It is a distinguishing mark of the man, and we are confident that this case will be no different.