Medicine is as much an art as a science when it comes to predicting how long a cancer patient will survive, particularly when the patient is the president of the United States.

National Cancer Institute surgeon Steven Rosenberg, who served on President Reagan's medical team, predicted that Reagan has a better than 50 percent chance of being cured permanently.

Rosenberg stressed yesterday that he was looking at the long-term chances. However, statistics are more favorable when looking only at the five-year survival rate. Prognoses depend on the system used to classify the tumor's severity and whether the cancer has spread.

Rosenberg said that the tumor removed Saturday from the president's large intestine had spread into the initial layers of the intestinal wall but had not penetrated the outer layer. He graded it "Dukes B," under a system that bears the name of a British doctor and ranks tumors from A, least severe, to D.

White House spokesman Larry Speakes said yesterday that the tumor fell into the least severe subcategory, B-1.

Other National Cancer Institute studies use a numerical staging system ranging from 1, least severe, to 4. Although the Dukes and numerical systems are not completely comparable, statistician Lynn Ries said yesterday that the institute's Seer program, which tracks cancers diagnosed between 1977 and 1981, suggests that the five-year relative survival rate is 79 percent for Stage 2, 87 percent for Stage 1.

Available information suggested that Reagan's tumor may have fallen between the two, perhaps in the lower end of stage 1. Ries said Stage 1 tumors have progressed into the initial layers of the muscle, while Stage 2 tumors involve all layers of the bowel wall, with or without invasion of the tumor to adjoining tissues.

The numerical staging system is based on guidelines from the American Joint Committee on Cancer published in 1983, Ries said.

Institute spokesmen said another system, the PDQ data base, suggests that the five-year survival rate is 75 percent or better in the Stage 1 category, which is said to include tumors of the B-1 type. Under this system, Stage 2 tumors have a 50 to 75 percent five-year survival rate.

Rosenberg, who briefed reporters Saturday and Monday, made clear yesterday that his prediction had been based on the "entire medical literature for the kind of lesion the president had."

Rosenberg said that in a 1954 study reported in the Annals of Surgery, patients with B-1 tumors of the colon had a 66 percent chance of surviving the first five years. He added, however, that 30 percent of colon-cancer recurrences take place after five years.

All of these predictions are based on large populations, and it is impossible to predict the course of a particular case.

About 96,000 cases of colon cancer will be diagnosed this year, and 52,000 people will die from the disease, said the institute's John Horm. About half of those diagnosed with colon cancer this year are expected to be alive at the end of five years. Horm said the chance of colon cancer increases with age, with half of all newly diagnosed cases occurring in people over 71.

The average white 74-year-old man is likely to live 9.6 more years, said Dr. Henry Rosenberg of the National Center for Health Statistics. Emphasizing that there was no way to know what would happen in the president's case, Rosenberg said that cancer generally would reduce the probability of surviving that long. For those 65 and older, heart disease accounts for 45 percent of all deaths, while cancer accounts for nearly 20 percent, he said.