Inconsistency among the different systems for classifying the severity of colon cancer led President Reagan's physicians to give misleadingly pessimistic figures after his surgery about chances that his tumor might recur, according to pathologists.

National Cancer Institute surgeon Steven Rosenberg, a member of Reagan's medical team, said last July that the president's intestinal tumor was graded "Dukes B" and that the president's chances of surviving five years without a recurrence were between 50 and 75 percent.

However, Rosenberg's description of the tumor as extending only partly through the muscle layer of the intestinal wall would lead most pathologists to classify it as "Dukes A," a less ominous cancer with an 80 to 95 percent chance for five-year survival, according to Dr. Michael Kyriakos, professor of surgical pathology at the Washington University School of Medicine.

The confusion apparently arose because Cuthbert Dukes, originator of the grading system, defined "Dukes B" one way in 1929 and another way in 1932 and because his definitions have since been modified by other pathologists, according to Dr. Robert V.P. Hutter, chairman of the American Joint Committee on Cancer.

To eliminate such ambiguities, Hutter's committee has helped to develop a new international classification system for colon cancer, described in the May issue of the Archives of Pathology and Laboratory Medicine. Today's issue of the Journal of the American Medical Association decribes the confusion over the classifying of Reagan's cancer.

Inconsistency among current systems led to conflicting statements by medical experts, the White House and the media about Reagan's prognosis, according to Dr. William W. McLendon, the Archives of Pathology editor.

"If you used one system, it would imply he has a 50 percent chance of survival, and in the other one he has a 90-plus percent chance of not having any further problem," McLendon said, adding, "I don't think the treatment would have been any different, but . . . from the standpoint of the historical record and the current political scene, there was some confusion about what his prognosis would be."

In a subsequent attempt to clear up the mystery, White House press secretary Larry Speakes called the tumor a "Dukes B1" and added that pathologists had used a modification of the Dukes system called "Astler-Coller" after the doctors who developed it. No official pathologist's report on Reagan's cancer has been released.

Kyriakos said Rosenberg's description of the tumor's extent corresponds with a grade of Astler-Coller B1 or Dukes A.

The chances of a surgical cure, measured by surviving five years without a recurrence, are "anywhere from 80 to 95 percent, depending on patient age and differentiation of the tumor . . . . It's nowhere near the 50 to 60 percent" mentioned at the time of Reagan's surgery, he said.