The tumor removed from President Reagan's abdomen Saturday was determined yesterday to be a malignant cancer. However, there was no immediate sign that it had spread to other parts of the body.

Reagan's doctors estimated that the chances were "greater than 50 percent" that he will have no recurrence of cancer and that, after recovering from the surgery that removed the tumor, he will be able to live a normal and vigorous life. They said no further cancer treatment would be necessary.

Because the cancer had begun to penetrate into the muscle wall lining the lower intestine, or colon, the chances of a recurrence are greater than if the cancer had been detected and removed sooner, Dr. Steven Rosenberg said. Rosenberg, chief of surgery at the National Cancer Institute, assisted in Saturday's operation and was in the group that informed Reagan of the laboratory findings at about 3:15 p.m. yesterday.

Rosenberg said that Reagan was in bed reading a book when interrupted with the news and that he took it calmly.

"I'm glad that that's all out," Reagan replied, as quoted by Dr. Dale Oller, chief of general surgery at Bethesda Naval Hospital. Oller led the surgical team.

Oller said the doctors conferred with the president for only about five minutes. Just before telling him that his tumor had been malignant, they informed Nancy Reagan, meeting with her in the sitting room outside the president's bedroom at the hospital for about 35 minutes. Oller said Mrs. Reagan received the news "very calmly."

"The president had a cancer confined to the wall of his colon, or bowel," Rosenberg said at a hospital news briefing. "It had not spread in the local area outside the bowel wall. There is every expectation that the local problem has been cured."

Rosenberg noted, however, that there is a chance of recurrence and said that Reagan would have to undergo a variety of repeated tests in years to come. These would include colonoscopies, the procedure that discovered the tumor last Friday, and CAT scans, computerized X-ray examinations that provide visual cross-sections of parts of the body.

"There is a possibility that the tumor can return," Rosenberg said. Asked to be more specific about the chances of recurrence, he indicated that they would be somewhere between 25 percent and about 50 percent.

When he first announced the lab findings, Rosenberg said, "The president has cancer." Although he later explained that he did not mean to imply Reagan is known for certain to still have the disease, the language did convey what doctors generally feel: that once a person has been diagnosed as having had cancer, the person must be regarded as in a special category of higher risk.

Yesterday's findings were based on detailed laboratory examination of slices of the removed tumor, the adjacent colon wall, more distant parts of the removed colon, and 15 lymph nodes that were removed along with two feet of Reagan's colon. Lymph nodes are bean-shaped nodules throughout the body that filter undesirable matter from the watery fluid, called lymph, that is extracted from blood.

If malignant cells from Reagan's cancer had begun to spread, a process in which individual cells break way from the primary tumor and circulate through the lymph and blood systems, there is a good chance some would have lodged in the lymph nodes. But microscopic examination showed that all the nodes were normal, as were other parts of the colon away from the tumor.

The tests were done not only by pathologists at the naval hospital but by others from the National Cancer Institute and the Armed Forces Institute of Pathology.

Despite all the tests, however, there is no direct way to tell whether some cells had spread through the blood vessels. This is why doctors cannot say conclusively that the president has been cured.

When colon cancer cells spread, they typically show up first in the liver, Rosenberg said. Cancer cells that lodge in the liver begin multiplying, seeding new tumors. Once cancer has begun in the liver, the prognosis is extremely poor, with survival usually measured in months rather than years.

Reagan's liver was examined visually during the operation and X-rayed with a CAT scan before the operation. Neither step revealed evidence of cancer. The surgeons did not remove a piece of liver for laboratory analysis.

Rosenberg said that Reagan's cancer, found inside a tumor called a villous adenoma, was of a type called an adenocarcinoma and that because it had begun to invade the colon wall, it would be classified as a "Dukes B" tumor. The Dukes system of classification, developed by C.E. Dukes, helps doctors estimate a patient's survival chances. Had the tumor been removed in the earlier stage called a Dukes A, there would have been at least a 90 percent chance of surviving five years beyond surgery.

Rosenberg said there was no way to tell when, if ever, new tumors might develop. He said it could be five or 10 years, or never. If new tumors developed, however, they would most likely be in the liver. In the meantime, he said, Reagan could return to a normal life.

The president was already beginning to do this yesterday, meeting with aides and signing routine documents. All his vital signs continued normal and he took more short walks in his room and up and down the hall and sat in a chair for brief periods. The bladder catheter was removed as no longer necessary. Reagan also visited with his wife, met with aides and received a national security briefing.

Vice President Bush, who has not yet met with the president, canceled a two-day trip and met with White House chief of staff Donald T. Regan to discuss new duties he will assume during Reagan's recuperation. "Bush will sit in when it's appropriate until the time the president returns to the Oval Office," said White House spokesman Larry Speakes.

Speakes also said that shortly after midnight Sunday Reagan finished reading the Louis L'Amour novel that had engrossed him much of the day. Yesterday morning he asked for another book to continue with his reading.

And doctors not connected to the Reagan case said the president appeared to be recovering from his surgery about as well as any 74-year-old in good physical condition, the White House continued to describe the recovery in unusually glowing terms. Speakes said Oller "is running out of superlatives to apply to his patient."

He quoted Oller as saying, "The president continues to progress superbly from his surgery. His vital signs are the same as a person who has not had surgery." The one exception, however, is that Reagan's body temperature has remained slightly above normal, which is normal for a person who has had surgery. Speakes refused to say what the temperature was, saying, "We don't go into exact figures on this."

Speakes also would not divulge any other details, including the pathologists' reports on the laboratory findings. "The Reagans have very strong feelings about doctor-patient relationships, as I think you would if you were in the hospital," he told reporters. "Go fly a kite," he concluded.

"The patient's only complaint," Speakes said, "is a bit of abdominal pain or discomfort when he gets out of bed." This is normal for post-operative patients, but Speakes said Reagan has not had any pain-killing medication since the one injection of morphine in an area near his spine just after the operation.

There was one indication that the president's hospitalization might not be as speedy as implied. Since the operation, doctors have said Reagan would stay in the hospital for "seven to 10 days." Yesterday, Rosenberg said the president would probably stay for the full 10 days.

Mrs. Reagan, who remained alone with her husband for a short time after they heard the findings, returned to the White House where she substituted for the president at a Boston Pops Orchestra concert for the Washington diplomatic corps. "My husband is sorry he can't be with you and I'm sort of a stand-in," she told the audience.

At news briefings throughout the president's stay at the hospital White House officials have made a point of showing that the office of the president is still in business, even if its headquarters is at the hospital. Aides distributed routine announcments of minor presidential appointments every day and yesterday Speakes said Reagan had just created a new commission to study the Defense Department.

Late yesterday, however, Speakes said the White House press office would move back downtown today and that the scores of telephones and other facilities hastily installed at the hospital would be dismantled. White House officials, however, would continue commuting to Bethesda to meet with the president for brief periods.