One of the stubbornest human cancers -- cancer of the rectum -- seems to be yielding to a new three-way treatment.

In a five-year study of 200 patients at seven medical centers, the common recurrence rate of more than 50 percent has been cut in half by combined use of surgery, radiation and an 18-month barrage of anti-center drugs.

"This opens up new vistas" in the treatment of this far-too-common disease, Dr. Vincent DeVita, head of the National Cancer Institute, said in an interview yesterday. One cancer in every 25 is rectal, and there will be 35,000 new cases in the United States this year.

"It will take us another year or two to see if there is an improvement in long-term survical as well as a decline in recurrence," Dr. Philip Schein of Georgetown University said. "But after four decades of almost no progress, our data look promising."

Schein is co-chairman of the nationwide group that has conducted the study with federal NCI funds.

And in fact, he said, the group has temporarily halted its comparative study because of the apparent superiority of various treatment over the commonest past practice -- use of surgery alone.

The new results are especially good news for older people, DeVita said during a conference on aging and cancer here.

The diesease's average age of onset is in the late 50s. But rectal cancers are far from uncommon in younger adults. From age 35 to 44, cancers of the colon and rectum, taken together, rank behind only lung cancer in men, and only after breast and lung cancer in women.

The cooperative study started in 1975 at Georgetown University, the Mayo Clinic, Mount Sinai Medical Center in New York, the university of Miami, the University of California at Los Angeles, the Sidney Farber Cancer Center in Boston and Roswell Park Memorial Insititute in Buffalo.

The current results, Schein said, are based on a Sept. 1 review of fortunes of four groups of patients, all which disease spread outside the rectum or rectosigmoid area alone. The rectosigmoid is the border region between the rectum and colon or large intestine.

After an average 2-to-2 1/2-year follow-up, there has been a 52 percent recurrence rate in patients treated by surgery alone, but only a 39 percent rate in those given chemotherapy alone, 32 percent in those given radiation alone and 21 percent in those who got the triple treatment: surgery followed by four to six weeks of radiation followed by a year and a half of chemotherapy.

Actually, there is no statistically significant difference between the last three recurrence rates, said Schein, "so we can only say all three seem to be an improvement on surgery alone."

"But we stopped doing surgery alone in our study last Februrary," he added. "And if one were going to select a therapy today, I think one would go with" the triple treatment.

Next, he said, the cooperative group will compare this combination with some other possibilities "we're not yet ready to discuss."

Even with the best treatment, either rectal or colon cancer is still a formidable disease. There are usually no symptoms at onset and the disease is discovered only when it has already begun its advance. Many patients must have a colostomy -- an opening in the side of the body to replace the excised colon or rectal tissue.

Eventually, DeVita sid, the new combination approach might help avoid a colostomy in some patients. But today, Schein stressed, "every patient who needs one" must still have an "optimal operation," even when it means a colostomy.