Two widely used treatments for breast cancer -- radiation and the drug Herceptin -- can cause heart problems, researchers are reporting.

But the problems can usually be treated or prevented, and they should not scare women away from needed cancer therapies, the doctors say.

The findings, from two studies being published today in The Journal of Clinical Oncology, expand on earlier research linking the treatments to heart trouble.

The first study involved 961 women with early-stage breast cancer who were treated from 1977 to 1994 at the University of Pennsylvania; 477 had cancer in the right breast, and 484 in the left.

Those who had radiation to the left breast had an increased risk of heart disease up to 20 years later, apparently because the radiation hit parts of the heart and coronary arteries. The women's risk of coronary artery disease was 25 percent, compared with 10 percent in the women who had radiation on the right.

Similarly, 15 percent of the women treated on the left side had heart attacks, compared with only 5 percent treated on the right.

The problems did not usually show up until 10 or more years after the radiation. Women who already had heart disease, hypertension or high cholesterol before starting cancer treatment had the greatest risk.

Despite the problems, the women did not have higher death rates from heart disease.

Dr. Eleanor E. R. Harris, the lead author of the study, said that in one sense, the findings reflected good news: many women with breast cancer are now living long enough to be concerned about the long-term effects of treatment.

Dr. Harris, who has moved from Pennsylvania to the H. Lee Moffitt Cancer and Research Institute in Tampa, said her study was more important for women who had radiation in the past than for those being treated today.

Those treated five or more years ago need to be especially careful about treating high blood pressure and cholesterol, she said.

Nationwide, researchers estimate that there are about 370,000 women who had radiation to the left side of the chest for breast cancer more than 10 years ago.

Women today have less risk, Dr. Harris said, because improved radiation techniques and equipment have made it possible to avoid exposing the heart in nearly every case. But she said it was important for women to be treated at centers with the most advanced techniques.

Some women who have the breast removed completely do not need radiation. Those who have lumpectomies, removing just the tumor, almost always need it.

But Dr. Harris said she would not recommend that women choose mastectomy just to avoid radiation and its risks. The operation has complications of its own, and patients sometimes need radiation anyway. She said the first priority should be picking the most effective treatment for the cancer.

Dr. Larry Norton, a breast cancer expert at Memorial Sloan-Kettering Cancer Center in New York, who was not involved in the study, said the effects of radiation on the coronary arteries were relatively small but of concern to cancer doctors. He said more data and recommendations about the problem would become available later this year.

The second study, at the M. D. Anderson Cancer Center in Houston, involved 173 women with cancer that had spread beyond the breast. The women had tumors that were sensitive to the drug Herceptin, which can help 20 percent to 25 percent of patients but is known to cause heart problems in some.

The patients took the drug for at least a year; in 49 women, or 28 percent, the heart lost some of its pumping ability, usually while they were still taking the drug. More than half had no symptoms, but some felt short of breath. But the problem could usually be reversed with medicines like beta blockers and ACE inhibitors.

Most of the women recovered fully, but a few had lingering problems, and one died from heart failure.

The director of the study, Dr. Francisco J. Esteva, said that over all, the findings were encouraging because the heart problems were reversible and women could even take the drug again once their hearts recovered. But women who have already had heart failure may need to avoid Herceptin.