Breast cancer patients who undergo radiation therapy have a slightly increased risk of ischemic heart disease within five years, and the risk persists for at least twenty years, researchers from Oxford University, England, the Karolinska Institute, Sweden, and Copenhagen University Hospital, Denmark, reported in NEJM (New England Journal of Medicine).

The authors added that the risk of developing ischemic heart disease and/or dying prematurely from heart disease is more closely linked to those with other heart disease factors, including diabetes, obesity, and smoking when radiation therapy was administered.

Ischemic (British spelling: ischaemic) heart disease is characterized by ischemia of the heart muscle – ischemia is the restriction in the blood supply to tissue. In most cases, ischemic heart disease is caused by coronary artery disease. Overall, in the general population, it is more common among men, people with close relatives with ischemic heart disease, regular smokers, individuals who have high cholesterol levels, diabetes, and/or hypertension (high blood pressure). Ischemic heart disease is also known as myocardial ischemia.

Doctors have always presumed that women with breast cancer who undergo radiation therapy have a higher risk of heart disease later on. However, not much was known about the specifics of the risk and whether susceptibility to ionizing radiation might affect some patients more than others.

In this study, the scientists set out to determine what impact radiotherapy (and its doses) might have on heart disease risk among women who were treated for breast cancer.

The study involved 2,200 female patients who had been administered radiation therapy for breast cancer from 1958 to 2001. The researchers used data from radiotherapy charts and medical records to estimate the mean radiation dosage to the heart. They also gathered and examined data regarding each woman’s heart disease risk factors as well as their medical histories.

The team found a clear association between the radiation dose and ischemic heart disease later in life. Although the risk is real, the authors stressed that it is still small.

A higher risk of heart disease among women receiving radiation therapy for breast cancer was more evident among those with:

  • angina
  • other heart diseases
  • chronic obstructive lung disorder
  • diabetes
  • obesity
  • a short distance between the chest wall and the heart
  • and women who smoked

The risk peaked during the first ten years after radiation therapy, and then dropped, but remained higher-than-normal for at least twenty years.

The risk of developing ischemic heart disease was influenced by the woman’s age when radiation therapy was given, as well as previous disorders, and heart dose.

This does not necessarily mean that a breast-cancer patient has a considerable risk of heart disease if she undergoes radiation therapy. A 50-year-old patient who received a 3 Gy heart dose during radiotherapy has a 2.4% chance of dying from ischemic heart disease before she is 80, compared to 1.9% among breast cancer patients of the same age who had no radiation.

Professor Per Hall, from the Karolinska Institutet, said:

“The results confirm what we have long suspected, that irradiation increases the risk of myocardial infarction, and that women with other known risk factors for ischemic heart disease are more susceptible than others. This gives us stronger grounds on which to weigh the pros of irradiation against the cons of its harmful effects on the heart for individual patients.”

This is the first study to correlate radiation therapy doses for breast cancer paitents with heart disease risk. The authors say that their findings will help doctors decide on how best to treat patients.

Professor Sarah Darby, from the University of Oxford, said:

“We carried out this work because doctors could not reliably estimate the risk of heart disease in women treated with radiotherapy for breast cancer. Doctors can now estimate the risk and know that in most cases it will be very small so that they can reassure their patients. In addition, the few women for whom radiotherapy poses undue risk can now be identified, so that alternative techniques can be considered.”

Dr. Carolyn Taylor, who works at Oxford University’s Clinical Trial Service Unit, and was also involved in the study, said “Already our work is being used in cancer centres throughout the world and we’ve heard it’s making a difference to patients. It’s important to remember that for most women treated today the benefits of breast cancer radiotherapy far outweigh the risks.”

Radiation therapy is measured in Gy (Gray). For a breast-cancer patient with malignancy on the right-side, most therapy regimes expose the heart to 1 or 2 Gy. It is usually slightly higher for breast cancer on the left side.

Kate Law, director of clinical research at Cancer Research UK (one of the sponsors of the study), explained that radiation therapy is very effective for treating breast cancer and has played a crucial part in pushing up survival rates to over 85%. Several studies have shown that, for example, breast cancer patients benefit from a single dose of radiation at the time of surgery. This latest study means that oncologists can reassure most patients that the risk of heart disease from radiation therapy is small. “Also, for the first time, women who would be at substantial risk if they had radiotherapy can be identified and alternative techniques considered.”

Two scientists published an article in Lab Investigation in 1971 stating that radiation to the breast could damage the capillaries and restrict blood flow to the heart. Over the last 40 years studies have come up with contradictory findings. Another study published in the International Journal of Radiation Oncology/Biology/Physics (May 2007) found that older breast cancer patients with early stage disease who undergo radiation therapy have no increased risk of a heart attack. They added that “Interestingly, pre-existing cardiac risk factors such as diabetes, hypertension and hyperlipidemia do not potentiate the effects of radiation on the heart.”

Written by Christian Nordqvist