BOSTONWomen who receive postlumpectomy radiotherapy to their left breast are not at greater risk of heart attacks, according to a study presented by Katherine Vallis, MD, at the annual meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO).
Dr. Vallis, a radiation oncologist at Princess Margaret Hospital, Toronto, Ontario, and her colleagues found nearly identical rates of myocardial infarction (MI) in 1,074 women treated for left-sided breast cancer and 1,054 for right-sided breast cancer between 1982 and 1988 at Princess Margaret Hospital. The rates were also similar to those found in the general population.
"Whenever the left breast is treated, a small part of the heart receives some radiation," Dr. Vallis noted. "There was concern that the radiation might cause subtle damage to the heart, therefore increasing the risk of heart attack in these women."
The Princess Margaret results contradict previous studies that found women were at greater risk after radiotherapy for cancer of the left breast. Also from Ontario, a population-based study published in 1999 found that these patients had twice the risk of their counterparts treated for cancer of the right breast.
"Some of the earlier studies included mainly postmastectomy radiotherapy patients," Dr. Vallis said, "and some of them have included patients treated with radiation therapy techniques that would now be considered obsolete." She also speculated that one reason her results differ from the province-wide study might be that the groups used different databases and methods.
The Princess Margaret study linked to the Canadian Institute for Health Information Hospitalization File and the Ontario Mortality Database.
It validated the cardiac events with the original hospital records, using diagnostic criteria from the Multinational Monitoring of Trends and Determinants in Cardiovascular Disease (MONICA) Project.
Dr. Vallis said that an Ontario-wide study using methods similar to the Princess Margaret Hospital study is now underway.
The median follow-up in the Princess Margaret study was 10.7 years, and the median age of the 2,128 women in the cohort was 54. All had lumpectomies for early-stage breast cancer; 63% were node negative. Most patients (92%) received a radiotherapy dose of 40 Gy in 16 fractions delivered using a pair of coplanar tangential opposed fields. The other 8% received 50 Gy in 25 fractions.
A five-fraction 12.5 Gy boost to the tumor bed followed the shorter protocol in 84% of cases. In 6%, treatment was also delivered to the supraclavicular fossa, the axilla, or both; the internal mammary nodes received therapy in 2.5% of cases. The overall survival rates were 81% at 5 years and 67% at 10 years.
The researchers identified 70 MIs in 56 patients following radiation therapy. Of these, 53 met MONICA criteria for definite MIs, 6 were possible MIs, and 11 definitely were not MIs.
Definite or possible MIs were recorded for 26 patients treated to the left breast. These included 8 fatal MIs. In the right-breast group, 23 patients had definite or possible MIs, of which 6 were fatal. There were more MIs in women over 60, which, Dr. Vallis noted, was to be expected.
"We can say conclusively from this study of a large group of patients that, using these modern techniques, we haven’t found any increased risk for heart attack even many years later," Dr. Vallis concluded. "And if there is an effect, it must be a small one because we haven’t been able to detect it."