SAN ANTONIO — The sizable excess in cardiovascular mortality caused by older radiotherapy regimens for breast cancer appears to be greatly diminished with more modern ones, Sarah C. Darby, Ph.D., reported at a breast cancer symposium sponsored by the Cancer Therapy and Research Center.
She analyzed nearly 309,000 cases of early breast cancer enrolled in the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) registry in 1973-2001.
A total of 37% of the SEER cases received radiotherapy. Heart disease was the main cause of death in that group; however, comparing cardiovascular mortality in women who did or did not get radiotherapy wouldn't be appropriate, since patients weren't randomized to this treatment.
A better comparison would use the fact that the heart is located slightly left of center; thus, patients with cancer of the left breast will get a higher radiation dose than those with right-sided cancer, said Dr. Darby of the University of Oxford (England).
Sure enough, patients who received radiotherapy and subsequently died of heart disease showed a highly significant 16% excess of cancers of the left breast. After 15 years or more, this risk climbed to 53%. In patients who didn't get radiotherapy, death due to heart disease was equally common in those with left- and right-sided breast cancer.
The key question is whether the excess cardiovascular risk associated with radiotherapy is a relic of older techniques or is still present with more recent regimens. A statistically significant downward trend in excess cardiovascular mortality is seen in left-sided breast cancer patients grouped by decade of cancer diagnosis. (See box.)
However, it's too soon to say the hazard has gone away completely, since there are no 10-year follow-up data on the most recent radiotherapy group, she said.