SAN ANTONIO — The considerable 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.
This was the principal conclusion of her analysis of 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 population received radiotherapy.
Heart disease was the leading cause of death in radiotherapy-treated patients; however, a straightforward comparison of cardiovascular mortality rates in women who did or did not get radiotherapy wouldn't be appropriate, since SEER participants were not randomized to this treatment.
A better comparison would take advantage of the fact that the heart is located slightly left of center in the body; thus, radiotherapy recipients with cancer of the left breast will get a higher radiation dose than those with right-sided cancer, explained Dr. Darby of the University of Oxford (England).
Sure enough, among SEER participants who received radiotherapy and subsequently died of heart disease, there was a highly significant 16% excess of cancers of the left breast. In the subset of radiotherapy-treated patients followed for 15 years or more, this excess risk climbed to 53%.
In contrast, among those patients who did not get radiotherapy, death due to heart disease was equally common in women with left- and right-sided breast cancer, the study found.
The key question is whether the excess cardiovascular risk associated with radiotherapy is a relic of outdated techniques or is still present with more recent regimens. A statistically significant downward trend in excess cardiovascular mortality in left-sided breast cancer patients was evident when SEER radiotherapy recipients were stratified by decade of cancer diagnosis. (See chart.)
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. And it's by no means clear that regimens just now being introduced—such as intensity-modulated radiation therapy—are going to be free from cardiovascular risk, she said at the meeting.