The research, published in The British Medical Journal, found that for women who underwent breast cancer treatment, the risk of a second primary cancer — meaning a cancer originating outside the breasts — increased by only about 2 percent compared with the general population.
But the relatively low increase in risk should be “very reassuring” for women with a history of breast cancer, says Dr. Mouabbi, who was not involved with the study.
Risk of a Different Type of Cancer Only Marginally Higher for Breast Cancer Survivors
Researchers analyzed data from the British National Cancer Registration and Analysis Service, identifying more than 475,000 women with breast cancer as their first invasive cancer from 1993 to 2016. Of these women, the researchers followed 86 percent for at least five years.
About 65,000 developed a second primary cancer, mainly in the uterus or lungs. That’s the equivalent of 13.6 percent of participants — 2.1 percent more than what’s expected for the general population.
Another 5.6 percent developed breast cancer in the opposite breast (called contralateral breast cancer) — about 3 percent more than the general population. This risk was higher in younger women, the study noted.
“Our study confirms that the risks of new cancers are higher for breast cancer survivors than for other women in the general population,” says lead author David Dodwell, MD, senior clinical research fellow in the population health department and consultant clinical oncologist at the University of Oxford in England.
“However, these additional risks are small in comparison to the risks of recurrence and breast cancer death in the great majority of women diagnosed with early invasive breast cancer, despite the substantial improvements in the treatment of early breast cancer recently reported.”
How Treatment Types Affect Secondary Cancer Risk
The study focused on women who received breast-conserving surgery or mastectomy as their first treatment. Researchers grouped the women based on the type of adjuvant therapy they got (or additional treatment after surgery) to examine the connections between treatment type and risk of secondary cancers.
They found that radiation therapy was associated with higher rates of contralateral breast and lung cancer. Endocrine (hormone) therapy was linked to an increased risk of uterine cancer, but decreased risk of contralateral breast cancer. Chemotherapy was tied to an increased risk of acute leukemia.
While this may seem alarming, it’s important to note that breast cancer treatments today have advanced far beyond those available in the ‘90s and even a decade ago, says Hope Rugo, MD, division chief of breast medical oncology at City of Hope in Duarte, California, who was not involved with the study.
These improved treatments could reduce the risk of secondary cancers, she adds.
For example, in the past, doctors delivered radiation straight down onto the chest where it often reached the lungs, which are behind the breasts, Dr. Rugo explains. Today, they deliver radiation sideways and isolate the breast to protect the lungs.
Even though some cancer treatments can have side effects, Mouabbi says any small risks outweigh the benefits.
“It’s important to note that the treatment is what’s preventing other cancers from popping up,” he says.
The Study Has Limitations
Researchers noted that some cancer registry data were incomplete in certain areas, such as related to adjuvant treatments. The data analyzed also lacked information on participants’ family history, any genetic predispositions to cancer, and lifestyle factors like smoking.
Less than 5 percent of participants were recorded as nonwhite, and men weren’t included, so the findings may not be generalizable to other groups.
Because the study was observational — meaning there was no control group or intervention — the findings don’t prove that any of the adjuvant therapies caused secondary cancers, only that there is a link.