When a patient has breast cancer in only one breast, known as unilateral cancer, they’ll often choose to have a double mastectomy in an effort to lower the risk of developing contralateral breast cancer, or cancer in the other breast. While research indicates that getting a double mastectomy does decrease the risk of this happening, a new study shows it may not have much of an impact on mortality.

Researchers at Women’s College Hospital in Toronto recently compared 20-year survival rates among more than 108,000 women with unilateral breast cancer who either had lumpectomies, unilateral mastectomies, or double mastectomies. An equal number of women who’d received each treatment type were included. The findings, published in JAMA Oncology, showed that while patients who underwent double mastectomies had a significantly lower risk of developing contralateral breast cancer, each group had roughly the same breast cancer mortality rate after 20 years.

Dr. Vasily Giannakeas, the study’s lead author and researcher at the Women’s College Hospital Research and Innovation Institute, says, “Traditionally, it is assumed that a contralateral breast cancer is a new primary cancer with the potential to metastasize and thus impact overall survival negatively. However, the similar mortality rates across all surgical groups suggest that the occurrence of contralateral cancer may be more of a marker of an already existing higher risk of mortality rather than an independent cancer event that drives an increase in mortality.”

When it came to developing cancer in the other breast over the 20-year follow-up period, the study showed that there was a 6.9% risk in both the lumpectomy and unilateral mastectomy groups, while there was only a 0.7% risk for those who had a double mastectomy. However, mortality rates hovered around 1 in 6 for all three groups. For lumpectomy, it was 16.3%, 16.7% for unilateral mastectomy, and 16.7% for double mastectomy. This is despite the fact that 32.1% of those who’d developed contralateral breast cancer had died within 15 years, compared with 14.5% of those who had not developed the disease in the other breast.

So, though this does show a double mastectomy provides a significant protective benefit against developing contralateral breast cancer, it also shows that it may not be providing as much protection against breast cancer death overall as would be hoped by doctors and patients. It also shows how deadly contralateral breast cancer can be. The team says the findings may mean some changes to treatment technique are needed.

Dr. Giannakeas explains, “About one-third of patients with contralateral breast cancer are treated with chemotherapy. However, it now seems we are trying to treat the first cancer and chemotherapy may not be as effective. These are compelling questions that we hope to address in future studies.”

“For now, this research’s findings are valuable to the patient and their physician as they look to decide on surgical options to treat breast cancer.”