But with high survival rates and other benefits, that may be the wrong question to ask.
Bilateral mastectomy does not reduce breast cancer mortality, a large longitudinal study has confirmed – but that is unlikely to reverse the rise in women choosing the procedure for other reasons.
In a finding that appears paradoxical, the study found that this surgical option fails to increase survival even though it substantially reduces the incidence of contralateral breast cancer, which is associated with a fourfold higher risk of death from breast cancer.
The Canadian matched cohort study, published in JAMA Oncology, is the largest and longest study to date on bilateral mastectomy, also known as a contralateral prophylactic mastectomy, for unilateral breast cancer – a treatment option that is gaining popularity, US data shows, “despite consensus guidelines discouraging the procedure in women of average risk”.
Using registry data on breast cancer diagnoses between 2000 and 2019, the researchers found 36,000 women who had had bilateral mastectomy and matched them with equal numbers of women who had breast-conserving surgery, aka lumpectomy, or unilateral mastectomy. The patients were followed up for an average of seven years and maximum of 20 years.
There were some differences at baseline among the groups: patients with bilateral mastectomy were younger and more likely to have advanced clinical features. There was no data on endocrine therapy, BRCA status or screening.
Ninety-seven patients in the bilateral mastectomy group developed a contralateral cancer compared with more than 700 patients in the other two groups (a 20-year cumulative incidence of 7%).
The mortality rate in those with contralateral invasive breast cancer was four times those without.
Yet the 20-year cumulative mortality from breast cancer was equal across all three groups at 16.3-16.7%.
“It is generally presumed that a contralateral breast cancer is a new primary tumour with the potential to metastasise,” the authors wrote. “Our findings question this interpretation. If the increase in deaths after a contralateral breast cancer were due to metastasis of the second cancer, we would expect bilateral mastectomy to be beneficial.”
Associate Professor Rhea Liang, a general and breast surgeon on the Gold Coast and clinical subdean at Bond University medical school, said with this interpretation, contralateral cancers, rather than generating extra mortality risk, reflected a woman’s baseline risk – they were a marker, rather than a cause.
“If we thought that each new cancer adds its own separate risk, then you might expect that bilateral mastectomy would be helpful, because it would remove the risk of the second cancer,” she told Oncology Republic.
“But what they’re surmising is that the women have an underlying field of risks, and that whether they get one cancer or two cancers on two different breasts, they’re just expressing their baseline risk. And that’s why bilateral mastectomy doesn’t change things.”
She added that women who have had breast cancer are closely surveilled, so subsequent cancers are picked up sooner and at a less lethal stage. Second cancers are also more likely to be treated with more effective therapies because breast cancer treatments improve over time.
Professor Liang said the evidence was “very clear” that rates of bilateral mastectomy were increasing worldwide and in Australia, and that survival was not the only consideration when survival rates were already high. Australia’s five-year survival rate of 92% is among the best in the world.
“It’s really hard to show an improvement in mortality when the mortality is so low already,” she said. “If mortality has ceased to become the main measure of our success, it’s understandable that women are starting to prioritise things like quality of life, anxiety, ongoing screening, the worry about getting a second cancer.
“The statement that bilateral mastectomy doesn’t provide any benefit is based on quantitative data regarding mortality, but there’s a lot of qualitative [considerations] that are important to this decision.
“People might not want to take the chance of having to go through the challenges of treatment for a second cancer, or they may understand that although those mortality figures apply to the whole population, they on a personal level – for reasons of family history or unbearable distress from watching a loved one die of breast cancer – want to minimise their risk as much as they can, as they perceive it.
“With the rapid advances in breast reconstruction, there are some considerations in terms of achieving symmetry between two breasts that are, on a surgical, technical level, somewhat easier when you do both at the same time rather than just one.”
Professor Liang said this study was “another piece of the puzzle” of how to counsel patients about operative decisions.
“If they are saying ‘I want a bilateral mastectomy because I don’t want to die’, you can reassure them that actually the mortality risk is the same whether they have a unilateral or a bilateral mastectomy, or if they’re suitable, a breast-conserving operation like lumpectomy.
“However, that doesn’t mean the decision [to have a bilateral mastectomy] might still not be valid for a whole bunch of qualitative reasons.”
Professor Christobel Saunders, James Stewart Chair of Surgery and director of research at Melbourne Medical School, said the study “confirms our advice to patients that, in the absence of other risk factors such as a genetic mutation, undergoing contralateral prophylactic mastectomy does not improve survival for most women with unilateral breast cancer.
“However, it leaves us with lots of intriguing questions such as whether there are certain groups of women who may benefit, the place of surveillance after unilateral treatment for breast cancer and how treatments may alter the long-term risk of subsequent breast cancer.”
Professor Saunders described the “fairly high” 7% rate of contralateral cancers as an important finding.
“Avoiding a second cancer and its treatment is often an important motivator for women to consider contralateral mastectomy, even understanding there is no survival benefit, and I do believe that if women are well informed and counselled, they are able to make the choice for which treatment they would prefer.”
