Infertile men may be at twice the risk of developing breast cancer than those without fertility issues, according to new research. While the potential causes of this remain unknown, the finding could help pave the way for much-needed research into the disease.

The study, published in the journal Breast Cancer Research, is one of the largest ever to look at the disease, having included almost 2,000 men with breast cancer.

“Our study suggests that infertile men may be twice as likely as those without fertility issues to develop breast cancer,” study author Dr Michael Jones of The Institute of Cancer Research, London, said in a statement.

“The reasons behind this association are unclear, and there is a need to investigate the fundamental role of male fertility hormones on the risk of breast cancer in men. We hope this could lead to insights into the underlying causes of male, and possibly even female, breast cancer.”

Male breast cancer is rare, with just 1 percent of all breast cancers in the US occurring in men. This year, around 2,710 American men will be diagnosed with breast cancer – and of these, 530 are expected to die from the disease, The American Cancer Society estimates. In the UK, where the current study was based, 370 men are diagnosed each year. 

Due to its lower prevalence, much less research is dedicated specifically to male breast cancer, and treatment decisions are often based on studies conducted on women.

“The causes of breast cancer in men are largely unknown, partly because it is rare and partly because previous studies have been small,” Jones said in a statement.

The new study set out to change that: Jones and colleagues interviewed 1,998 men recently diagnosed with breast cancer in England and Wales over a 12-year period, as part of the Breast Cancer Now male breast cancer study.

They asked the men questions to ascertain their fertility. These included whether or not they had biological children, if they or their partners had ever experienced problems conceiving, and whether they’d ever sought professional help for fertility concerns. 

They then compared the responses to those of 1,597 men with no history of breast cancer, revealing that men with breast cancer were more likely to report fertility issues than those without. The risk of invasive breast cancer – cancer with the ability to spread – was significantly linked to male infertility, they report.

The same was true even after accounting for known risk factors for infertility and breast cancer that could affect the results. 

Significantly more men with breast cancer didn’t have children compared to the controls, the team found – though this cannot be taken as a measure of infertility as people choose not to have children for a whole host of reasons.

The study was somewhat limited by its self-reporting of fertility, allowing room for misclassification. Fertility can be affected by either member of a couple, for example, or men may have failed to report children outside of a marriage. Using medical records would provide future studies with a more tangible and accurate measure of infertility. 

With any luck, these future studies could help us on our way to better understanding the causes of male breast cancer.

“Discovering a link between infertility and male breast cancer is a step towards us understanding male breast cancer and how we could find more ways to diagnose and treat men – and possibly women – with this devastating disease,” Dr Simon Vincent, Director of Research, Support and Influencing at Breast Cancer Now, said.

“Importantly, we hope the knowledge we have gained from this study reaches more men who might benefit from being aware of male breast cancer,” he added.