The persistent disparity, derived from an analysis of data from the National Cancer Database, suggests a possible distinct cancer biology, less effective treatment or compliance issues, and perhaps unhealthy lifestyles among men may be responsible for the lower overall survival rates, said Xiao-Ou Shu, MD, PhD, MPH, the study's senior author.
The five-year mortality rate for men was 19% higher than for women. Additional studies are needed to identify the causes, she said, but any such studies would likely require international consortia because male breast cancer is so rare, accounting for less than 1% of cases.
"It is so rare, it would be extremely difficult for any single institute to recruit a sufficient number of patients for research," said Shu, associate director for Global Health and co-leader of the Cancer Epidemiology Research Program at Vanderbilt-Ingram Cancer Center.
The study used 11 years of registry data from Jan. 1, 2004, to Dec. 31, 2014, which included 1.8 million female patients but only 16,025 male patients.
About 85% of male breast cancer is ER-positive, a proportion that is higher than female breast cancer patients (75%).
"That is a cancer type where patients usually fare better because we have a hormonal treatment," Shu said. "We have a lot of treatment options for that type of breast cancer. In theory, men should have better outcomes and have lower mortality as women do if the treatment is equally effective.
"The question is whether there are some other biological differences between ER-positive male breast and ER-positive female breast cancer. We don't know," Shu said.
Previous studies have shown that men may not be as compliant with hormonal treatments as women, Shu said. Other factors that may influence mortality rates among men could be lifestyle factors, such as smoking, alcohol consumption, physical inactivity, and obesity.
Clinical characteristics and undertreatments were associated with 63% of the sex-related mortality disparity.
"The bottom line is that we need more studies specifically focused on male breast cancer," Shu said.