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Ten years after breast cancer diagnosis, women with multiple sclerosis (MS) had higher mortality than women without MS, a Canadian case-control study showed.

MS was associated with a 28% increased hazard for all-cause mortality (HR 1.28, 95% CI 1.08-1.53), but not with cancer-specific survival (HR 0.98, 95% CI 0.65-1.46), reported Ruth Ann Marrie, MD, PhD, of the University of Manitoba in Winnipeg, and colleagues in .

"Although multiple sclerosis and its complications remain the most common cause of death in people with MS, cancer is the second or third most common cause of death," Marrie said in a statement. "Our study looked at whether survival rates for women after a breast cancer diagnosis were different for those with MS and those without it."

Earlier Canadian research showed the incidence of breast and colorectal cancers was similar between people with and without MS. Breast cancer incidence is of particular interest since several cases of in ocrelizumab (Ocrevus) phase III clinical trials. The drug, which was approved for MS in 2017 in the U.S., carries a .

In their study, Marrie and colleagues identified female MS patients in population-based administrative data in Manitoba and Ontario from 1994 through 2016, linking the MS cohorts to cancer registries. For each woman with MS and breast cancer, the researchers selected four breast cancer controls without MS, matched on birth year, cancer diagnosis year, and region.

The analysis included 779 MS cases (691 from Ontario and 88 from Manitoba) and 3,116 controls with a mean age of 58 at diagnosis. Most women were diagnosed with stage I or II breast cancer. About half of the study group was diagnosed in 2010 or later, and most lived in urban areas.

The primary outcome was overall survival after breast cancer diagnosis truncated at 10 years, based on the assumption that deaths 10 years after diagnosis were less likely due to breast cancer and more likely due to other health conditions.

After adjusting for age at cancer diagnosis, cancer diagnosis period, socioeconomic status, region, and , MS was associated with an increased hazard for death due to any cause, but not with an increased hazard of cancer-specific death. When the analysis was stratified by cancer stage at diagnosis, point estimates for associations between MS and death were similar.

In the Ontario group, disability status was tied to earlier death (HR 3.26, 95% CI 2.24-4.74). Having MS was not associated with cancer-specific survival after adjusting for disability status.

"Survival from breast cancer, thankfully, remains very good overall," noted Robert Bermel, MD, of the Cleveland Clinic in Ohio, who wasn't involved with the study. "These data should be placed in the context of similar studies from other countries, some of which showed a lower mortality in MS patients with breast cancer," he told MedPage Today.

"One of the factors explaining greater mortality appeared to be neurological disability, which was strongly associated with earlier death," Bermel added "This study is an important step in helping us understand how MS affects overall health and underscores the importance of treating MS to reduce the likelihood of physical disability."

The study was not able to include information about race or ethnicity, factors associated with differential breast cancer survival, Marrie and colleagues said. Some relevant comorbidities may have been missed, they added, and how MS disease-modifying therapy may have influenced cancer survival could not be determined.

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