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Incidence of breast and colorectal cancers was similar between people with and without multiple sclerosis (MS), but MS patients had a higher incidence of other cancers, including bladder cancer, according to a population-based study.

Among 54,000 persons with MS and 267,000 people without the disease, risk of breast cancer (HR 0.92, 95% CI 0.78-1.09) or colorectal cancer (HR 0.83, 95% CI 0.64-1.07) did not differ, reported Ruth Ann Marrie, MD, PhD, of the University of Manitoba in Winnipeg, Canada, and colleagues in .

"This is good news for people with MS, because earlier studies have shown a link between MS and breast and colorectal cancers," Marrie said in a statement. "While we did not find that link, our study did show that people with MS had a 72% greater risk of developing bladder cancer."

"The increased risk of bladder cancer in people with MS may have to do with the fact that people with the disease are more likely to have urinary tract infections and use catheters," she added. "However, more research is needed to confirm our findings."

In 2015, a found that cervical, breast, and digestive system cancers had the highest incidence in MS, but findings about relative risks of cancer among MS patients were inconsistent. "Studies published since 2014 continue to report disparate findings regarding cancer risk in MS," Marrie and colleagues wrote.

Incidence rates are needed to support pharmacovigilance in clinical trials of disease-modifying therapies (DMTs), they added: at times, the only way to determine whether a number of cancer cases is excessive is by comparing it to MS-specific incidence rates. Breast cancer incidence is of particular interest since several in ocrelizumab (Ocrevus) phase III clinical trials. The drug, which was approved for MS in 2017 in the U.S., carries a .

In their analysis, the researchers identified MS cases in Manitoba and Ontario and matched each case to five controls without MS on birth year, sex, and region, linking the cohorts to cancer registries. Records in Manitoba were from 1984-85 to 2017-18; in Ontario, they were from 1994-95 to 2017-18.

In total, the researchers evaluated 53,984 MS cases and 266,920 controls; 70% of both groups were female.

Neither cancer incidence nor mortality differed between MS patients and controls for breast and colorectal cancer. This was consistent across two time periods, from 1998 to 2007 when DMTs including interferon-beta and glatiramer acetate (Copaxone) were first introduced for MS, and 2008-2017 when second-generation DMTs including natalizumab (Tysabri), dimethyl fumarate (Tecfidera), fingolimod (Gilenya), teriflunomide (Aubagio), and alemtuzumab (Lemtrada) were used.

From 2008-2017, the incidence of bladder cancer was 25.7 cases per 100,000 person-years among MS patients and 14.60 cases among controls (IRR 1.72, 95% CI 1.28-2.30).

Ovarian cancer incidence was elevated in the MS cohort from 2008-2017 (IRR 1.50, 95% 1.01-2.24). Cervical and uterine cancer incidences were lower among MS cases than controls. Central nervous system cancer incidence consistently was elevated in MS patients during 1998-2007 and 2008-2017, even after excluding meningiomas from the analysis (2008-2017 IRR 2.14, 95% CI 1.49-3.07).

The findings "add to our knowledge, helping to provide background rates that can inform how we monitor patients and choose treatments," said Robert Bermel, MD, of the Cleveland Clinic in Ohio, who was not part of the study.

"The data on bladder and ovarian cancers underscore the importance of monitoring patients with MS for symptoms suggestive of these diseases," he told MedPage Today. "When questions arise about whether specific MS treatments may increase the risk of a certain type of cancer, we rely on studies such as this for establishment of the disease-specific risk from which to plan additional studies and for guidance to patients."

Though Marrie and colleagues adjusted data for comorbidities, they couldn't incorporate health behaviors like smoking, diet, and physical activity into their analyses. They also couldn't determine how clinical MS characteristics -- a relapsing onset versus a primary progression course, for example -- were linked with cancer incidence. Prescription data were not available in Ontario and the researchers were unable to assess how DMT use may be related to cancer.

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