March 31 (HealthDay News) -- People with multiple sclerosis may have a lower overall risk of developing cancer, Swedish researchers report.
The study, which tracked the medical records of more than 20,000 MS patients for 35 years, "found that they had some 10 percent decreased overall cancer risk compared with those without the disease," said lead researcher Dr. Shahram Bahmanyar, from the Clinical Epidemiology Unit at the Karolinska Institute in Stockholm.
The study did find an increased risk among MS patients for certain malignancies, such as brain and bladder cancers.
Prior studies into the risk for cancer for people with MS has produced a hodgepodge of findings. The disease has been associated with a reduced risk for digestive, respiratory, prostate and ovary cancers, as well as non-Hodgkin lymphoma, but other studies have noted increased risks for urinary tract and nasopharyngeal cancers. To complicate matters even further, breast cancer risk has been reported as higher, lower or unchanged among those with MS, the researchers noted.
The new study sought to settle the matter. So, the Stockholm team compared the cancer risk in 20,276 people with MS to almost 204,000 people without the disease.
The reduction in cancer risk that they discovered was even more pronounced in women, according to the report, which appears in the March 31 issue of Neurology.
The trend wasn't all positive: The risk of developing brain tumors, plus bladder and other urinary cancers, was increased by up to 44 percent among MS patients compared to those without the illness, the researchers noted.
So, were genes that influence MS also keeping overall cancer rates down? "To determine whether there was a possible genetic link between multiple sclerosis and lower cancer risk, we studied the risk among parents of MS patients," Bahmanyar said. However, he and his colleagues "found that [the parents] did not have a notably increased or decreased overall cancer risk," suggesting that genes are not the causative factor.
Instead, the researchers speculate that the lower risk for cancer among people with MS could be due to other factors.
"It could be a result of lifestyle changes, such as having healthy diet and regular exercise," Bahmanyar said. "It is well-known that high body-mass index [BMI] is a risk factor for several types of cancer and people with MS have on average a lower BMI than the general population, so the lower body weight may explain some of the reduction in cancer risk."
In addition, MS treatment may play a role, Bahmanyar said. Finally, he said, "it is also possible that some reduction in cancer risk results from the way the body responds to this disease."
But one expert questioned the findings.
"I think one of the key criticisms of the study is the disparity of the groups in terms of the number of patients analyzed for cancer," said Dr. Abhijit Chaudhuri, a consultant neurologist at Queen's Hospital in Romford, U.K. "MS patients constituted 10 percent of the size of control population, and the lower incidence of systemic cancer in MS patients is very likely to represent a statistical error when in reality, none exists in clinical practice."
According to Chaudhuri, the only statistically significant observation was the higher risk of brain tumors in MS patients. "This is more likely to be true and meaningful, and the clinical experience supports the view that MS patients do have a higher risk of primary brain tumors," he said.
But Dr. William Sheremata, director of the MS Center at the University of Miami School of Medicine, believes the reduction in overall cancer risk may be a valid finding.
"This study does reflect the clinical experience of physicians that there is a decreased risk of malignancy," he said.
Sheremata said that he participated in a study where researchers found a 78 percent decreased risk of cancer among MS patients. "When we looked at the patient records, cancer occurred only in [MS patients who were] smokers," he said. Sheremata also wondered if MS treatments might be influencing cancer risks.
Patricia O'Looney, vice president for biomedical research at the National Multiple Sclerosis Society, said the reasons for the reduced risk of cancer remain unclear, and she was concerned that the study did not include data on the treatment MS patients received.
However, "the study serves as a reminder for keeping careful watch over one's overall health," she said.
SOURCES: Shahram Bahmanyar, M.D., Ph.D., Clinical Epidemiology Unit, Karolinska Institute, Stockholm, Sweden; Abhijit Chaudhuri, M.D., consultant neurologist, Queen's Hospital, Romford, U.K.; William Sheremata, M.D., professor, neurology, and director, MS Center, University of Miami School of Medicine; Patricia O'Looney, Ph.D., vice president, biomedical research, National Multiple Sclerosis Society, New York City; March 31, 2009, Neurology