Associations between Cancer and Inflammation of the Muscles

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The summary below is from the full report titled "Incidence of Malignant Disease in Biopsy-Proven Inflammatory Myopathy. A Population-Based Cohort Study." It is in the 19 June 2001 issue of Annals of Internal Medicine (volume 134, pages 1087-1095). The authors are R Buchbinder, A Forbes, S Hall, X Dennett, and G Giles.

What is the problem and what is known about it so far?

The muscles of the body sometimes become inflamed, resulting in symmetrical weakness of the legs and arms. (When the cause of inflammation is not known, it is referred to as idiopathic.) This weakness is unrelated to exercise and may be accompanied by muscle aching or tenderness. The two main types of idiopathic muscle inflammation are known as polymyositis and dermatomyositis. Skin lesions or rashes often accompany dermatomyositis. Since the early 1900s, researchers and clinicians have noted that the risk for cancer seems to be increased in people with idiopathic inflammatory muscle disease compared with the general population. Just how great the risk for cancer is in people with inflammatory muscle disease has not been clear, however.

Why did the researchers do this particular study?

To describe as exactly as possible the risk for cancer in people with inflammatory muscle diseases.

Who was studied?

The study included 537 patients living in Australia who had newly diagnosed inflammatory muscle disease that was proved by biopsy. Their ages ranged from 1 to 89 years, and 55% were women.

How was the study done?

Researchers reviewed files from a central pathology service located in Victoria, Australia, to identify all new cases of biopsy-proven idiopathic inflammatory muscle disease from 1981 to 1995. They also reviewed records up to 1997 from the Victorian Cancer Registry to identify patients in whom cancer had been diagnosed. Researchers then determined how often cancers were found among patients with inflammatory muscle disease compared with how often cancers were seen in the general population.

What did the researchers find?

Of the 537 people with inflammatory muscle disease, 104 had cancer. In approximately one third of these patients, the cancers were identified several months to years before muscle disease was diagnosed. Cancer was found at the same time as or later than muscle disease diagnosis in the remaining two thirds of patients. Compared with the general population, people with dermatomyositis had a sixfold higher risk for cancer, and people with polymyositis had a twofold higher risk.

What were the limitations of the study?

This biopsy-based study may not be representative of all patients with inflammatory muscle diseases because some patients with those diseases may not be referred for biopsy, and because biopsies may not always show such muscle disease even when it is present ("false-negative" biopsies). Also, the study did not describe how cancer was diagnosed in patients with inflammatory muscle disease and did not indicate whether patients found to have inflammatory muscle disease should undergo multiple tests to look for cancer.

What are the implications of the study?

People with idiopathic inflammatory muscle diseases, such as dermatomyositis or polymyositis, are clearly at increased risk for cancer.