Association with malignancy
Nearly one-third of adult myositis patients additionally have some type of associated cancer, observed most commonly in dermatomyositis patients. The relationship between cancer and myositis is generally considered a paraneoplastic phenomenon; that is, in one way or another, the two conditions are related. A parallel course of the diseases—myositis improves after cancer is cured and when cancer recurs, myositis worsens—is considered a classical paraneoplastic criterion. Nevertheless, this parallel clinical course does not always occur in true cancer-associated myositis, or perhaps it cannot be seen because of the interference of therapy. Based on epidemiologic studies, the current gold standard for the diagnosis of cancer-associated myositis is a temporal criterion: diagnosis of the two diseases within a 3-year period, although to a certain extent, this is an arbitrary standard.