Our study aimed to investigate the incidence, risk factors and time to occurrence of
malignancy in patients with
dermatomyositis (DM) and
polymyositis (PM). The electronic medical records of 1100 patients with DM and 1164 patients with PM were studied between January 2001 and May 2019.
Malignancies after
myositis were diagnosed in 61 (5.55%) patients with DM and 38 (3.26%) patients with PM. The cumulative incidence of
malignancies in patients with DM were significantly higher than patients with PM (hazard ratio = 1.78, log-rank p = 0.004). Patients with DM had a greater risk of developing
malignancy than those with PM at 40-59 years old (p = 0.01). Most
malignancies occurred within 1 year after the initial diagnosis of DM (n = 35; 57.38%).
Nasopharyngeal cancer (NPC) was the most common type of
malignancy in patients with DM (22.95%), followed by lung, and breast
cancers. In patients with PM, colorectal, lung and hepatic
malignancies were the top three types of
malignancy. The risk factors for
malignancy included old age (≥ 45 years old) and low serum levels of
creatine phosphokinase (CPK) for patients with DM and male sex and low serum levels of CPK for patients with PM. Low serum levels of CPK in patients with
myositis with
malignancy represented a low degree of muscle destruction/
inflammation, which might be attributed to activation of the PD-L1 pathway by
tumor cells, thus inducing T-cell dysfunction mediating immune responses in myofibers. A treatment and follow-up algorithm should explore the occurrence of
malignancy in different tissues and organs and suggested annual follow-ups for at least 5.5 years to cover the 80% cumulative incidence of
malignancy in patients with DM and PM.