The South Australian (SA)
myositis database has registered all patients with biopsy-proven inflammatory
myositis in SA from 1980 to 2009. We determined the incidence and associations of
malignancy in
myositis by linking this database with the SA
cancer registry. Standardized incidence ratios (SIR) for
malignancy were determined using the total SA population over the same time period, stratified by age and gender. The SIR for
cancer in the
myositis population (n = 373) was 1.39, p = 0.047. There was a trend towards an increased SIR in
dermatomyositis but no increased risk of
malignancy in
polymyositis or
inclusion body myositis.
Malignancies of the lung and prostate were the commonest and 28 % of
malignancies occurred within one year of IIM diagnosis. The odds of developing
cancer were significantly raised in the presence of a shawl sign, male gender, and in patients with overlap syndrome or
rheumatoid arthritis whilst
myalgia was a significant protective factor.
HLA-A28 allele was overrepresented in patients with
malignancy (11 vs 2 %, p = 0.006). Patients in SA with
myositis are at modestly increased risk for
malignancy. We report clinical and genetic risk factors allowing the identification of patients at greatest risk for
malignancy.