Cancer-associated myositis in Chinese patients with idiopathic inflammatory myopathies: Malignancy spectrum, risk factors, and validation of the IMACS guideline.

OBJECTIVE: To characterize malignancy patterns in Chinese patients with idiopathic inflammatory myopathies (IIM) and evaluate the real-world applicability of the International Myositis Assessment and Clinical Studies Group (IMACS) cancer screening guideline. METHODS: This multi-centre retrospective study included adult patients with IIM from five tertiary referral centers in China between January 2010 and December 2024. Cancer-associated myositis (CAM) was defined as malignancy diagnosed within 3 years before or after IIM onset. Logistic regression analyses were performed to identify factors associated with malignancy, and a simplified refinement score was developed to assess its incremental value beyond the IMACS guideline. RESULTS: Among 1980 eligible patients with IIM, 176 (8.9%) had CAM. Lung cancer (18.2%), breast cancer (14.8%), and thyroid cancer (11.9%) were the most common malignancies. Overall, 76.1% of cancers occurred within 1 year before or after IIM onset. According to the IMACS guideline, 65.9% of CAM patients were classified as high-risk, whereas 65.4% of non-CAM patients were also assigned to the moderate-risk category. Multivariable analysis identified anti-TIF1γ antibody positivity, anti-SAE antibody positivity, elevated C-reactive protein, and elevated CA125 as independent risk factors, whereas interstitial lung disease was inversely associated with malignancy. A refinement score incorporating anti-SAE antibody positivity, elevated C-reactive protein, elevated CA125, and absence of interstitial lung disease further stratified malignancy risk within IMACS categories and improved discrimination compared with IMACS alone (AUC 0.762 vs. 0.699, P < 0.001). CONCLUSION: The IMACS guideline effectively identifies patients at increased malignancy risk, and a simple refinement score may further improve risk stratification.