How comparable are rates of malignancies in patients with rheumatoid arthritis across the world? A comparison of cancer rates, and means to optimise their comparability, in five RA registries

background The overall incidence of cancer in patients with rheumatoid arthritis (RA) is modestly elevated. The extent to which cancer rates in RA vary across clinical cohorts and patient subsets, as defined by disease activity or treatment is less known but critical for understanding the safety of existing and new antirheumatic therapies. We investigated comparability of, and means to harmonise, malignancy rates in five RA registries from four continents. Methods Participating RA registries were Consortium of Rheumatology Researchers of North America (CORRONA) (USA), Swedish Rheumatology Quality of Care Register (SRR) (Sweden), Norfolk Arthritis Register (NOAR) (UK), CORRONA International (several countries) and Institute of Rheumatology, Rheumatoid Arthritis (IORRA) ( Japan). Within each registry, we analysed a main cohort of all patients with RA from January 2000 to last available data, and sensitivity analyses of subcohorts defined by disease activity, treatment change, prior comorbidities and restricted by calendar time or follow-up, respectively. Malignancy rates with 95% CIs were estimated, and standardised for age and sex, based on the distributions from a typical RA clinical trial programme population (fostamatinib). Results There was a high consistency in rates for overall malignancy excluding non-melanoma skin cancer (NMSC), for malignant lymphomas, but not for all skin cancers, across registries, in particular following age/sex standardisation. Standardised rates of overall malignancy excluding NMSC varied from 0.56 to 0.87 per 100 person-years. Within each registry, rates were generally consistent across sensitivity analyses, which differed little from the main analysis. Conclusion In real-world RA populations, rates of both overall malignancy and of lymphomas are consistent.