Background Emerging evidence links inflammation and immune competence to cancer progression and outcome. Few studies addressing cancer survival in the context of rheumatoid arthritis (RA) have reported reduced survival without accounting for the underlying mortality risk in RA. Whether this increased mortality is a cancer-specific phenomenon, an effect of the decreased lifespan in RA or a combination of both remains unknown.

Methods Using Swedish register data (2001–2009), we performed a cohort study of individuals with RA (N=34 930), matched to general population comparators (N=169 740), incident cancers (N=12 676) and deaths (N=14 291). Using stratified Cox models, we estimated HRs of death associated with RA in the presence and absence of cancer, by stage and time since cancer diagnosis, for all cancers and specific sites.

Results In the absence of cancer, RA was associated with a doubled mortality rate (HR=2.1, 95% CI 2.0 to 2.2). In the presence of cancer, the relative effect of RA on mortality was varied by stage. For cancer (tumour, node, metastases) stages I and II at diagnosis, the relative effect of RA on mortality was the same as in the absence of cancer. For cancers diagnosed at advanced stages with absolute higher mortality, the effect decreased (HR=1.2, 95% CI 1.1 to 1.3). These associations remained across time since cancer diagnosis and were reasonably similar across cancer sites.

Conclusions Much of the increase in mortality in patients with RA diagnosed with cancer seems to reside with effects of RA independently of the cancer.