Do Rheumatoid Arthritis and Lymphoma Share Risk Factors? A Comparison of Lymphoma and Cancer Risks Before and After Diagnosis of Rheumatoid Arthritis

Objective. Patients with rheumatoid arthritis (RA), in particular those with the most severe disease, are at increased risk of developing malignant lymphoma. Whether this increase is entirely a consequence of the RA disease and/or its treatment or is reflective of shared susceptibility to the two diseases remains unclear. We undertook this study to assess whether patients with RA are already at increased risk of lymphoma or of other cancers before the diagnosis of RA, and if the relative risk increases with time since RA diagnosis. Methods. Patients with incident RA (symptom duration <1 year) (n = 6,745) registered in the Swedish Early Arthritis Registry from 1997 through 2006 were identified. For each patient, 5 general population controls were randomly matched by sex, age, marital status, and residence (n = 33,657). For all study subjects, inclusion in the nationwide Swedish Cancer Register in 1958-2006 was determined. Relative risks (RRs) (with 95% confidence intervals [95% CIs]) of lymphoma and of cancer overall, before and after diagnosis of RA, were estimated using conditional logistic regression and Cox regression, respectively. Results. Before diagnosis of RA, there was no observed increase in the risk of lymphoma (RR [odds ratio] 0.67 [95% CI 0.37-1.23]) or other cancers (RR 0.78 [95% CI 0.70-0.88]). During the first 10 years following diagnosis of RA, the overall RR (hazard ratio) of lymphoma development was 1.75 (95 % CI 1.04-2.96). Conclusion. These findings indicate that overall, a history of cancer, including lymphoma, does not increase the risk of subsequent RA development. Shared susceptibility to RA and lymphoma may thus be of limited importance. In contrast, increased lymphoma risks were observed within the first decade following RA diagnosis.