Patients with rheumatoid arthritis (RA) are at increased risk for malignant lymphomas. Both conditions display a familial aggregation, and there are reports of RA and malignant lymphomas occurring in the same families. This study was undertaken to determine the risk of malignant lymphomas in first-degree relatives of RA patients, in order to investigate whether the increased risk of malignant lymphomas in RA could be due to genetic or environmental risk factors common to both conditions, rather than being a consequence of the rheumatic disease.
Using Swedish nationwide and population-based registers, we identified 76,527 patients hospitalized with RA in 1964–1999 and 70,290 first-degree relatives of a subset of these patients. These subjects were followed up for more than 3 decades, and information on cancer occurrence was recorded.
Patients with RA had a significantly increased risk of malignant lymphomas (535 cases; standardized incidence ratio [SIR] 2.00, 95% confidence interval [95% CI] 1.83–2.17), which was apparent for up to 2 decades of followup. Among the first-degree relatives without RA, no increased risk of malignant lymphomas was found overall, although modest and nonsignificantly elevated risk estimates were observed in subgroups. With respect to childhood cancer (0–14 years of age), we observed an increased risk of Hodgkin's lymphoma (5 cases; SIR 3.18, 95% CI 1.03–7.42).
Patients with RA are at a markedly, but possibly time-limited, increased risk for malignant lymphomas. There is little to suggest a prominent role for coinherited or common environmental risk factors in malignant lymphomas arising in the context of RA. Instead, lymphomas complicating RA appear to be a direct consequence of the inflammation or its treatment.