Ankylosing spondylitis, psoriatic arthritis, and risk of malignant lymphoma: a cohort study based on nationwide prospectively recorded data from Sweden.

OBJECTIVE: Data on lymphoma risk in ankylosing spondylitis (AS) and psoriatic arthritis (PsA) are scarce. This study was undertaken to assess the risk of lymphoma in AS and PsA overall and in relation to therapies, including tumor necrosis factor inhibitor (TNFi), for which lymphoma risks are a concern.
METHODS: Through the Swedish National Patient Register we assembled nationwide prevalence cohorts of patients with AS (n = 8,707) and patients with PsA (n = 19,283) for whom data were obtained between 2001 and 2010. Each cohort member was matched to 5 population comparator subjects. Linkage with the nationwide Cancer Register identified all lymphomas recorded from 2001 to 2010. Through the Swedish Biologics Register (Anti-Rheumatic Therapy in Sweden [ARTIS]), we identified patients exposed to TNFi in the AS cohort (n = 1,908) and the PsA cohort (n = 2,605) before lymphoma diagnosis. Hazard ratios (HRs) for lymphoma were estimated by Cox regression. Crude incidences of lymphoma in TNFi-exposed and TNFi-naive patients were compared.
RESULTS: For AS patients, the HR of having lymphoma versus the general population was 0.9 (95% confidence interval [95% CI] 0.5-1.6) (14 lymphomas). For PsA patients, the corresponding HR was 1.2 (95% CI 0.9-1.7) (45 lymphomas). For PsA patients treated with methotrexate and/or sulfasalazine, the HR of having lymphoma was 1.7 (95% CI 1.0-3.1). The numbers and incidence of lymphoma were not materially different in TNFi-exposed versus TNFi-naive AS and PsA patients, although the numbers of lymphomas were small.
CONCLUSION: In contrast to rheumatoid arthritis, the average risks of lymphoma in AS or PsA are not elevated, although increased risks in a subset of PsA patients cannot be excluded. Our findings indicate that TNFi does not affect the risk of lymphoma in AS or in PsA.