Risk for malignant lymphoma in ankylosing spondylitis: a nationwide Swedish case-control study

Background: Several inflammatory conditions are associated with an increased risk of lymphoma. The specific features of inflammation that mediate this risk are unknown. There are few studies on whether ankylosing spondylitis increases the risk of lymphoma. Besides inflammation-lymphoma aetiology, information on risk of lymphoma in ankylosing spondylitis is particularly important as a benchmark in the evaluation of, for example, tumour necrosis factor inhibitors. Methods: The association between ankylosing spondylitis and malignant lymphomas overall, and separately for non-Hodgkin's lymphoma, Hodgkin's lymphoma and chronic lymphocytic leukaemia, was assessed in a nationwide, population-based case-control study of 50 615 cases of lymphoma and 92 928 matched controls by using prospectively recorded data on lymphomas from the Swedish Cancer Register (1964-2000) and data on pre-lymphoma hospitalisations for ankylosing spondylitis from the Swedish Inpatient Register (1964-2000). The odds ratios (ORs) associated with pre-lymphoma hospitalisation for ankylosing spondylitis were calculated using conditional logistic regression. Results: 23 (0.05%) patients with lymphoma and 41 (0.05%) controls had a pre-lymphoma hospitalisation listing ankylosing spondylitis, relative risk = 1.0 (95% confidence interval (CI) 0.6 to 1.7). The number of discharges and the mean latency between ankylosing spondylitis and lymphoma were similar in patients and controls. Analyses restricted to lymphomas diagnosed during the 1990s showed similar results (OR = 1.3, 95% CI 0.6 to 2.5, number of exposed cases/controls = 14/21). Conclusions: On average and in the absence of tumour necrosis factor inhibitors, patients hospitalised with ankylosing spondylitis do not appreciably show an increased risk of lymphoma.