Aim  Hwang et al. aimed to evaluate the risk of malignancy among individuals with eczema, allergic rhinitis (AR) and asthma, compared with the general Taiwanese population.

Hypothesis  People with atopic conditions, including eczema, have an altered risk of malignancy.

Setting and design  This was a prospective nationwide cohort study. The authors used the Taiwanese National Health Insurance Research Database (NHIRD) to compare the incidence of cancers among people with established allergic disease relative to the risk in the general population.

Study exposure  Exposure was the presence of one or more atopic conditions (eczema, AR or asthma). Data were extracted on 997 729 randomly selected people registered on the NHIRD at any time point between 1996 and 2008. Eczema was identified via ICD-9-CM codes with the diagnosis being made by a dermatologist, paediatrician or allergist. Follow-up was until 2008, date of first cancer or death.

Outcomes  The outcome was a new diagnosis of malignancy, identified via catastrophic illness insurance certificates, again using ICD-9-CM diagnostic codes.

Primary outcome measure  Standardized incidence ratios (SIRs) for cancers overall and different types of malignancy among patients with eczema, AR or asthma were calculated against the expected number of cancer cases in the general population, adjusted for age and sex.

Results  The number of patients identified with eczema, AR and asthma was 34 263, 225 315 and 107 601, respectively. Overall cancer rates in patients with these conditions were not significantly different from those in the general population [SIR eczema = 0·97 (95% confidence interval 0·87–1·09), SIR AR = 1·02 (0·98–1·05) and SIR asthma = 1·01 (0·97–1·04)]. However, when the results for eczema were stratified by age, people aged between 20 and 39 years appeared to have a 56% increase in risk in relation to ‘any cancer’ [SIR = 1·56 (1·13–2·09)]. Looking at individual cancer types in patients with eczema, only the risk of brain cancer was significantly raised [SIR = 2·52 (1·15–4·79)]. Patients who had had all three allergic conditions had a reduced SIR for ‘cancers overall’ [SIR = 0·59 (0·37–0·88)]. This inverse association was less strong for those with eczema and asthma [SIR = 0·73 (0·55–0·97)] or asthma and AR [SIR = 0·79 (0·73–0·84)] and statistically only of borderline significance for those with eczema and AR [SIR = 0·85 (0·67–1·07)].

Conclusions  Hwang et al. conclude that the relationship between allergic diseases and cancer risk is complex and site specific. The risk of malignancy was highest in all atopic conditions in the 20–39-year age group. In patients with eczema, the incidence of brain cancer was higher than expected, which the authors note is at odds with previous studies. However, numbers were too small to allow stratification by histological subtypes. The authors warn against deriving conclusions for rarer cancers and that borderline SIRs must be interpreted with caution.