Pneumonia diagnosis in childhood and incidence of leukaemia, lymphoma and brain cancer: a Danish nationwide cohort study

OBJECTIVES: There is an ongoing debate on the possible association between infections in early childhood and subsequent cancer risk, but it remains unclear if a hospital admission for infection is associated with risk of childhood cancer diagnosis. We examined if a hospital-based diagnosis of pneumonia was a clinical marker of the three most common childhood cancers. DESIGN: Population-based cohort study. SETTING: Denmark, hospital diagnoses, 1994–2013. METHODS: Using national health registries, we compared the observed incidence of leukaemia, lymphoma and brain cancer among 83 935 children with a hospital-based pneumonia diagnosis with that expected among children in the general population. We calculated absolute cancer risks and standardised incidence ratios (SIRs) as a measure of relative risk. RESULTS: The cancer SIRs were substantially increased during the first 6 months of follow-up; lymphoid leukaemia: 6.2 (95% CI 3.5 to 10.3); myeloid leukaemia: 14.8 (95% CI 6.0 to 30.6); Hodgkin’s lymphoma: 60.8 (95% CI 26.2 to 120), non-Hodgkin’s lymphoma: 15.9 (95% CI 5.2 to 37.2) and brain cancer: 4.4 (95% CI 1.9 to 8.7). The 6-month absolute risks of leukaemia, lymphoma and brain cancer were all low, reaching 0.05% when combined. An increased risk persisted beyond 5 years for non-Hodgkin’s lymphoma and brain cancer. However, the 5-year absolute cancer risk was 0.14%. CONCLUSIONS: The short-term incidence of leukaemia, lymphoma and brain cancer was higher than expected and persisted beyond 5 years for non-Hodgkin’s lymphoma and brain cancer. However, the absolute cancer risk was low.