The research question was supported by clear inclusion criteria relating to the intervention, outcome and study design. The literature search was limited to one electronic database and restricted to articles published in English. No attempt was made to locate unpublished material or to assess publication bias. This means that we cannot be certain that all relevant studies were included in the review. Two reviewers appeared to select primary studies for inclusion, although it is not known whether this process was independent. The authors do not report how the data were extracted or whether validity was assessed, thus it was not possible to assess the likelihood of reviewer error or bias being introduced at these stages. Very few participant or setting details were provided, making it difficult to assess the generalisability of the findings.
SIRs were calculated from event rates in the general population. Therefore, it might be useful to know more about the population on which these are based. The data were synthesised by meta-analysis and statistical heterogeneity was assessed. However, clinical differences between the studies, in terms of study design, treatment duration and length of follow-up exist, which may mean that a quantitative summary was not appropriate. In addition, very few lymphomas were identified (11 in 3,891 participants) and the rates were low in each study (range: 0 to 3).
The authors acknowledged that their results were based on observational studies and, as such, the true effect of treatment is uncertain. It was also unclear whether the increasing risk may be due to other factors, such as the severity of the underlying disease.