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In a nested case-control study on breast cancer among airline cabin attendants, Kojo and colleagues1 state this policy implication: “There is no need to take occupational factors into account in breast cancer prevention among cabin attendants”. With respect to breast cancer risk among cabin attendants, Kojo et al have not shown the absence of effect of an occupational exposure. There is a well known definition of a negative study, but the study of Kojo et al does not fit into that definition. A true negative study must be large and sensitive, and it must have accurate exposure data.2 The study is lacking in these aspects. The material contains 27 cases. Information on exposure was collected retrospectively and therefore 8 cases (18%) and 24 non-cases (2%) were deceased. Thus proportionally more cases than non-cases were lost because of high mortality due to the disease under study. Furthermore, there was 52% participation rate in the questionnaire survey on exposure.3
In attempt to evaluate possible bias because of poor participation, Kojo et al calculate the odds ratio for breast cancer for all subjects in the cohort with known start and end of cabin work. They calculate active work year and combine this with the estimated mean annual cosmic radiation dose by calendar period3 to obtain a crude estimate of cosmic radiation dose for every person, explained in the Methods.1 Not surprisingly they get a similar odds ratio in the matched case-control study as in the analysis when they calculate the odds ratio using all subjects in the cohort in the Results.1 Both exposure estimates involve information from the questionnaire survey with the poor participation rate. However, this does not keep the authors from concluding the similarity of the two odds ratio in the Discussion.1 Here we seem to be facing a phenomenon called arguing in a circle. Arguing in a circle occurs when two or more unproved propositions are used to establish each other.
Kojo et al excluded cabin attendants who had worked for less than two years; this appears not to have involved cases. This exclusion introduces bias towards the null hypothesis. The range of radiation exposure is shown in the paper: 0–103.5 mSv for women 50 years of age and younger, and 0–136.8 mSv for women over 50 years of age. It is rather confusing to see that the range goes down to zero, given that cabin attendants with a career of less than two years were excluded.
Kojo et al extended the follow up of their cohort in the cancer registry; the increased incidence of breast cancer persisted. They suggest the breast cancer risk to be related to known risk factors of breast cancer such as family history of breast cancer. The authors do not put this conclusion in the light of former studies of cabin attendants, thus ignoring a benevolent recommendation given long ago.4
We thank Dr Rafnsson for valuable comments on our paper. Rafnsson finds our policy implications surprising. In the light of present evidence, we do not find further measures justified for reducing radiation exposure among cabin crew. The justification for this view is the fact that exposure limits common for all radiation workers also apply for the cabin crew. Dose monitoring indicates that the cosmic radiation doses are within the exposure limits. We see no reason to depart from the general radiation protection principles.
Cohort studies have shown an excess risk of breast cancer in cabin crew, in particular among those with long employment, with 1.5–3.4-fold incidence compared with the general population.1 Nevertheless, the radiation doses received are low and the expected effect based on previous literature is very small, with relative risk well below 1.1.2 Neither previous studies nor our study have been able to identify the cause for the excess incidence of breast cancer. Lack of association in our study does not exclude the contribution of cosmic radiation in the development of breast cancer, but it implies that other risk factors are likely to have a greater role.
Rafnsson finds our approach to occupational radiation dose estimation crude. For cabin attendants, the only available source of information on the number of flights during their career is the cabin attendants themselves; thus the questionnaire approach in exposure assessment was appropriate. We used self-reported numbers of flights by route and calendar period. We feel this is an improvement compared to previous studies,3–5 none of which have had any estimates of the individual cosmic radiation dose. They have been based on surrogate indicators such as length of employment or flight route type assignment.
Rafnsson claims that we did not consider the possibility that breast cancer can influence the subjects’ answers. Recall bias is intrinsic in all case-control studies with subjects as source of information; the issue was discussed in our paper.
We excluded cabin attendants who worked for less than two years because they had negligible exposure (due to very short period of cabin work). In addition, several studies have shown that short term employees differ in terms of mortality and cancer risk from those with more stable employment. Therefore, they are commonly excluded from occupational cohorts.
Our study has shortcomings inherent to retrospective case-control studies and to sparse data. Therefore, it cannot provide conclusive evidence but does nevertheless supply new information. A large prospective follow up study with a large data set would be valuable. Currently, a retrospective study, combining all the Nordic cabin crew cohorts with comprehensive cancer incidence registration systems and improved dose estimation algorithm is ongoing, and may be able to provide further insight to the issue.