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Aircrew studies have the potential to inform on cancer risks from several occupational exposures: cosmic radiation, electromagnetic frequency radiation (in studies of cockpit crew), and circadian disruption due to night work and rapid transmeridian travel. Six years ago, a commentary1 in this journal queried whether there is more yet to learn about cancer from studies of aircrew cohorts and outlined some possible avenues for research, including the assembly of large cohorts from individual employers, evaluation of cancer types that are not strongly related to reproductive and other behaviours, and measurement of circadian disruption exposure in addition to cosmic radiation. The publication by Dreger and colleagues2 in this issue offers an opportunity to assess progress in understanding cancer risk among aircrew. Dreger and colleagues2 report on cancer mortality in a nearly 27 000-person cohort of cockpit and cabin crew from a large German airline, extending follow-up by 10 years. In this study, they estimated cosmic radiation doses through linkage to a national dose registry (since 2003) and by conducting the more challenging retrospective assessment required to estimate doses for cabin crew before 2003. They compared mortality to the general population of Germany and conducted dose-response assessment within the cohort for certain cancers. As noted for previous follow-up of the same cohort,1 3 4 large deficits in mortality for most causes were observed compared with the general population, in particular deficits of 16% to 73% for all-cause, all-cancer and lung cancer mortality, which suggests a continued strong healthy worker effect. Dreger and colleagues2 observed an excess of mortality from cancers of brain and colon and of melanoma of skin compared with the general population; however, positive associations were not seen between cosmic radiation and any studied outcome.
Some of the strengths of the new study include the lengthy added follow-up, low loss-to-follow-up (although, with a high emigration rate) and especially the extensive lengths taken to improve the early dosimetry for the cohort members (in particular, the cabin crew). Limitations include that the cohort is still quite young (80% still alive), and that only mortality was studied, which is less informative for several cancers, including melanoma and cancers of breast, colon and prostate. Lack of control for confounders remains a substantial limitation. This is especially problematic for outcomes (eg, breast cancer, melanoma) with other known risk factors that may be correlated with cosmic radiation dose or work as a member of aircrew and less so for outcomes with few known risk factors (eg, prostate and brain cancer). The main confounders include recreational sun exposure for melanoma and other skin cancers; reproductive history for breast and gynaecologic cancers; and, for several cancer types, smoking and body mass index (likely lower for aircrew) and alcohol consumption.
Studies of aircrew also exhibit key challenges in estimating occupational exposures: lack of individual flight history data for cabin crew in some studies necessitates group-level exposure assessment, such as use of era-specific and domicile-specific estimates.5 These sources of error (termed ‘Berkson error’) generally lead to greater imprecision but do not bias point estimates if the group means are measured accurately. For many of the cohorts in which long-haul flights predominate, there are also very strong (r>0.9) correlations of aircrew employment duration, night work (eg, as measured by travel during the standard sleep interval of 22:00–08:00), measures of circadian disruption (eg, number of time zones crossed) and cosmic radiation dose.2 5 6 This problem was previously noted by Schüz in the earlier commentary.1 Thus, it may be challenging to ascribe any positive findings to specific occupational exposures.
Another limitation concerns the use of the ‘effective dose’ as the main exposure metric in this and other recent studies.7–9 Effective dose is a derived radiation protection quantity that is poorly suited for epidemiological purposes, given additional uncertainty in tissue and radiation weighting that is used. This is especially true for high-linear energy transfer cosmic radiation, for which there has been some debate on radiation weighting. These assumptions may differ among studies and over time. Notably, recent empirical research on cosmic radiation dosimetry has suggested a radiation weighting factor of 2–2.5 for cosmic radiation measured on board aircraft.10 11 Lower radiation weighting may also imply that aircrew may experience lower doses, and the studies may have lower power, than originally anticipated. Furthermore, the cumulative dose distribution in most aircrew cohorts is fairly narrow (eg, in the study by Dreger and colleagues2 the IQR was 13–52 mSv), which also affects study power.
Where does the field go from here? Elevated melanoma rates among aircrew suggest opportunities for primary or secondary prevention through reductions in recreational sun exposure and enhanced screening efforts. Beyond this, given the evolving understanding of the radiation weighting factors for cosmic radiation, suggesting that dose estimates for aircrew may be lower than previously estimated, it may be especially important to evaluate the role of circadian disruption resulting from night work and rapid transmeridian travel in cancer hazard assessment. A recent review by the International Agency for Research on Cancer12 re-affirmed the classification of night shift work as “probably carcinogenic to humans” (Group 2A). The re-evaluation (which included evidence from aircrew studies) noted there is limited evidence from studies in humans for a causal association of night shift work with breast, prostate and colorectal cancer. Given the expansion of new cancer sites that may be related to night shift work exposure, researchers studying aircrew are encouraged to include these cancer sites in future studies. In particular, studies of cockpit crew (overwhelmingly male) should evaluate prostate cancer. Such studies should also emphasise cancer incidence, preferably evaluating effect modification by tumour subtypes (eg, ER/PR/HER2 status for breast cancer, tumour aggressiveness for prostate cancer and tumour location for colorectal cancer) and should collect information on important potential confounders. Ultimately, however, identifying and studying aircrew populations for which night work or rapid transmeridian travel may be separable from cosmic radiation exposure remains an elusive goal, as do recommendations to reduce cancer risk in the aircrew workforce.
The author thanks Robert D. Daniels for helpful discussions on issues related to aircrew dosimetry.
Contributors The sole author was responsible for conceiving and drafting this article in its entirety.
Funding Funding for this work was provided by the International Agency for Research on Cancer.
Disclaimer Where authors are identified as personnel of the International Agency for Research on Cancer/WHO, the authors alone are responsible for the views expressed in this article and they do not necessarily represent the decisions, policy or views of the International Agency for Research on Cancer/WHO.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; internally peer reviewed.
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