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Rafnsson et al 1 reported an excess in malignant melanoma especially among commercial airline pilots flying over five time zones. The authors hypothesised that a disturbance of the circadian rhythm might be responsible for this increased risk and suggest that circadian rhythm should be taken into consideration in future studies. The authors are well aware of the problems of their study that included only limited numbers of malignant melanoma (n = 5).
We recently performed a cross sectional case-control study on possible risks of nightshift work on the development of malignant melanoma among women in South Germany.
In this interview survey we included 137 cases with histologically diagnosed malignant melanoma from outpatients at the Department of Dermatology and Allergy at the Ludwig-Maximilians-University of Munich, Germany. Also, 137 female outpatients and inpatients of the Department of Surgery at the same university without previous diagnosis of cancer were interviewed.
Besides ever working in nightshifts* and factors related to shiftwork (sleep during night shifts*, lightning at the workplace*, duration of shiftwork*) well established risk factors for the development of malignant melanoma (number and size of nevi, skin type, number of sunburns in childhood (<15 years of age), use of artificial exposure to ultraviolet (UV) light*, occupational and private history of exposure to sunlight*, relatives with skin cancer) were assessed.
Among the cases, 9.5% had ever worked in nightshifts whereas 16.1% of controls had ever been working in night shifts (p=0.10, χ2). The median duration of nightshift work was 6 years for cases (range 1–34 years) and 13.5 (2–33) years for controls. After adjusting for known risk factors confirmed in univariate analyses (skin type, smoking, >50 nevi, nevi >5 mm, relatives with skin cancer, use of artificial UV light) the odds ratio (95% confidence interval (95% CI)) for nightshift work was 0.48 (0.19 to 1.22). Therefore, we could not confirm an increased risk for the development of malignant melanoma by disturbance of the circadian rhythm among this randomly selected group of women in South Germany. To increase the power a case-control survey with more cases and controls is now going to performed.
Authors' reply—Referring to our article,1-1 Radon et al have contributed to the discussion of the significance of disturbance of the circadian rhythm for the risk of malignant melanoma of the skin. We welcome this discussion. Their preliminary results of a case-control study, although it is exclusively about women, does not support the hypothesis that disturbance of the circadian rhythm leads to malignant melanoma after adjusting for different confounding factors. This study may be too small to be considered a negative study; however, the authors state that attempts are being made to increase its power.
Those pilots in our study1-1 who experienced travel though five time zones and thus possible disturbance of the circadian rhythm also had the greatest exposure to cosmic radiation. We should like to draw attention to other possible aetiological factors contributing towards our finding, and those of others, of an increased risk of malignant melanoma among pilots.1-1-1-6 Cohort studies of pilots may be limited due to detection bias, and frequent medical examination of pilots may lead to earlier diagnosis of cancers. Despite that, the studies of pilots uniformly indicate increased risk for malignant melanoma. A documentation of pilots' sunbathing habits and exposure to ultraviolet (UV) radiation, is needed as the importance of the cosmic radiation seems to be increasing. A recently published study of cancer incidence among Norwegian airline pilots1-2 is the first study to show a clear dose-response relation between cumulative block hours and risk of malignant melanoma; and cumulative ionising radiation (mSv) and risk of malignant melanoma. No increased trend was found for rates of non-melanoma skin cancer with radiation dose. These results were interpreted with great caution and the authors concluded that UV radiation other than in the cockpit1-7—that is, experienced during leisure time—seemed to be a more likely explanation for the increased risk for malignant melanoma, than conditions at the workplace.
Cohort studies on skin cancer are complicated due to the fact that these cancers have good survival (handicapping mortality studies) and skin cancer is assumed to be underreported to cancer registries (introducing different problems). However, this is not of concern in the preliminary case-control study on shift workers. Nevertheless, the risk of basal cell and squamous cell carcinoma of the skin has been found to be increased by occupational and therapeutic radiation1-8 and among survivors of the atomic bomb in Japan.1-9 There are some indications that the skin susceptibility to radiation carcinogenesis also involves increased risk of malignant melanoma1-8; the survivors of the atomic bomb had, for malignant melanoma, a relatively high point estimate based on only 10 cases, and thus there was a wide confidence interval.1-9 This may, however, be important as the incidence of malignant melanoma is low in the Japanese population compared with European populations.1-10
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