In the present analysis focusing on central nervous system tumours, we left censored the subscription date of individuals with a subscription before 1987 (1.8% of all subscribers) to 1 January 1987 because handheld handsets were introduced in Denmark only in 1987 and cranial exposure from car phones (available from 1982) is much lower than exposure from mobile phones held to the head.
In the previous follow-ups of the subscriber study information on socioeconomic factors was not available on an individual level, and the comparison of the average income of subscribers and non-subscribers showed higher average income in the subscriber group, thereby suggesting potential confounding.8 To overcome this limitation by obtaining access to individual data on socioeconomic factors, we conducted our study in the CANULI (“cancer og social ulighed”) cohort, a register based Danish cohort study conducted at the Institute of Cancer Epidemiology on social inequality and cancer.9 This approach of restricting the mobile phone study to the CANULI cohort has been applied previously.12 The CANULI study included all Danes born in Denmark in 1925 or later and alive in 1990 who did not emigrate from the country before 1 January 1990.
Sex specific incidence rate ratios estimated with log linear Poisson regression models adjusted for age, calendar period, education, and disposable income.
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Additional investigations showed that early male subscribers probably constituted a unique subgroup of people with a higher income and therefore a potentially distinct risk profile, particularly lower tobacco consumption.8We followed up the mobile phone subscriber study to 2007, with a focus on tumours of the central nervous system.
Longer follow-up increased the numbers of person years for subscribers, particularly in long term subscribers (≥10 years), in whom the total of person years under risk increased from 170 000 to 1.2 million.
Figure 2 shows the definition of the observation periods of collection of exposure data and follow-up for cancer outcome by age and calendar time in the study cohort.
We excluded from analyses any people with a history of cancer before entry into the study (except for non-melanoma skin cancer); this led to the exclusion of 3117 subscribers from analyses (370 of whom had cancer after their first subscription).
The CANULI cohort is therefore a nationwide cohort of all Danes aged 30 or older and born after 1925 in Denmark.