Since 1 April 1968, all Danish residents have been registered in the central population register.
At birth they are assigned a unique personal identification number that is used in all national registers, ensuring accurate linkage of information among these registers.9 This system enables researchers to conduct purely register based cohort studies for exposure data available from respective registries, as the follow-up of vital status, migration, and many health outcomes, in particular cancer, can be done by computerised linkage on an individual level with an exact calculation of person years at risk.
We had 3.21 million Danes for follow-up and the number of the mobile phone subscribers was reduced by 54 350 individuals compared with the previous design.
From the CANULI study we obtained information on highest attained education and disposable income from the population based Integrated Database for Labour Market Research9 from 1990 onwards.
Disposable income was calculated from household income after taxation and the number of people in the household, deflated according to the 2000 value of the Danish Crown.
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).
Entry into the CANULI cohort was at age 30 because younger people might have still been in the educational system.
Descendants of immigrants were not included, as they comprised a small and heterogeneous group, and information on their education, if acquired abroad, was not systematically recorded in the register.Results for long term mobile phone users (≥10 years) remain scarce, and all epidemiological studies are based on few cases.4 In addition, most studies have been retrospective case-control studies with self reported data on mobile phone use, which are prone to bias, particularly random reporting bias and differential recall bias for cases and controls, which hampers the risk estimation and precludes firm conclusions.5 6The only cohort study investigating mobile phone use and cancer to date is a Danish nationwide study comparing cancer risk of all 420 095 people who had signed a mobile phone contract with a phone company (subscribers) from 1982 (the year such phones were introduced in Denmark) until 1995, with the corresponding risk in the rest of the adult population with follow-up to 19967 and then 2002.8 The study found no evidence of any increased risk of brain or nervous system tumours or any cancer among mobile phone subscribers.There was, however, a decreased risk (standardised incidence ratio 0.66, 0.44 to 0.95) of developing a tumour of the brain or nervous system in people who had had a subscription for more than 10 years, but this result was based on only 28 cases.8 In addition, it was observed that male mobile phone subscribers were at a lower risk (standardised incidence ratio 0.88, 0.86 to 0.91) of developing tobacco related cancers.Information on cancer diagnosis was available from the Danish Cancer Register, which provides accurate and virtually complete nationwide ascertainment of cancers since 1943, including benign tumours of the central nervous system.13 Cancers were classified according to a modified Danish version of ICD-10 (the international classification of diseases, 10th revision).14 Topography and morphology were categorised according to the first revision (ICD-O1) (until 2003) and third revision (ICD-O3) (2004-2007) of the international classification of diseases for oncology.15 Date of birth, sex, date of emigration, or date of death were available for each cohort member from the Danish central population register.For the present analysis, follow-up for the occurrence of cancer started at age 30 or 1 January 1990, whichever occurred later, and ended on the date of first diagnosis of cancer (except for non-melanoma skin cancer), date of death, date of emigration, or 31 December 2007, whichever came first.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.