Authors’ reply to Khurana and to Philips and Lamburn

• Published in: BMJ 2011-12, Vol.343 (dec06 4), p.d7912-d7912
• Main Authors: Frei, Patrizia, Poulsen, Aslak H, Johansen, Christoffer, Olsen, Jørgen H, Steding-Jessen, Marianne, Schüz, Joachim
• Format: Article
• Language: English
• Subjects: Brain cancer; Cellular telephones; Subscribers; Subscriptions; Tumors
• ISSN: 0959-8138; 1468-5833; 1756-1833
• DOI: 10.1136/bmj.d7912

Khurana asks about the number of brain tumour cases in the cohort, exposure assessment, and cohort ascertainment. 1 Brain tumour cases-The 10-fold increase among long term subscribers since the previous update is a mixture of the ageing of the cohort and a major increase in person years under risk in subscribers of 10 or more years (1.2 million compared with 170000 person years 2 ) who were in the 5-9 years category of the last follow-up. Cohort ascertainment-Children are not included in the cohort because subscriptions are not in their name. [...]our group investigated the risk of brain tumours in children with a different design. 4 Corporate subscriptions not registered in the name of an individual could not be considered; the effect of this misclassification and its impact on risk estimation have been addressed in previous papers. 5 6 The expected dilution would not allow the detection of a small risk increase but shows or rules out moderate or large risks. To obtain information on education and income the current update is based on a linkage of the subscriber cohort and a nationwide cohort on social inequality in cancer; therefore the overall study population is smaller than the whole Danish population owing to the exclusion of some subjects, as described. 2 The percentage of the contribution of exposure to other radio frequency sources mentioned by Philips and Lamburn refers to far field sources and does not reflect exposure to the head. 9 Competing interests: