Danish cohort study: Questions regarding selection, exposure, and tumour incidence
In this era, the value of long-term, large cohort studies examining
any association between mobile phone use and primary brain tumour
incidence cannot be overstated. While the importance of the study by Frei
et al. [Ref. 1] is widely recognised, the authors' responses to the
following questions regarding participant selection, exposure and tumour
incidence would be appreciated:
1. SELECTION: To which subpopulation do the study's results actually
apply? It is plausible that mobile phone-using subpopulations such as (i)
corporate users (which might diversely include business executives,
government officials, telecommunication industry field workers, real
estate agents, media staff, lawyers and doctors whose subscriptions happen
to be on corporate accounts), (ii) children and adolescents, and (iii)
people with a family history of (i.e., genetic predisposition to) cancer
might be at a higher risk of developing a brain tumour following near-
field exposure to mobile phone electromagnetic radiation over a long-term
(e.g., 10-20 years). However, as indicated in Figure 1 of their study
[Ref. 1], all of these perhaps more tumour-predisposed subpopulations (in
total accounting for 206 174 or nearly 30% of the initial 723 421 eligible
records) were excluded from participation and/or statistical analysis. The
recording and analysis of data from these important subgroups could surely
have been expected to enhance our knowledge.
2. EXPOSURE: If actual phone records could not be obtained, why was
the amount of mobile phone usage in this large cohort not estimated or
extrapolated by some means? This omission is particularly important given
that the 13-nation INTERPHONE study [Ref. 2] found a significantly
increased risk of glioma among the highest decile (> 1640 hours) of
cumulative time that mobile phones were recalled as being used, a finding
that supported a preceding meta-analysis of brain tumour risk in long-term
(>= 10-year) mobile phone users [Ref. 3].
3. INCIDENCE: Does the greater than 10-fold increase in the number of
brain tumours among long-term subscribers over the additional 5 years of
follow-up between the authors' present and previous publications reflect
an actual increase in yearly tumour incidence within the cohort? The most
recent primary brain tumour incidence rates in Western populations are
reported to range from 11 [Ref. 4] to 19 [Ref. 5] primary brain tumours
per 100,000 person-years. In their previous publication [Ref. 6] with
follow-up to 2002, the authors reported 28 cases of brain tumours in long-
term subscribers. In their present publication [Ref. 1] with follow-up to
2007, they report 316 cases. What was the yearly incidence of primary
brain tumours in this cohort over the duration of follow-up?
[Ref. 1:] Frei P, Poulsen AH, Johansen C, et al. Use of mobile phones
and risk of brain tumours: update of Danish cohort study. BMJ 2011;343.
[Ref. 2:] INTERPHONE Study Group. Brain tumour risk in relation to mobile
telephone use: results of the INTERPHONE international case-control study.
Int J Epidemiol 2010;39:675-94.
[Ref. 3:] Khurana VG, Teo C, Kundi M, et
al. Cell phones and brain tumors: a review including the long-term
epidemiologic data. Surg Neurol 2009;72:205-14; discussion 14-15.
4:] Dobes M, Shadbolt B, Khurana VG, et al. A multicenter study of primary
brain tumor incidence in Australia (2000-2008). Neuro Oncol 2011;13:783-
[Ref. 5:] CBTRUS Statistical Report. Primary brain and central
nervous system tumors diagnosed in the United States in 2004-2007. Central
Brain Tumor Registry of the United States.
http://cbtrus.org/reports/reports.html (downloaded 22 October 2011).
6:]. Schuz J, Jacobsen R, Olsen JH, et al. Cellular telephone use and
cancer risk: update of a nationwide Danish cohort. J Natl Cancer Inst
Competing interests: No competing interests