A search strategy consists of the keywords and databases used. For this systematic review, a comparison of several important databases was carried out and the publications identified were evaluated for the relevance of the topics identified.
Keywords
Both intuitively relevant terms and MeSH (Medical Subject Headings) in PubMed were used as keywords. For exposure, the MeSH terms were “cellular phone”, “radio frequencies” and “electromagnetic fields”, but “mobile phone”, “radio waves” and “cell phone” were also used. To assess the impact of different words for telephone both “phone” and “*phone” were tried and gave identical numbers of hits. The keyword “telephone” give substantially fewer hits and was taken as included in “*phone”. As outcome parameter the MeSH was
“neoplasms”, but “tumour” and “cancer” were also used. These keywords individually resulted in different numbers of hits, therefore they were all included in the search strategy. For methodology “epidemiology” (a MeSH term) and “exposure assessment” plus “dosimetry” were added.
Databases
Initially both PubMed Central and PubMed were searched and compared for the number of hits. Since PubMed appeared a broader database then PubMed Central, only PubMed was used.
Searches
Initial searches were performed in the week of 20 July 2009 and fully repeated on 15 August 2011. The results of the search from August 2011 are presented in tables B.1 and B.2, where the number of hits for the different keywords and combinations of keywords is given.
Using the combinations of the search terms that were evaluated, a combined search was conducted. The combined search used the terms:
cellular phone* OR mobile phone* OR cell phone* OR radio waves OR electromagnetic fields OR radio frequency AND human AND (tumour OR cancer OR neoplasms) AND (epidemiology OR dosimetry OR exposure assessment).
There were no restrictions on years, language or any other placed on this search. This resulted in 2083 hits.
Based on title 420 papers were identified as possibly of interest. The rest was discarded as animal or cell studies (73), extremely low frequency fields (339), radio- or tv- or GSM masts (11), SAR (8), ionising radiation or therapy (583), using mobile phones as research tool (17), other (76), and tumours not in head or brain (537).
The 420 remaining papers were evaluated using the abstracts. This resulted in 76 publications on original studies of interest, 108 editorials, 68 reviews and 14 of potential interest as theory forming papers. The rest was discarded as animal or cell studies (2), extremely low frequency fields (54), tumours not in head or brain (12), ionising radiation or therapy (27), language not English, French, German or Dutch (16), not mobile phones (43).
The resulting list of 76 publications was evaluated using full text publications. Thirty-three were set aside as validation studies (6), supporting papers (14), meta-analyses (2), not mobile phone studies (7), not tumour studies (3) and case study (1).
The remaining 41 publications were checked for completeness by an expert (EvR) and compared to the reference lists of recent reviews as well as searching for other publications by the main authors. This identified a further 27
Table B.1 PubMed search results in number of hits per single or two-term combination.
solo + H + T + C + N + Epi + EA + D Cellular phone* 2639 2247 314 343 2914 367 138 266 Mobile phone* 1799 1366 203 230 1768 266 177 192 Cell phone* 716 551 65 80 61 95 12 24 Radio waves 15322 7098 2230 2228 2065 402 204 1279 Electromagnetic fields 14720 8381 1843 1890 1657 1044 557 1570 Radio frequency 7779 4538 1743 1780 1644 1863 123 399 Human (H) 12116038 - - - - Tumour (T) 2568569 2111767 - - - 269665 5566 23874 Cancer (C) 2537766 2075920 - - - 280169 6737 25520 Neoplasms (N) 2274624 1918981 - - - 263885 5121 22233 Epidemiology (Epi) 1391216 - - - - Exposure Assessment (EA) 37814 - - - - 9300 - - Dosimetry (D) 106491 - - - - 5080 - - * operator term allowing for plural + : operator term “AND”;.?????
Table B.2 PubMed search results in number of hits per multiple term combination. Cellular
phone*
Mobile phone*
Cell phone* Radio waves Electromagnetic fields Radio frequency + T/C/N 362 249 84 2361 2040 1854 + H + T 276 178 60 1839 1565 1591 + H + C 314 202 72 1842 1598 1612 + H + N 266 167 59 1748 1446 1545 + H + T/C/N 321 208 73 1915 1691 1649 + H + Epi) 362 253 93 366 998 1754 + H + EA 125 101 9 160 483 113 + H + D 198 151 18 670 940 286 + H + Epi + T 149 98 24 210 667 979 + H + Epi + C 157 101 29 213 692 995 + H + Epi + N 149 97 24 210 663 973 + H + Epi + T/C/N 157 101 29 213 694 999 * operator term allowing for plural + : operator term “AND”, / : operator term “OR”, ????
searching needs to include a snowballing component and cannot solely rely on protocol-driven search strategies, as has also been observed by others.180
This resulted in a total of 85 publications on original or pooled studies that were to be analysed. A complete list of all publications identified at any of the stages of the search is available upon request. The full flow of searches, decisions and numbers is presented in Figure 1 in the main text of this report. Duplicate publications
It can be argued that overlapping publications should be excluded to avoid double counting and overweighting limited evidence. However, this would exclude potentially important evidence, so pooled and overlapping evidence is included in the extracted papers with due recognition of the problem. After the extractions were performed, a selection of papers was made that were used for presenting unique results. In some cases information on e.g. methods of numbers of cases and controls was obtained from related publications, but the data on odds ratios was taken from one single publication per study, to avoid overweighting.
Updating search
As there were many ‘later identified’ publications and the whole process took a long time, an update search was conducted on 10 July 2012 with a limited timeframe starting 01 January 2011. This confirmed the identification of 15 ‘later’ publications which were all included in the evaluation process taking the total of publication evaluated to 85.
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Data extraction
Table C1 was used for the extraction of data from the selected studies.
Table C1 Data extraction items.
Reference no. for extraction; file no. EMFcommittee, Reference no. document 1st Author (Year)
Title (short) GENERAL
A 1 Why was the study done?
2 What were the prior hypotheses, if any? 3 What hypotheses were actually tested? B 1 What type of study was done?
2 Was this design appropriate to the study question? 3 How might some other design have been better? 4 What was the follow-up period?
5 Was the follow-up period relevant to the study questions? C 1 How was the size of the study population determined?
2 How might some other size have been better?
3 Was a power based assessment of adequacy of sample size done? D 1 How was the ratio case/controls or exposed/non–exposed determined?
2 How might some other ratio have been better?
E 1 What would be possible ethical issues in relation to the design and conduct of this study? 2 Was the study cleared by an ethics committee?
DATA COLLECTION
F 1 What was the source of the subjects? 2 How might another source have been better?
G 1 What were the response rates?
2 Were the response rates adequate for interpretation of the results? 3 What were the final numbers in the study?
4 What was the percentage of follow-up?
5 Was the follow-up percentage adequate for interpretation? 6 What was the follow-up no. of years?
H 1 Could there have been selection bias?
2 What was the likely effect of selection bias on the data if identified? I 1 Could there have been responders bias?
2 What was the likely effect of responders bias on the data if identified? J 1 Could there have been information bias?
2 What was the likely effect of the information bias on the data if identified? K 1 Could there have been observation bias?
2 What was the likely effect of the observation bias on the data if identified? L 1 What confounding bias was possible?
2 Were the confounders measured?
M 1 Could there have been misclassification bias? 2 What were the sources of misclassification bias? N 1 How was exposure measured?
2 Would other exposure measures have been better?
3 Do exposure measures reflect person-dose or population-dose? 4 What was the exposure among the controls?
5 Can the exposure measures allow for a dose gradient measure? O 1 Was there a major influence of measurement error?
P 2 Was there a major influence of random error? ANALYSIS
Q 1 What were the methods used to control confounding bias? 2 Would other methods have been better?
R 1 What were the methods used to measure the association between exposure and disease? 2 Would other methods have been better?
S 1 What were methods used to measure the stability of the association between exposure and disease?
2 Would other methods have been better?
T 1 Was there internal consistency among the data presented in the paper? INTERPRETATION
U 1 What were the major results of the study? 2 What were the key results in numbers? 3 Was the temporal relationship correct? 4 Was there a dose-response gradient?
V 1 How might bias including confounding have affected these results? W 1 How might misclassification have affected these results?
X 1 Are the references up to date and relevant? 2 Are there any glaring omissions in the references? Y 1 To whom may the results of this study be generalised? Z 1 Is the interpretation of the data conservative? note1
note2
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