Sato Y, Akiba S, Kubo O, Yamaguchi N. A case–case study of mobile phone use and acoustic neuroma risk in Japan. Bioelectromagnetics. Oct 28, 2010. Ahead of print.
Although mobile phones emit electromagnetic fields of extremely low intensity, potential effects in tissues close to the mobile phones during its use need to be clarified. Acoustic nerve tissue from which acoustic neuroma originates is located very close to the mobile phone. Results of case-control studies on mobile phone use and the risk of acoustic neuroma are inconsistent. Case-control studies are vulnerable to selection and recall biases. Because acoustic neuroma is unilateral in most cases, the case-case design can be used to study this disease. With this design, the affected ear is regarded as the case side and the opposite ear – as the control side. Therefore, the same patient plays the role of both case and control, and the effect of selection and recall biases can be evaluated more precisely.
The objective of the study was “to examine, using a case–case design, the association between various parameters of mobile phone use and the risk of acoustic neuroma, taking into account the more frequently used ear and the location of acoustic neuroma”.
Sixty eight hospitals throughout Japan were invited to participate, and 22 agreed. Eligible for inclusion were all patients who were alive at the time of the invitation and who were diagnosed with an acoustic neuroma in the participating hospitals between January 2000 and December 2006. Information on mobile phone use (the year of first use, average daily number of calls and call duration, proportion of calls using the left and the right ear) was obtained from the patients using mailed questionnaires. Clinical information about the tumor was obtained from neurosurgeons. Reference dates were set at 1 and 5 years before diagnosis. Mobile phone use before these reference dates was analyzed.
Of 1,589 eligible cases, 804 (51%) answered the questionnaire. Four cases were double-enrolled via two hospitals. Of the remaining 800 cases, 9 with tumors on both sides and 4 with missing information on tumor location were excluded, leaving 787 participants for the analysis. Risk ratio for regular mobile phone use until 1 year before diagnosis was 1.08 (95% CI 0.93-1.28) and for regular mobile phone use until 5 years before diagnosis 1.14 (95% CI 0.96-1.40). The risk was significantly increased for age at diagnosis <40 years. There were more heavy users in this age category compared to those diagnosed at ages 40-60 and ≥60 years. There were increasing trends in acoustic neuroma risk with increasing number of calls per day, with increasing duration of one call and with increasing daily call duration. These trends were statistically significant for 5 years before diagnosis, but not for 1 year before diagnosis. Significant risk ratios were seen for daily duration of calls >20 min 1 year before diagnosis (RR=2.74; 95% CI 1.18-7.85) and 5 years before diagnosis (RR=3.08; 95% CI 1.47-7.41). Heavy mobile phone use on the affected ear could increase the chance of the patient noticing slight changes in hearing due to disease. Therefore, cases with ipsilateral mobile phone use could be diagnosed at earlier stages of the disease than cases with contralateral use. Indeed, tumor size tended to be smaller in cases with ipsilateral mobile phone use suggesting a detection bias. Also, analysis of the distribution of tumors by side suggested an effect of “tumor-side-related recall bias” at 5 years before diagnosis. Cases with tumors on the left side were more likely to associate their tumors with mobile phone use than cases with tumors on the right side because of recall bias.
Interpretation and Conclusion
The authors have concluded that the observed increased risk of acoustic neuroma in individuals with average call duration >20 min/day should be interpreted with caution because detection and recall biases could shift the results away from the null. However, there was no convincing evidence that biases could entirely explain the observed increase in acoustic neuroma risk. Therefore, the possibility that mobile phone use increased the risk of acoustic neuroma could not be excluded.