Re: "cellular telephone use and risk of acoustic neuroma".

RE: " CELLULAR TELEPHONE USE AND RISK OF ACOUSTIC NEUROMA " Christensen et al. (1) recently presented results on the use of cellular telephones and the risk for acoustic neuroma. However, in our opinion, several issues need to be clarified in the paper. Our main concern is that the study did not take into account exposures from cordless phones, and associations specific for ever use of analog or digital phones were not addressed. Furthermore, a truly nonexposed group was not used for comparison. Of 130 eligible cases, 107 (82 percent) were interviewed. Two individually matched controls were used for each case. The response rate for controls was 64 percent or 214 controls. Thus, 334 controls must have initially been selected, a number that does not correspond to 1:2 matching. Conditional logistic regression analysis was based on 106 cases and 212 controls, but it is not clear whether these numbers represented complete matched triplets. Use of cord-less telephones was not assessed in contrast to our study (2– 5). The output power of cordless phones is of the same order as that of digital mobile phones, and the calling times on the cordless phones are much longer, so it is an important exposure that is neglected in the study. Furthermore, the investigators did not have a clean group of " no exposure, " since subjects reporting less than two calls per week and less than 6 months' use were included among the unexposed. This low-exposure group should be reported. Only first use of the telephone operating system was reported (1, table 2). Certainly the results should have been given for subjects using both analog and digital telephones. In fact, in our large study (2–5), only 54.3 percent of the cases and 50.9 percent of the controls had used analog phones only, that is, no digital phones. The corresponding results for digital telephones were 73.3 percent and 75.3 percent for cases and controls, respectively. Of the cases, 45 reported use of cellular telephones, but only four (8.9 percent) of them had used the analog type. This is a low number; for comparison, refer to the Danish cohort study of mobile telephone users (6). Of the 154 brain and nervous system tumor cases, 84 (54.5 percent) used analog phones, 20 (13.0 percent) used analog and digital, and 50 (32.5 percent) used digital phones; 104 (67.5 percent) had ever used an analog telephone. This number …