Response: Potential Bias in Denver Childhood Cancer Study

two neighborhoods developed within the last 50 years were classified as W-L high wire code (6). Based on these Columbus data, if cases from the inner city are matched with controls from newer neighborhoods, odds ratios exceeding a value of 3.0 could be produced, falsely associating high wire code with childhood cancer. Evidence exists that the cases in the 1988 Denver study may be from older neighborhoods relative to the controls. For example, of the homes classified as W-L high wire code based on secondary powerline construction, 63% of the cases and only 33% of the controls had the older "open wire" construction (7). This finding is suggestive of a failure to match cases and controls by neighborhood, which could have resulted in the creation of a false association between W-L high wire code and childhood cancer. As compared to the W-L wire code, the authors' new wire code appears to place an even greater proportion of older powerline constructions in the high wire code category and a greater proportion of newer constructions in the low category (Table 1). Based on the authors' Table 3, the most important result may be the movement of spun wire secondaries (present standard construction which was introduced in Columbus in the 1950s) within 50 feet of a residence from the W-L "high" category to the new "low" category (23% of the W-L "high wire code" controls, while only 9% of the W-L "high wire code" cases were so reclassified). If cases are from older neighborhoods relative to the controls, application of this new wire code would likely result in a stronger but false association between the new high wire code and childhood cancer. Adjustment for age of neighborhood should remove this possible bias. In conclusion, the fact that the new wire code is only weakly correlated with magnetic field measurements (in the same manner as the original W-L wire code) suggests that the newly reported stronger association with childhood cancer is likely due to factors other than magnetic fields. Differential residential mobility and differential residential age are two possible explanations and are suggestive that the reported association may be false.