Spatial and temporal analysis of breast cancer and potential exposures in Marin County, California
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Marin County has one of the highest incidences of breast cancer in the California Bay area. Case-control studies completed to date provide multiple sources of information on covariates and risk factors, as well as the residential histories of each participant over their life course. However, the geocoding completeness of the residential histories in these earlier studies is relatively low in some time periods due to incomplete residential histories (recall bias) and changes in place names and road networks over the years. Therefore, the analysis of geocoding completeness is crucial. The first objective of this study aims to identify those time periods with sufficient geocoding completeness to support the space-time analysis of breast cancer while accounting for residential mobility. Serpentine soils, pits and quarries are hypothesized as two potential exposure sources for metals (including cadmium, chromium and mercury) implicated as breast cancer carcinogens. The second objective of this study uses serpentine soil sites and quarries in a focused analysis of breast cancer to assess whether there is a statistically significant excess of breast cancer cases near such locations. We identified 1980 to 1999 as that time period with geocoding completeness greater than 80%, and analyzed breast cancer case clustering in this interval using the Q focused test. The focused Q statistic assessed clustering of breast cancer cases in the vicinity of serpentine soil and quarries, and did not find an excess of breast cancer cases in the vicinity of these sites when all of these were evaluated together in a global test. Although one quarry site (Larkspur) was found to be a highly significant focus, it was not significant when multiple testing was accounted for. This study systematically evaluated potential place-based causative exposures and global local clustering through time, using case-control residential histories and demonstrates a comprehensive application of the Q focused statistic in the study of breast cancer.