Analyzing geographic patterns of disease incidence: Rates of late-stage colorectal cancer in Iowa

This study, using geocodes of the locations of residence of newly diagnosed colorectal cancer patients from the Iowa Cancer Registry, computed continuous spatial patterns of late-stage rates of colorectal cancer in Iowa. Variations in rates in intrahospital service regions were as great as interhospital service regions, shown by analysis of variance tests. Some of the spatial variations observed could be explained, using a general linear regression model on individual-level data, by spatial variations in attributes of individuals and their relationships to health resources. We show how this source of variation can be removed from the original map leaving a new map showing the remaining variation in late-stage rate not explained by these relationships. We argue that it would be more appropriate to organize prevention and control activities targeted at the areas with higher than expected late-stage rates shown on this map. The originality of this approach is in the integration of geocoded data from a cancer registry with methods of spatial analysis that provide considerable geographic detail in the cancer rate while controlling for rate stabilization and reliability due to the small number problem.