Using Spatially Adaptive Filters to Map Late Stage Colorectal Cancer Incidence in Iowa

Abstract

Disease rates computed for small areas such as zip codes, census tracts or census block groups are known to be unstable because of the small populations at risk. All people in Iowa diagnosed with colorectal cancer between 1993 and 1997 were classified by cancer stage at the time of their first diagnosis. The ratios of the number of late-stage cancers to cancers at all stages were computed for spatial aggregations of circles centered on individual grid points of a regular grid. Late-stage colorectal cancer incidence rates were computed at each grid point by varying the size of the spatial filter until it met a minimum threshold on the total number of colorectal cancer incidences. These different-sized areas are known as spatially adaptive filters. The variances analyzed at grid points showed that the maps produced using spatially adaptive filters gave higher statistical stability in computed rates and greater geographic detail when compared to maps produced using conventional fixed-size filters.