Breast Cancer Surveillance using Gridded Population Units, Connecticut, 1992 to 1995

PURPOSE: To assess geographic variation in invasive breast cancer across Connecticut using gridded population areas to enumerate cases and the population at-risk. METHODS: The stateʹs land mass was divided into 5168, 1-by-1 square mile areas and the population of women, 20+ years of age, within each location was estimated by areal interpolation of the 1990 US Census Block Group STF-3A data file. Using information on breast cancer incidence, 1992 to 1995, from the Connecticut Tumor Registry, latitude-longitude coordinates for place of residence at the time of breast cancer diagnosis were determined for 8530 records and assigned to appropriate grid locations. A spatial scan statistic was used to detect variation in incidence and test the significance of observed differences across the state. Standardized Incidence Ratios (SIRs) described the proportional change in the age-adjusted breast cancer incidence rate across gridded locations. RESULTS: The statewide age-adjusted invasive cancer incidence rate was 163.6/100,000 women/year. The spatial scan statistic identified three locations around Connecticut with significantly low incidence rates and four places where rates were significantly high. The most probable place of low incidence was rural Northeastern Connecticut where risk of disease, relative to elsewhere around the state, was 0.73 (p = 0.001). The most probable location of elevated incidence was a suburban location in Southwestern Connecticut with a relative risk of 2.02 (p = 0.001). CONCLUSIONS: Visual representation of disease incidence and underlying populations at-risk according to gridded units provides a useful tool for assessing small area variation in disease patterns.