Objectives. To examine the geographic variation in the use of radiation to treat nonmetastatic prostate cancer and to identify factors that explain the variation in the United States. Methods. This study was based on data from the nine geographic regions of the Surveillance, Epidemiology, and End Results Program for 1983 through 1996. Patients with localized or regional prostate cancer who did not undergo surgical treatment were included in the analysis. Logistic regression analysis was used to investigate the influence of geographic and demographic factors on the use of radiation. The squared multiple correlation coefficient R 2 was used to measure the proportion of variation in the selection of radiation explained by each factor of interest. Results. Compared with San Francisco, the adjusted odds ratios for 6 of the 8 geographic areas had highly significant P values, suggesting the use of radiation therapy varies from region to region. However, geographic location only explained less than 3% of the total variation in the use of radiation. The geographic location explained a much higher proportion of variation in the youngest (younger than 55 years) and the oldest (80 years old or older) groups. Overall, age was the most important factor that influenced the use of radiation. Conclusions. The finding that geographic location explains a significant proportion of the variation in the use of radiation in the youngest and oldest age groups demonstrates the outcome of longstanding controversies in the nonsurgical treatment of prostate cancer. Documenting the impact of the interaction of age and geographic location on the treatment approaches provides for better understanding of the impact of patients and physicians making clinical decisions in the management of nonmetastatic prostate cancer.
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