Purpose: Compared to white women, African-American women have had a higher breast cancer mortality rate despite the lower incidence rate for almost thirty years now, and this disparity continues to grow. The objective of this study is to evaluate differences in the natural history of breast cancer between African-American and white women using the University of Wisconsin Breast Cancer Simulation Model (UWBCS), a previously validated discrete-event, stochastic simulation model of breast cancer epidemiology in the U.S. female population from 1975 to 2000 developed by our research team as part of Cancer Intervention and Surveillance Modeling Network (CISNET).
Method: We used an acceptance sampling process for model calibration. Specifically, we determined biologically plausible ranges for each input parameter and ran the UWBCS for all possible input combinations for white and African-American women separately. From each run's output, we calculated age-adjusted incidence rates for each breast cancer stage and compared them with age-adjusted incidence rates calculated based on stage-specific incidence rates as reported in the SEER database. We selected input vectors that generated incidence rates sufficiently close to the observed ones. The calibrated input parameters included Onset Proportion (the ratio of assumed age-specific biologic onset rate to age-specific incidence rate in the absence of screening), Mean Gamma and Var Gamma (simulated tumors grow according to Gompertz-type function with growth rate for individual tumors having a gamma distribution with mean Mean Gamma and variance Var Gamma expressed in months), Percent 4 Nodes and Percent 5 Nodes (percentage of tumors classified as regional and distant at onset, respectively).
Result: Onset Proportion was 0.88 for African-American and 0.80 for white women. Mean Gamma and Var Gamma were 0.13 and 0.025 for African-American vs. 0.10 and 0.008 for white women, respectively. Percent 4 Nodes and Percent 5 Nodes were 0.02 and 0.04 for African-American vs. 0.01 and 0.02 for white females, respectively.
Conclusion: Our results suggest that breast cancer tumors in African-American women have higher growth rates, higher variation in growth rates and may be more likely to become fatal than those occurring in white women. This raises the question whether the screening guidelines should be adjusted to accommodate those differences in natural history of breast cancer between white and African-American women.
Candidate for the Lee B. Lusted Student Prize Competition