The Costs of Adjuvant Chemotherapy in Early Stage Breast Cancer Patients: Comparison of Attributable Cost and Microcosting Approaches OBJECTIVES: To evaluate and compare the costs of adjuvant chemotherapy in early stage breast cancer patients utilizing attributable cost and microcosting approaches. METHODS: For the microcosting analysis, we determined utilization based on clinical guidelines using adriamycin and cyclophosphamide as adjuvant chemotherapy. Resource prices were based on reimbursements for drugs and procedures from Regence Blue Shield, a managed-care organization in the Pacific Northwest. The attributable cost approach estimates cancer-related costs as the difference between total costs of female breast cancer patients and costs of age- and gender-matched control subjects without breast cancer. Attributable costs were derived using the Kaplan-Meier Sample Average Estimator. Breast cancer cases were identified from a linked database of Regence Blue Shield claims records and the Surveillance, Epidemiology and End Results (SEER) cancer registry for Western Washington State. We hypothesized that attributable costs would be higher than microcosting because it more extensively accounts for treatment of chemotherapy-associated complications. RESULTS: We identified 1356 breast cancer cases for the analysis. Utilizing the microcosting approach, we estimated that adjuvant chemotherapy costs, including costs of chemotherapy and anti-nausea agents and surgical, laboratory and other procedures, were approximately $4013. Attributable costs of adjuvant chemotherapy were estimated to be $5330 in preliminary analyses. CONCLUSIONS: The integration of managed care claims data with clinical data from the SEER registry offers a unique opportunity to derive potentially more accurate disease burden costs in oncology. The results of our study will be useful for better understanding the differences between costing methods, informing cost-effectiveness models in breast cancer and evaluating the economic burden of the disease.