Background: CPS + EG staging, which incorporates estrogen receptor (ER) status and tumor grade with pretreatment clinical stage (CS) and post-treatment pathologic stage (PS), has been reported to have better correlation with outcome than classic TNM staging for patients treated with neoadjuvant chemotherapy (NAC). Our goal was to evaluate the performance of CPS + EG staging system in an external cohort treated with NAC. Methods: We reviewed patients with stages I–IIIC breast cancer treated with NAC and surgery at our institution between 1988 and 2014. ER status, Nottingham grade, treatment, American Joint Committee on Cancer (AJCC) CS before NAC and PS after NAC, and follow-up data were collected. The discrimination of CPS + EG and pathologic AJCC stage were assessed using area under the curve (AUC) for survival data. Results: A total of 769 patients were analyzed with a median follow-up of 2.6 (range 0.0–19.4) years; 103 patients died of breast cancer. Overall, the 5-year breast cancer cause-specific survival was 81.5 % [95 % confidence interval (CI) 77.6–85.5]. The 5-year, cause-specific survival by CPS + EG score was 93.8 % score 0, 89.9 % score 1, 90.7 % score 2, 84.8 % score 3, 67.7 % score 4, and 43.4 % score 5/6. CPS + EG score was significantly associated with cause-specific survival (p < 0.001) with an AUC of 0.69 (95 % CI 0.62–0.77) at 5 years. This was higher than the AUC of 0.63 (95 % CI 0.56–0.70) for AJCC PS (p = 0.10). Conclusions: This study validates the CPS + EG staging system using Nottingham grade in an external cohort. Addition of tumor biology and treatment response shows promise in improving survival estimates for patients treated with NAC.
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