Depth of Response and Early Tumor Shrinkage for Predicting Clinical Outcomes in HER2-Positive Metastatic Breast Cancer Treated with Trastuzumab

Background: To evaluate whether the depth of response (DepRe) and early tumor shrinkage (ETS) are predictive factors of clinical outcomes in HER2-positive metastatic breast cancer (mBC) patients treated with trastuzumab.

Methods: We performed a retrospective study of 100 HER2-positive mBC patients who received trastuzumab combined with chemotherapy as first-line treatment. ETS and DepRe were calculated. We employed Youden's index to determine the optimal cutoff value of ETS and DepRe for predicting progression-free survival (PFS) and overall survival (OS). We used Kaplan-Meier analysis, Log-rank tests, and Cox proportional hazards regression models to evaluate the impacts of ETS and DepRe on clinical outcomes.

Results: The optimal cutoff values were 30% for ETS and 40% for DepRe; ETS and DepRe were observed in 51.0% (51/100) and in 56.0% (56/100) of patients, respectively. Both ETS≥30% and DepRe≥40% were significant tumor-size metrics for predicting PFS (ETS: median 1.43 vs 0.69 years, hazard ratio [HR] = 0.384; 95% confidence interval [CI]: 0.245 to 0.601; P=0.000030; DepRe: median 1.43 vs 0.59 years, HR = 0.390; 95% CI: 0.250 to 0.609; P=0.0000034), but only DepRe≥40% was a significant predictor for OS (median 4.02 vs 3.07 years, HR = 0.484; 95% CI: 0.255 to 0.919; P = 0.027). Multivariate analyses also identified DepRe as an independent prognostic factor for PFS (HR = 0.52; 95% CI: 0.29 to 0.93; P = 0.028) and OS (HR=0.37; 95% CI:0.15 to 0.90; P = 0.029).

Conclusion: ETS≥30% and DepRe≥40% were significant predictors of better clinical outcomes in mBC patients treated with first-line trastuzumab-based chemotherapy. Further validation in prospective trials with larger patient populations is needed.