156P The impact of Ki-67 on prognosis and chemotherapy benefit in breast cancer patients with ER-positive/HER2-negative micrometastatic disease in sentinel lymph nodes: A register-based cohort study

Previous studies concerning benefit of adjuvant chemotherapy in breast cancer with sentinel node micro-metastatic disease (pN1mi), has shown diverging results. Using a large set of real-world data (RWD), we investigated whether adjuvant chemotherapy offers survival benefit in patients with pN1mi and whether proliferation marker Ki-67 (at different cut-off levels) could be used as a predictive marker of chemotherapy benefit. Using Breast Cancer Data Base Sweden (BCBaSe 3.0), a retrospective cohort study was performed, including all Swedish breast cancer patients 2008 – 2020. The cohort included 2310 patients with estrogen receptor(ER)-positive/HER2-negative breast cancer. Multivariable Cox regression models were applied using clinicopathological parameters as covariates. In total, 778 patients (34%) received adjuvant chemotherapy. This patient group significantly more often presented with T2-T3 disease, grade III tumors, and higher levels of Ki-67. In multivariate analysis, adjuvant chemotherapy did not result in statistically significant improvement in distant disease free survival (DDFS; Hazard Ratio (HR): 0.81; 95% Confidence Interval (CI): 0.44 – 1.49) or overall survival (OS: HR 0.79; 95% CI: 0.36 -1.73). Three different cut-off levels of Ki-67 (> 14%, > 20%, > 30%) were analyzed in terms of predictive value for chemotherapy benefit without any statistically significant association. In an unselected cohort of patients with ER-positive/HER2-negative pN1mi breast cancer, the use of adjuvant chemotherapy was not associated with improved survival. Furthermore, Ki-67 could not predict chemotherapy benefit, regardless of cut-off-level applied. These results indicate the need of further prognostic and predictive biomarkers when tailoring adjuvant treatment to patients with micrometastatic lymph node disease.