Prognostic impact of proliferation for resected early stage ‘pure’ invasive lobular breast cancer: Cut-off analysis of Ki67 according to histology and clinical validation

Abstract Introduction The intent of this analysis was to investigate and validate the prognostic potential of Ki67 in a multi-center series of patients affected by early stage ‘pure’ invasive lobular carcinoma (ILC). Methods Clinical-pathological data of patients affected by ILC were correlated with overall survival and disease-free survival (OS/DFS); data from a parallel invasive ductal carcinoma (IDC) patients' cohort were gathered as well. The maximally selected Log-Rank statistics analysis was applied to Ki67 continuous variable to estimate the appropriate cut-off. The Subpopulation Treatment Effect Pattern Plot (STEPP) analysis was performed as well. Results Data from overall 1097 (457/222 ILC: training/validation set; 418 IDC) patients were gathered. The identified optimal Ki67 cut-offs were 4% and 14% for DFS in ILC and IDC cohort, respectively. In ILC patients, the Ki67 cut-off was an independent OS predictor. Ten-years OS and DFS were 89.9% and 77.2% ( p  =  0.007 ) and 79.4% and 69.2% ( p  =  0.03 ) for patients with Ki67 ≤ 4% and >4%, respectively. In IDC patients, 10-years OS was 93.8% and 71.7%, p  =  0.02, DFS was 84.0% and 52.6%, p  =  0.0003 , for patients with Ki67 ≤ 14% and >14%, respectively. In the validation set, the optimal Ki67 OS cut-off was 5%. The STEPP analysis showed that in the presence of low Ki67 values, IDC patients have a better DFS than ILC patients, while with the increase of values the prognosis tends to overlap. Conclusions Despite the retrospective design of the study, the prognostic relevance of Ki67 (as well as its optimal cut-off) seems to significantly differ according to breast cancer histology.