The Prognostic Significance of Ki-67 Expression in Breast Cancers Developing Malignant Effusions

Introduction: The aim of this study was to investigate the prognostic significance of Ki-67 in breast cancer patients developing malignant effusions. Methods: In this retrospective study, 76 breast cancer patients developing a malignant effusion were enrolled. The Ki-67 immunohistochemistry (IHC) findings from both initial tissue and paired metastatic effusion samples of these patients were grouped into the following three categories: ≤5, 6-29, and >30%. Results: Triple-negative breast cancer (TNBC) showed higher Ki-67 levels at both initial diagnosis (p < 0.001) and malignant effusion samples (p = 0.015), compared to the non-TNBC phenotypes at the same tumor evolution phases, respectively. A Ki-67 <30% at initial diagnosis was associated with a shorter overall survival (p = 0.031; long-rank test), in addition to an earlier development of a malignant effusion (p < 0.001; longrank test). A Ki-67 >5% at malignant effusion cell block samples was associated with a longer post-effusion survival (p = 0.015; long-rank test). Lastly, a Ki-67 index >5% at the effusion samples showed a significantly lower risk ofmortality following the development of malignant effusions (HR: 0.355 [0.150-0.842; p = 0.019]), after being adjusted for the intrinsic subtype covariate (TNBC vs. non-TNBC). Conclusion: Estimating the Ki-67 IHC expression in malignant effusion cytology samples may have a prognostic significance, when evaluating patients with metastatic breast cancer. However, larger scale prospective studies would be necessary to provide more evidence on this topic.