In oncology clinical trials, the primary endpoint is often time to an event of clinical interest, e.g., time to disease progression or time to death. As a result, progression-free survival (PFS: the time from initiation of treatment till disease progression or death whichever occurs first) and overall survival (OS: the time from initiation of treatment till death) are the focus of statistical analysis in comparison of two treatment arms. It is often argued that PFS may serve as a surrogate endpoint for OS, while the validity of such surrogacy is still under debates in different types of cancer. In practice, one may observe a significant difference in PFS but no significant difference in OS; or vice versa. We provide a concordance index (C-index) to measure the degree of concordance between PFS and OS, and elaborate on the PFS vs OS discrepancies using the C-index using simulation studies and real trial analysis.