Integration of Ki-67 index into AJCC 2018 staging provides additional prognostic information in breast tumours candidate for genomic profiling

Background: The Eighth edition of the American Joint Committee on Cancer (AJCC) staging system (2018) for breast cancer (BC) introduced the prognostic stage. Moreover, multigene assessment has been indicated to tailor staging in T1/T2/N0, ER-positive/HER2-negative BC. However, many National Health Systems do not provide reimbursement for routine testing. The aim of this study was to assess whether Ki67 proliferation index is prognostically relevant for patients' candidacy for molecular testing. Methods: A retrospective series of 686 ER+/HER2- BC were reclassified using AJCC 2018, and in the group of 521 patients for which AJCC 2018 recommends molecular evaluation, we assessed the prognostic efficacy of a prognostic stage enriched by Ki67 (Ki67-PS), considering Ki67 <20% an alternative to recurrence score <11 provided by Oncotype DX. Results: We found that a group of BCs (35.6%, 58/163) assigned to IB stage by prognostic score were down classified to IA with Ki67-PS. The outcome of these 58 cases overlapped with that of lesions classified as stage IA using prognostic stage, showing a significantly better prognosis compared to IB tumours (HR = 2.79, p = 0.003). Conclusions: These data suggest that Ki67 may be a reliable marker to enrich the 2018 AJCC prognostic score in BC patients' candidacy for genomic profiling.