Abstract 150P
Background
Ki67 immunohistochemistry (IHC) with a cut-off of 20% by MIB-1 PharmDx assay has been approved as a companion diagnostic for adjuvant abemaciclib in high-risk ER+HER2- breast cancer. We addressed the genomic risk profile of high Ki67 (≥20%) ER+HER2- breast cancer using a 21-gene multigene assay (Oncotype DX test). In addition, we investigated survival outcomes and identified factors associated with low genomic risk in the high Ki67 group.
Methods
We collected clinical and pathologic information in 2,295 patients who underwent Oncotype DX testing. High genomic risk was defined by the 21-gene recurrence score (RS ≥26). Ki67 IHC examination with a cut-off of 20% was performed locally using MIB-1.
Results
The Ki67 IHC assigned 870 (38%) to the high and 1,425 (62%) to the low groups, while 21-gene RS assigned 347 (15%) to the genomic-high and 1,948 (85%) to the genomic-low. Of these, 263 of 870 (30%) of the high Ki67 had high genomic risk, whereas 84 of 1424 (6%) of the low Ki67 had it (p<0.0001). Average 21-gene recurrence score was significantly higher in the high Ki67 than in the low Ki67. In survival analyses, high Ki67 and 21-gene RS were significant prognostic factors. When the patients were classified into three groups (genomic-high, high-Ki67/genomic-low, low-Ki67/genomic-low), survival outcome of the high-Ki67/genomic-low was better than the genomic-high, whereas it was worse than the low-Ki67/genomic-low. Within the high Ki67 group, a multivariable analysis demonstrated that PR+, age ≤50, and histologic grade I-II were associated with low genomic risk.
Conclusions
Although about 60% of tumors with high Ki67 expression had low genomic risk by 21-gene multigene assay, the patients with high Ki67 have a substantial risk of relapse regardless of genomic risk. In high Ki67 tumors with PR+ and/or lower histologic grade, the multigene assay would be useful to evaluate their genomic risk.
Clinical trial identification
Editorial acknowledgement
Legal entity responsible for the study
The authors.
Funding
Has not received any funding.
Disclosure
All authors have declared no conflicts of interest.