Ki67 in Breast Cancer: The Effect of Different Length of Fixation on Ki 67 Index
Hala F Faragalla, Linda Feeley, Anita Bane, Anna Marie Mulligan. University Health Network, University of Toronto, Toronto, ON, Canada; Cork University Hospital, Cork, Ireland; Juravinski Cancer Center, McMaster University, Hamilton, ON, Canada
Background: Ki67 has been shown to be a prognostic marker in invasive breast cancer with high levels are associated with poorer outcome. Furthermore, patients with HR positivity and a high Ki67 LI may benefit from adjuvant chemotherapy. A major obstacle is lack of a standardized assay approach. The aim of this study was to address specifically, the effect of counting methods and fixation length on the LI.
Design: Paired tumour blocks were selected from 70 tumors for short (SF) and prolonged fixation periods (PF) and stained with MIB1. Two visual counting methods were utilized: actual count (VC) of positive to negative cells in three different areas and a visual estimate (VE), similar to scoring HR. Statistical analysis was performed using t-test with a P-value < 0.05 considered significant.
Results: The mean fixation period for the SF group was 13hrs18 min (range 10hrs33 min–17hrs45min) and for the PF group, 79 hrs35min (range 73 hrs33min-102hrs30 min). There was no statistically significant difference between the LI for the SF vs PF groups using either counting methods; however, 23% had >10% difference in the LI between the fixation groups. When 14% was used as the cut-off to assign cases to high or low Ki67 LI, 34% of cases were assigned into different risk categories depending on fixation length. In both analyses, there was no trend towards an increased or decreased LI according to fixation length, i.e. these cases were evenly distributed between those that showed a higher or lower LI with greater fixation. A significant difference between VC and VE was not seen; in 80% of cases, an underestimation of LI was seen with VE compared with VC. In 9% of cases the difference was >10%; however a difference in risk category (high vs low) was seen in 14% of cases (VE range for incorrectly assigned cases = 6-15%).
Conclusions: Fixation does not appear to affect Ki67 LI in BC. While a high number of cases were reassigned to different risk categories based on fixation length, this reassignment was in both directions. This is likely a result of intratumoral heterogeneity in Ki67 LI, further emphasizing the need to determine the most appropriate approach to selecting areas to be counted. In most cases VE accurately predicted LI; however, a visual count is recommended when the LI is estimated to be low (approximately < 20%).
Tuesday, March 5, 2013 1:00 PM
Poster Session IV # 54, Tuesday Afternoon