[1969] Impact of 96-Hour Formalin-Fixation on Assessment of Ki-67 Labeling Index

Beth Z Clark, Mohamed M Desouki, David J Dabbs, Kim McManus, Rohit Bhargava. Magee-Womens Hospital of University of Pittsburgh Medical Center, Pittsburgh, PA

Background: Ki-67 labeling index (LI) by immunohistochemistry is used in invasive breast carcinoma as a prognostic indicator. ASCO/CAP guidelines recommend a duration of formalin fixation of 6-72 hours for hormone receptors but no upper limit of fixation is proposed for Ki-67. We have previously shown that fixation up to 96 hours does not significantly impact hormone receptor status (Am J Clin Pathol. 2012;137:691-698). In this quality assurance study of 46 invasive breast cancer cases, we determined Ki-67 LI using three antibody clones to Ki-67 following both routine fixation (8-48 hours) and prolonged (96 hour) fixation.
Design: Forty-six surgical resections for invasive breast carcinoma with sufficient tissue for diagnosis were included. One tumor section was fixed in formalin for 96 hours. Immunohistochemical stains for three antibody clones to Ki-67 (30-9, SP-6, and MIB-1) were performed. Ki-67 LI was manually examined by one pathologist on all clones for all cases (both routinely fixed and fixed for 96 hours) to avoid inter-observer variability. Mean Ki-67 LIs for each antibody clone were compared to each other for routinely fixed tissue and fixed for 96 hours using paired t-test.
Results: Using the 30-9 antibody clone to Ki-67, there was a statistically significant difference in mean Ki-67 PI for cases with routine fixation vs. 96 hours (p=0.036), while the duration of fixation did not affect the mean Ki-67 LI for antibody clones SP-6 (p=0.15) or MIB-1 (p=0.18). Although the differences with 30-9 clone were statistically significant only 9 of 46 cases had >10%age point difference. Moreover, 3 of these 9 cases actually had higher Ki-67 LI with 96 hours fixation. For SP6 antibody, 5 cases had >10%age point difference, of which 4 cases showed higher Ki-67 LI in tumors fixed for 96 hours. For MIB1 antibody, 7 cases had >10%age point difference, of which 1 case showed higher Ki-67 LI in tumors fixed for 96 hours.
Conclusions: A clinically significant difference was not observed in mean Ki-67 LIs with any of the antibody clones when invasive breast carcinomas were fixed for 96 hours in formalin. Our study validates performance of Ki-67 staining in tissue fixed for up to 96 hours.
Category: Quality Assurance

Tuesday, March 5, 2013 9:30 AM

Poster Session III # 283, Tuesday Morning


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