Comparison of immunostaining for estrogen receptor in breast cancer using the rabbit monoclonal antibody SP1 or the mouse monoclonal antibody 6F-11

Abstract

11562 Background: A high-quality assay to detect estrogen receptor (ER) expression is crucial as ER status guides many therapeutic decisions in breast cancer. Previously the rabbit monoclonal antibody (Ab) SP1 was shown to be more sensitive than the mouse monoclonal Ab 1D5. Results were similar in a comparison of mouse Abs 6F11 and 1D5. This study compares the ER status in breast cancer using SP1 on cases previously interpreted as negative with 6F11. Methods: All low grade (1 or 2), ER negative, invasive breast carcinomas, reviewed at the British Columbia Cancer Agency between 2002 and 2006 were reviewed. Cases that had been stained at the Vancouver site (VCC) with 6F11 were selected and those with available tissue blocks were re-stained with SP1. The patient’s oncologist was notified of any changes in ER status. Results: 123 cases of low grade, ER negative invasive breast tumors were initially selected. 49 of these cases had been stained at VCC with 6F11. Tissue blocks were available for 45/49 (92%) of cases. 5/45 cancers (11%) were grade 1 and 40/45 (89%) were grade 2. With SP1 the ER status changed from negative to positive in 3/45 (6.7%) of the cases. For 2 cases the ER status changed because current scoring criteria regard a test as positive even if only rare cells stain. Review of original 6F11 stained sections revealed these 2 cases would be scored as ER positive using the new criteria. The third case was a core biopsy with scant tumor and only focal staining. Review revealed the change in ER status was likely due to sampling difference in the deeper section. Conclusions: Of 45 previously 6F11 ER negative cases restained with SP1, the ER status changed in 2 because of revised more sensitive scoring criteria and in 1 because of a sampling difference between the original and subsequent sections. The rabbit monoclonal Ab SP1 appears to be as sensitive as the mouse monoclonal Ab 6F11 in detecting ER expression in breast cancer. This highlights the need for good quality ER testing with highly sensitive antibodies. Additional data for grade 3 cases will be presented. No significant financial relationships to disclose.