Endocrine therapy reduces recurrence risk by 30% to 50% in estrogen receptor (ER)-positive breast cancer. The ER-positive threshold recommended by the American Society of Clinical Oncology/College of American Pathologists is 1% based on studies using the ER-6F11 antibody. ER-SP1 antibody has a higher sensitivity and is more widely used.
We report interobserver concordance manually measuring ER in 264 breast cancers using ER-SP1 and 1D5 and 2 scoring methods (H-score and Allred score).
With both antibodies, 3% to 4% of cases have a low level of ER expression (1%-10%), more than previously reported (<1%). We find a high level of paired observer concordance with both antibodies and scoring methods (κ = 0.892-0.943) with no significant difference with method of scoring. Despite excellent concordance, positive/negative discordance was almost 5% among 3 observers using either antibody, an underappreciated clinically significant rate.
Discordance overwhelmingly reflected differing opinions recording the proportion of tumor cells positive with low levels of expression (<10% staining; 12/13 cases).