Quality assessment of HER2 testing by monitoring of positivity rates

Abstract

Interlaboratory variation in human epidermal growth factor receptor 2 (HER2) testing provides a challenge for targeted therapy in breast and gastric cancer. Assessment of positivity rates among laboratories could help monitor their performance and define reference values for positivity rates to be expected in a geographic region. Pathologists regularly determined the number of HER2-positive cases (HER2 3+, HER2 2+/amplified or amplified) in their laboratory, and figures were continuously entered into a central website. The overall positivity rate of each participant was calculated and compared with the average rates of all other institutes (n = 42). A total of 18,081 test results on breast cancer and 982 on gastric cancer were entered into the system. Positivity rates for HER2 in breast cancer ranged from 7.6% to 31.6%. Statistically, the results from six institutions qualified as outliers (p < 0.000005). From the remaining institutions encompassing 10,916 assessments, the mean proportion of positive cases was 16.7 ± 3.2% (99% confidence interval 16.6-16.8). The results from six institutions were in between the 95% and 99.5% confidence intervals. For gastric cancer, there was one outlier and the mean positivity rate was 23.2 ± 5.7%. The proportion of HER2-positive breast cancer cases is considerably lower than could have been expected from published studies. By assessing the positivity rates and comparing them with that of all breast or gastric cancers in a given population, pathologists will be alerted to a potential systematic error in their laboratory assay, causative for over- or underestimation of cancer cases suited for anti-HER2 therapy.

Fig. 1

The number of HER2-positive breast cancer cases (HER2 3+, HER2 2+/amplified, amplified) per institute of pathology in relation to the number of cases investigated was plotted on a logarithmic scale. The 99.5% confidence interval is indicated by red lines. In institutions with a low number of assessments, the confidence interval is broader. The expected rate calculated from the mean value of 36 institutions within the 99.5% confidence interval is demonstrated by a blue line. There are six institutions outside the 99.5% confidence interval (indicated by red crosses). Four of these potentially underestimate HER2 and two have a higher positivity rate than could have been expected. Institutes with a positivity rate within the 99.5% confidence interval are represented by white circles

Fig. 2

The number of HER2-positive gastric cancer cases (HER2 3+, HER2 2+/amplified) per institute of pathology in relation to the number of cases investigated was plotted on a logarithmic scale. The 99.5% confidence interval is indicated by red lines. In institutions with a low number of assessments, the confidence interval is broader. The expected rate calculated from the mean value of 14 institutions within the 99.5% confidence interval is demonstrated by a blue line. There is one institution outside the 99.5% confidence interval (indicated by a black cross). Institutes with a positivity rate within the 99.5% confidence interval are represented by black points. The confidence interval might narrow over time when more assessments are available for consideration

Fig. 3

A high degree of variability between institutes was observed with regard to the HER2 2+ category. In particular, the differentiation between HER2 2+ and HER2 1+ might be handled differently. This case illustrates the borderline between the 2+ category (intraductal carcinoma, double arrow) and the 1+ category (invasive carcinoma, single arrow) (immunohistochemistry with 4B5 anti-HER2 monoclonal antibody, ×200)