Standardisation of EGFR FISH in colorectal cancer: Results of an international interlaboratory reproducibility ring study

Aims: Epidermal growth factor receptor (EGFR) gene copy number evaluated by fluorescence in situ hybridisation (FISH) can discriminate among KRAS wild-type patients those with better outcome to EGFR-targeted therapy in metastatic colorectal cancer, further enhancing selection of patients. Nevertheless, enumeration of gene copies is challenging and the lack of analytical standardisation has limited incorporation of the test into the clinical practice. We therefore assessed EGFR FISH interlaboratory consensus among five molecular diagnostic reference centres. Methods: A set of 12 colorectal cancer samples circulated among laboratories, and samples were scored according to commonly agreed guidelines. Reproducibility was quantified using the standard error of measurement (SEM). Results: A SEM of 0.865 and a within-subject coefficient of variation (WSCV) of 26.8% for mean EGFR gene/nuclei and a SEM of 0.235 and a WSCV of 19.4% for the mean EGFR gene/CEP7 ratio were observed. Measurement of the fraction of cells displaying chromosome 7 polysomy showed WSCV of 46.6%, 34.0% and 51.0% for percentage of cells displaying ≤2, ≥3 and ≥4 EGFR signals, respectively. Among different slides of the same specimen, the WSCV was 6.1% for mean EGFR gene/ nuclei and 3.9% for mean of EGFR gene/CEP7 ratios. Conclusions: Molecular diagnosis of EGFR gene copy number by FISH varied largely among pathology centres, with fluctuations covering the whole range of proposed cut-offs of predictive usefulness from literature. Definition of a detailed scoring system and implementation of comprehensive training programmes for laboratories are therefore necessary before including the test into clinical practice.