Performance of EGFR mutant-specific antibodies in different cytological preparations: a validation study

Molecular testing for epidermal growth factor receptor (EGFR) mutations is required to select the most appropriate treatment for advanced-stage non-small cell lung cancer (NSCLC). In routine practice, cytological samples are often the only specimens available for testing. When the number of neoplastic cells is large, DNA-based assays are the gold standard. When cytological samples contain only a few neoplastic cells, immunocytochemistry (ICC) using anti-EGFR mutant-specific antibodies may be more effective. We aim to assess the specificity and sensitivity of IHC staining in cytological specimens using mutated cell lines subjected to different cytopreparations and staining methods.

HCC827 (exon 19 p.E746-A750 del) and H3255 (exon 21 p.L858R) cell lines were subjected to different fixation (air dried, alcohol or CytoLyt(®)), staining (Diff-Quik(®) or Papanicolaou) and preparation (smears or cell blocks) methods before ICC. In a second set of experiments, mutated cells were mixed with EGFR wild-type cells to obtain low-level (10%) mutated cytological samples. The intensity and percentage of cells stained were evaluated against validated molecular techniques. Moreover, the cell lines were subjected to poor growing conditions to simulate routine specimens that are less optimal than in vitro samples.

The cytological preparations showing the most intense staining were formalin-fixed cell blocks and samples fixed with CytoLyt or alcohol, including Papanicolaou-destained samples. Conversely, air-dried slides showed the least intense staining. Mutant antibodies allowed the detection of mutated cells, even when representing only 10% of the total population. Although, in necrotic specimens, an aspecific background signal appeared, the viable cells still retained anti-mutant EGFR positivity.

All cytological preparations are suitable for ICC using anti-EGFR mutant-specific antibodies, in particular formalin-fixed cell blocks and alcohol- or CitoLyt-fixed samples. the method is also validated to detect even a few mutant cells in less than optimal samples.