HER2 diagnostics in gastric cancer-guideline validation and development of standardized immunohistochemical testing

Abstract

Trastuzumab-based therapy has been shown to confer overall survival benefit in HER2-positive patients with advanced gastric cancer in a large multicentric trial (ToGA study). Subgroup analysis identified adenocarcinomas of the stomach and gastroesophageal (GE) junction with overexpression of HER2 according to immunohistochemistry (IHC) as potential responders. Due to recent approval of trastuzumab for HER2 positive metastatic gastric and GE-junction cancer in Europe (EMEA) HER2 diagnostics is now mandatory with IHC being the primary test followed by fluorescence in situ hybridization (FISH) in IHC2+ cases. However, in order to not miss patients potentially responding to targeted therapy determination of a HER2-positive status for gastric cancer required modification of scoring as had been proposed in a pre-ToGA study. To validate this new HER2 status testing procedure in terms of inter-laboratory and inter-observer consensus for IHC scoring a series of 547 gastric cancer tissue samples on a tissue microarray (TMA) was used. In the first step, 30 representative cores were used to identify specific IHC HER2 scoring issues among eight French and German laboratories, while in the second step the full set of 547 cores was used to determine IHC HER2 intensity and area score concordance between six German pathologists. Specific issues relating to discordance were identified and recommendations formulated which proved to be effective to reliably determine HER2 status in a prospective test series of 447 diagnostic gastric cancer specimens.

Fig. 1

Photomicrographs of TMA examples. a–c Artifacts leading to potential mis-scoring on IHC: a intestinal metaplasia, b edge artifact at TMA border with granular (not linear) pseudo-membranous staining, and c cytoplasmic as well as nuclear staining. d–h Intensity scoring: d Score 3+ visible by naked eye with membranous staining clearly visible at low magnification (obj. ×5) being either complete, basolateral or lateral (e, ×10). f Photomicrograph of TMA sample showing distinction between 2+ and 3+ IHC using 4B5 antibody. Arrows indicate areas with clearly visible membrane staining at low magnification (i.e., 3+), focally in <10% of tumor); arrowheads indicate areas where membrane staining is only visible at ×10 magnification (i.e., 2+). g TMA core suspicious of some focal staining at ×5 which turned out to be a focally specific membranous staining in groups of at least five cells at medium magnification (h, ×20; see arrowheads). i Very weak staining where membranous staining is barely visible and could only be demonstrated using high magnification (i, ×40)

Fig. 2

Stepwise approach to IHC scoring in gastric cancer: tissue and quality issues (mod. acc. to [31])