30 Background: As some gastric (GC) and gastroesophageal junction adenocarcinoma (GEJC) overexpress HER2, HER2 testing should be part of routine assessment to decide target treatment initiation. Thus, the study objective was to assess the concordance of HER2 status between local and central laboratories. Methods: From July 2012 to February 2014, this observational study tested tumor samples from patients (pts) with GC or GEJC, regardless of disease stage. HER2 positive (+) status was defined as immunohistochemistry (IHC)3+ or IHC2+/In Situ Hybridization (ISH)+. The concordance was analyzed between HER2 status assessed by local laboratories (any technique) and centralized laboratory (IHC 4B5 and silver (S) ISH for all specimens). Results: 394 specimens from 367 pts were analyzed by 19 local laboratories. Pts' characteristics were: mean age 66±13 years, male 69%, GC 60%, intestinal type 55%. The specimens were surgical resections (52%), biopsies (41%), adenopathy (4%) and distant metastasis (3%). HER2+ status was found in 18% of locally tested specimens (69/394: 52 IHC3+; 17 IHC2+/ISH+) and in 19% of centralized specimens (73/394: 56 IHC3+; 17 IHC2+/SISH+). Among HER2+ specimens, 53% (39/73) were GC and 73% (53/73) were of intestinal histological type. Regarding amplification, 20% of centralized SISH tests (79/393) were amplified. The concordance between HER2 status (n = 393) assessed by local and centralized laboratories was acceptable according to Landis and Koch classification with a kappa coefficient of 0.69 (95%CI [0.60-0.78]). The discordance between HER2 status assessed by local and centralized laboratories was 9%. The rate of false negative was 27% and false positive 5%. Conclusions: The HERable study is the first large French study to evaluate concordance between HER2 status assessed by local and centralized laboratories. The cumulative rate of false negative and false positive in GC and GEJC is 9% (95% CI [6%; 12%]). Quality control should be setup to improve the quality of this test. [Table: see text]

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