15057 Background: Accurate HER2 testing is required to identify patients eligible for treatment with trastuzumab (Herceptin®). HER2 positivity is reported as 6–35% in gastric cancer (GC). This range is due to small sample sets and differing methods of evaluation or scoring. A specific HER2-testing process was established for the Phase III ToGA trial, which is evaluating trastuzumab added to chemotherapy in HER2-positive advanced GC. Methods: A validation study was completed to standardise IHC (HercepTest™) and FISH (PharmDx™) protocols, and to establish a scoring system specific for GC (M Hofmann et al. ASCO Gastrointestinal Cancers Symposium 2006. Abstract no. 24). Tumour samples for ToGA were then centrally tested by both IHC and FISH to identify patients eligible for enrolment. Results: To date, 1024 tumour samples have been assessed (243 HER2 positive and 781 HER2 negative) giving an overall HER2-positivity rate of 23.7%. Both IHC and FISH results are available for 960 patients, with 87% concordance. Differences were largely due to FISH-positive cases that were IHC 0/1+. HER2 positivity differed significantly by histological subtype: 36% in intestinal, 7% in diffuse and 23% in mixed. HER2 positivity also varied according to the site of the tumour: 36% (8/22) for gastro-oesophageal junction tumours and 21% (60/291) for gastric tumours. Sample numbers were very small so these results must be treated with caution. The HER2- positivity rate was similar in specimens obtained by biopsy (168/689; 24%) and surgery (71/322; 22%). Conclusions: Using validated methodology and based on the large sample set from the ongoing ToGA trial, the HER2-positivity rate observed in advanced GC is as high as in breast cancer: ∼24%. The first efficacy data from ToGA are expected in 2009. No significant financial relationships to disclose.
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