Explorative Phase II Study of Perioperative Treatment in Patients With Adenocarcinoma of the Gastroesophageal Junction or Stomach

Inclusion Criteria: \- Histologically confirmed adenocarcinoma of the gastroesophageal junction (AEG I-III) or the stomach (uT2, uT3, uT4, any N category, M0), or any T N+ M0 patient, with the following specifications: Endosonography and an esophageal-gastro-duodenoscopy; Categorization of gastroesophageal junction tumors according to the classification by Siewert (1987, cf. appendix 2) * Detection of an adenocarcinoma with HER2 3+ (IHC) or HER2 2+ (IHC) with amplification proven by FISH, SISH or CISH by an accredited local pathologist (for quality assurance tumor samples have to be available for a subsequent central review) * No preceding cytotoxic or targeted therapy * Male and female patients aged ≥ 18 years. If able to reproduce, patients must be willing to use highly effective methods of contraception during treatment and for 6 months after the end of treatment (adequate: methods fulfilling the requirements of the Note for guidance on non-clinical safety studies for the conduct of human clinical trials for pharmaceuticals \[CPMP/ICH/286/95 mod\]). Female patients with reproductive ability must have performed a negative pregnancy test within 7 days of study entry. * ECOG ≤ 2 * Exclusion of distant metastasis by CT of thorax and abdomen, bone scan or MRI (if osseous lesions are suspected due to clinical signs) * Laparoscopic exclusion of peritoneal carcinomatosis, if suspected clinically * Adequate haematological, hepatic and renal function parameters: Leukocytes ≥ 3000/…