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/mm³, platelets ≥ 100,000/mm3; Serum creatinine ≤ 1.5 x upper limit of normal, or GFR \> 40 ml/min; Bilirubin ≤ 1.5 x upper limit of normal, AST and ALT ≤ 3.5 x upper limit of normal, alkaline phosphatase ≤ 6 x upper limit of normal * Normal cardiac ejection fraction, as assessed by echocardiography * Written patient consent form Exclusion Criteria: * Known hypersensitivity against trastuzumab, murine proteins, 5-FU, leucovorin, oxaliplatin or docetaxel * Other known contraindications against trastuzumab, 5-FU, leucovorin, oxaliplatin, or docetaxel * Clinically significant active coronary heart disease, cardiomyopathy or congestive heart failure, NYHA III-IV * Clinically significant valvular defect * Past or current history of other malignancies not curatively treated and without evidence of disease for more than 5 years, except for curatively treated basal cell carcinoma of the skin and in situ carcinoma of the cervix * Known brain metastases * Severe dyspnoea at rest due to complications of advanced malignancy or requiring supplementary oxygen therapy * Other severe internal disease or acute infection * Peripheral polyneuropathy \> NCI Grade II * Chronic inflammatory bowel disease * On-treatment participation in another clinical study in the period 30 days prior to inclusion and during the study * Subject pregnant or breast feeding, or planning to become pregnant within 6 months after the end of treatment * Patients in a closed institution according to an authority or court decision (AMG § 40, Abs. 1 No. 4) * Any other concurrent antineoplastic treatment including irradiation