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