Timing of Comprehensive Local Treatment After First-Line Systemic Therapy in Oligometastatic Esophageal or Gastric Adenocarcinoma

Inclusion Criteria: * Age ≥18 years * Ability to provide written informed consent * Histologically confirmed esophageal, gastric or gastroesophageal junction tumor with oligometastatic (M1) disease defined according to the OMEC consensus statement: * One organ with ≤3 metastases or 1 involved extra-regional lymph node station (based on the TNM 8 classification) * ≤3 unilobar liver metastases or ≤2 bilobar liver metastases * ≤ 3 unilateral lung metastases * Unilateral adrenal gland involvement * Metastasis confined to 1 bone structure or 1 soft tissue compartment * Synchronous oligometastatic disease with a resectable primary tumor or metachronous oligometastatic disease (in the event of a locoregional recurrence this should be resectable) * Metastases should be deemed amenable by the international multidisciplinary expert team for radical local treatment * WHO performance status 0-2 * Indication for checkpoint inhibition and/or targeted therapy * PD-L1 with a CPS of 1 or higher as per local clinical practice for immunotherapy use * HER2 overexpression as per local clinical practice for trastuzumab use * Claudin 18.2 overexpression as per local clinical practice for zolbetuximab use. * Any other biomarker that allows targeted therapy in first line approved by EMA * No prior systemic therapy for metastatic disease * CT-scan ≤8 weeks prior to inclusion * Ability to undergo local treatment and start systemic treatment beyond 18 weeks of total systemic treatment. Exclusion Criteria: * Squamous cell carcinoma * Brain metastases * Peritoneal or pleural carcinomatosis * Patients with MSI dMMR * Uncontrolled immunodeficiency (e.g. AIDS) * Peripheral neuropathy \>CTCAE grade 1, precluding start of full dose oxaliplatin treatment * Both organ metastasis and extra-regional lymph node metastasis * Conditions precluding local treatment or systemic therapy for oligometastatic disease: * Serious medical comorbidities precluding local treatment (e.g., interstitial lung disease in patients with pulmonary metastasis) * Clinical or radiological evidence of spinal cord compression or epidural tumor within 2 mm of the spinal cord * Simultaneous other malignancy or previous other malignancy with a disease-free period of \<5 years, except adequately treated non-melanoma skin cancer or in-situ cancers * Uncontrolled (bacterial) infections * Significant concomitant diseases preventing the safe administration of study drugs or likely to interfere with study assessments * Uncontrolled angina pectoris, cardiac failure or clinically significant arrhythmias * Continuous use of immunosuppressive agents equivalent to \>10 mg daily prednisone * Concurrent use of the antiviral agent sorivudine or chemically related analogues, such as brivudine * Pregnancy or breast feeding * Patients (M/F) with reproductive potential not implementing adequate contraceptive measures