PD-L1 PET Imaging During Neoadjuvant (Chemo)Radiotherapy in Esophageal and Rectal Cancer

Inclusion Criteria: * 18 years of age and older * All sexes * Histologically confirmed carcinoma of the rectum or oesophagus (squamous cell carcinoma and adenocarcinoma, including oesophago-gastric junction cancers) * Medical need for a neoadjuvant CRT/SCPRT * Suitable to withstand the course of neoadjuvant CRT/SCPRT * Written informed consent form (ICF) for participation in the study Exclusion Criteria: * Metastatic disease, which is considered incurable by local therapies (expect for oligometastatic disease with a curative intend) * Previous surgery of the tumor other than biopsy * Pregnancy, breastfeeding or expectancy to conceive * Prior therapy with anti-CTLA-4, anti-PD-1, anti-PD-L1, anti-PD-L2 or any other agent directed against co-inhibitory T cell receptors or has previously participated in clinical studies with immunotherapy * Disagreement of participants with reproductive potential to use contraception throughout the study period and for up to 180 days after the last dose of study therapy * Hepatitis B or C * Human immunodeficiency virus (HIV) * Immunodeficiency * Allogeneic tissue or solid organ transplantation * Autoimmune disease that has required systemic therapy in the past 2 years with modifying agents, steroids or immunosuppressive drugs * Active non-infectious pneumonitis * Active infection requiring systemic therapy * Systemic steroids or any other form of immunosuppressive therapy within 7 days prior to the first dose of study treatment * Diagnosed and/or treated additional malignancy within 5 years of randomization, with the exception of curatively-treated basal cell or squamous cell carcinoma of the skin and/or curatively-resected in situ cervical and/or breast cancers * Treatment with botanical preparations (i.e. herbal supplements or traditional Chinese medicines) intended for general health support or to treat the disease under study within 2 weeks prior to randomization/treatment * Participants with serious or uncontrolled medical disorders * Uncontrolled or significant cardiovascular disease (myocardial infarction, uncontrolled angina, any history of clinically significant arrhythmias, QTc prolongation in males \> 450 ms and \> 470 ms in females, participants with history of myocarditis) * Allergies and adverse drug reaction (history of allergy or hypersensitivity to study drug components, contraindications to any of the study drugs of the chemotherapy regimen) * Other exclusion criteria: Prisoners or participants who are involuntarily incarcerated, participants who are compulsorily detained for treatment of either a psychiatric or physical (i.e. infectious disease) illness