The overall aim of this pilot study is to prospectively monitor programmed death-ligand 1 (PD-L1) expression dynamics in vivo, during neoadjuvant chemoradiotherapy (CRT) or short-course preoperative radiotherapy (SCPRT) in rectal and esophageal cancer by a positron emission tomography (PET) imaging approach.

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