CHEckpoint Inhibition in Combination With an Immunoboost of External Beam Radiotherapy in Solid Tumors

Inclusion Criteria: * Before patient registration, written informed consent must be given according to ICH/GCP and national/local regulations. * Histologically confirmed diagnosis of a solid tumour. * At least one extracranial tumour lesion available for radiotherapy administration. * Patient will receive a checkpoint inhibitor per standard of care in one of the following settings (locally advanced or metastatic): melanoma (1st - 3rd line nivolumab or pembrolizumab); renal cell carcinoma (2nd line nivolumab); non-small cell lung carcinoma (2nd or 3rd line nivolumab, pembrolizumab or atezolizumab); urothelial cell carcinoma ( 1st-3rd line nivolumab, pembrolizumab or atezolizumab); head-\& neck squamous cell carcinoma (1st-2nd line pembrolizumab, 2nd line nivolumab). * Karnofsky Performance status \> 60. * Age 18 years or older. Exclusion Criteria: * Prior radiotherapy preventing treatment with SBRT. * Prior treatment with an anti-PD-(L)1 antibody. * Has a known additional malignancy that is progressing or requires active treatment. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer or prostate cancer that has undergone potentially curative therapy and with normalized PSA. * Uncontrolled central nervous system (CNS) metastases at baseline (controlled = previously-treated CNS metastases (surgery ± radiotherapy, radiosurgery, or gamma knife) and who meet both of the following criteria: a) are asymptomatic and b) have no requirement for steroids or enzyme-inducing anticonvulsants), and/or carcinomatous meningitis. * Any condition requiring systemic treatment with corticosteroids (\> 10 mg daily prednisone equivalent) or other immunosuppressive medication within 14 days prior to the first dose of study drug. Inhaled steroids and adrenal replacement steroid doses \> 10 mg daily prednisone equivalent are permitted in the absence of active autoimmune disease. * Has a diagnosis of immunodeficiency or history of human immunodeficiency virus (HIV), Hepatitis B or Hepatitis C infection. * Mental condition rendering the patient unable to understand the nature, scope and possible consequences of the study. * Patient not likely to comply with the protocol; I.e. uncooperative attitude, inability to return for follow-up visits and unlikely to complete the study. * Contraindication for radiotherapy. * Female subjects of childbearing potential must be willing to use an adequate method of contraception for the course of the study through 120 days after the last dose of study medication.