Radiotherapy in combination with different forms of immune therapy improved consistently local tumor control and very interestingly, lead to better systemic tumor control and the induction of specific anti-cancer immunity with a memory effect. In small series, it has been shown that a new long-lasting remission can be induced by irradiating one tumor site in patients who showed cancer progression after an initial response to immune therapy. In these series, the original immune therapy was continued and the treatment was very well tolerated. In this study the progression-free survival after radiotherapy to a single lesion will be investigated in patients with stage IV non-small cell lung cancer (NSCLC), who have at least achieved stable disease with immune therapy alone or concurrent immune therapy and chemotherapy.

Radiation has consistently been shown to activate key elements of the immune system. Radiotherapy in combination with different forms of immune therapy such as anti-PD-(L)1, anti-CTLA4,immunocytokines, dendritic cell vaccination and Toll-like receptor agonists improved consistently local tumor control and very interestingly, lead to better systemic tumor control (the "abscopal" effect) and the induction of specific anti-cancer immunity with a memory effect. Moreover, as PD1/PD-L1 is upregulated by radiation and radiation can overcome resistance for PD-(L)1 blockage, their combination is logical.

In small series, it has been shown that a new long-lasting remission can be induced by irradiating one tumor site in patients who showed cancer progression after an initial response to immune therapy. In these series, the original immune therapy was continued and the treatment was very well tolerated. In this study the progression-free survival after radiotherapy to a single lesion will be investigated in patients with stage IV non-small cell lung cancer (NSCLC), who have at least achieved stable disease with immune therapy alone or concurrent immune therapy and chemotherapy.

Inclusion Criteria:

• Stage IV non-small cell lung cancer
• Initially a Complete Remission, Partial Remission or Stable Disease under immune therapy alone or concurrent immune therapy and chemotherapy and now progressive disease
• Able to continue the immune therapy

Exclusion Criteria:

• Not able to continue the already initiated immune therapy
• Patients with any grade 3 toxocity
• Patients in whom radiotherapy cannot be delivered, according to the radiation oncologist at the multi-disciplinary patient discussion