This study aims at enrolling patients with solid tumors at metastatic stage, considered long responders to immunotherapy (> 6 months) and displaying disease progression.

In this study, the investigator wants to evaluate specific modalities of stereotactic radiotherapy, with 3 sessions, each of 8 Gy, lasting 20 minutes and spaced 72 hours apart (Day 1, Day 4, Day 7). The radiotherapy device itself is not the subject of this study and will be used in accordance with its CE mark and indications.

The objective of the study is to assess the ability of stereotactic radiotherapy to restore the lost efficacy of immunotherapy. In particular, the abscopal effect will be assessed, i.e. the action of irradiating a particular target lesion and observing an effect on other distant metastases.

Inclusion Criteria:

1. Male or female, 18 years of age or older,
2. Patient with a metastatic solid tumor,
3. Patient with at least one target lesion (for radiotherapy) together with other secondary lesions that must be measurable,
4. Measurable disease according to RECIST 1.1 and iRECIST criteria,
5. Patient undergoing anti-PD1 immunotherapy as part of standard of care treatment,
6. Disease progression with immunotherapy (progression assessed by iRECIST criteria) but having received at least 6 months of immunotherapy,
7. Performance Status (PS) of 0 or 1,
8. Patient eligible for stereotactic radiotherapy,
9. Patient affiliated to or benefiting from a social security scheme,
10. Patient having been informed of the investigation and having signed the informed consent form prior to any study-specific procedures,
11. Patient willing and able to undergo all examinations and procedures in accordance with the clinical investigation plan.

Exclusion Criteria:

1. Patient having received previous radiotherapy since the start of immunotherapy,
2. Patient for whom first disease progression (pre-inclusion stage) is not confirmed secondarily (inclusion stage),
3. Patient with an active implantable medical device (e.g. pacemaker) or an active medical device worn on the body that could interact adversely with stereotactic radiotherapy treatment,
4. Patient with hematological cancer,
5. Symptomatic patient requiring a rapid change of systemic line according to the investigator's judgment,
6. Concurrent participation in another clinical trial, or patient having received an investigational product within 30 days prior to inclusion,
7. Pregnant or breast-feeding women,
8. Patient under legal protection.