Inclusion Criteria:  Histologically confirmed malignancy with at least one metastatic lesion referred for palliative radiotherapy Pathogenic mutation in ATM (somatic and germline allowed) ECOG performance status 0-2 Age ≥ 18 years. Able to provide informed consent. Patients at reproductive potential must agree to practice an effective contraceptive method during radiation treatments. Expected life expectancy of at least 6 months  Exclusion Criteria:  Previous radiotherapy to the intended treatment site that precludes developing a treatment plan that respects normal tissue tolerances. Serious medical co-morbidities precluding radiotherapy. Pregnant or breast-feeding women. Concomitant pathogenic mutation in p53 Lesions excluded from de-escalation include regions in which the risk of local disease progression is unacceptably high, including CNS disease and cord compression, and areas in which re-irradiation would not be feasible, including spinal cord overlap.