SBRT + PD-1/PDL-1 Inhibiting Therapy for Advanced Solid Tumors After Dz Control on PD-1/PDL-1 Tx

Inclusion Criteria: * Pathologically-proven diagnosis of a solid tumor malignancy * One of the following criteria must be met: * Clinical or radiographic evidence of disease control (defined as best response of stable disease (SD) or partial response (PR) or combination of both for ≥ 16 weeks) without evidence of complete response (CR) or progression OR * Clinical or radiographic evidence of disease progression during treatment with PD-1 or PD-L1 inhibiting therapy, following previous tumor response (CR, PR, or SD for ≥ 16 weeks) to PD-1 or PD-L1 inhibiting therapy, and, for patients who discontinued PD-1 or PD-L1 inhibiting therapy during response to therapy, disease progression must have occurred following at least 8 weeks of re-treatment with PD-1 or PD-L1 inhibiting therapy Note: Both the treating medical oncologist and radiation oncologist must be in agreement with determination of disease progression. * Administration of a PD-1 or PD-L1 inhibitor within 60 days prior to study registration * Determination by the treating radiation oncologist that the patient is a candidate for SRT (ie, radiation therapy with a stereotactic setup) Note: All brain metastases will receive stereotactic radiotherapy (SRT). * The total number of tumors requiring SRT must be ≤ 5 Note: Regardless of the number of brain metastases that will be treated with SRT, the brain metastases will be considered to be one tumor. * Measurable disease by RECIST v1.1 that will not undergo SRT and that is amenable to monitoring Note: all brain metastases will receive SRT. Therefore, a patient with brain metastases that will be treated with SRT must also have extracranial disease that will not undergo SRT and that is amenable to monitoring * Determination by the treating medical oncologist that the patient is a candidate to continue the PD-1 or PD-L1 inhibiting therapy that had previously provided disease control * Age ≥ 18 years * Karnofsky Performance Status score of ≥ 60 % * A woman of childbearing potential (WCBP), defined as a woman who is \< 60 years of age and has not had a hysterectomy, must have a documented negative serum pregnancy test within 14 days prior to initiating study treatment * Ability to understand and willingness to sign the consent form Exclusion Criteria: * Other anti-cancer therapy administered between the time of tumor response to PD-1 or PD-L1 therapy and time of study enrollment Note: Patients treated with a combination of PD-1 or PD-L1 inhibiting therapy and other immunotherapy are eligible; patients taking hormonal anti-cancer therapies or steroids for central nervous system (CNS) edema management that, in the opinion of the investigator, are appropriate to continue are eligible. * Any prior PD-1/PD-L1 therapy-related AE that, in the opinion of the investigator, warrants exclusion from participation in this trial * Administration of any investigational agent within 4 weeks prior to initiating study treatment * Known active hepatitis B or C * Pregnancy or breastfeeding * Medical, psychological, or social condition that, in the opinion of the investigator, may increase the patient's risk or limit the patient's adherence with study requirements