Stereotactic Ablative Radiation for Oligo-Progression of Urothelial Cancer

Inclusion Criteria: * Patients must be at least 18-years-old * ECOG performance status 0-2 * Patients must have pathology-proven metastatic urothelial carcinoma, with tissue sampling of at least the primary tumor. Tissue sampling of each presumed metastatic site is not necessary, provided that the patient already has a confirmed diagnosis of urothelial cancer. * Patients must be on immune checkpoint inhibitor therapy with radiographic scans to verify oligo-progression of at least one and ≤ 6 sites of disease per RECIST 1.1. * Any of the currently FDA-approved PD-1/PD-L1 inhibitors are allowed. These include pembrolizumab, nivolumab, atezolizumab, avelumab, and durvalumab. * At least 1 metastatic lesion must show stable disease, partial response or complete response per RECIST 1.1. * Patients must be able to understand and willing to sign written informed consent. * Patients must have acceptable tolerability of ongoing therapy as decided by the treating medical oncologist. * Patients must have a desire to continue ongoing therapy. * Women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for 90 days following completion of therapy. * A female of child-bearing potential is any woman (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: * Has not undergone a hysterectomy or bilateral oophorectomy, or has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months). * Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. Exclusion Criteria: * Inability to receive further immune checkpoint inhibitor therapy * Anticipated survival of fewer than 12 weeks * Daily steroid requirement of \> 10 mg prednisone or prednisone-equivalent. Steroid replacement therapy for adrenal insufficiency is permitted. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that, in the opinion of the investigator, would limit compliance with study requirements. * Subjects must not be pregnant or nursing due to the potential for congenital abnormalities and the potential of this regimen to harm nursing infants.