• Participants must be aged ≥ 18 years at the time of screening

• Written informed consent and any locally required authorization (e.g., Health Insurance Portability and accountability Act) obtained from the patient/legal representative prior to performing any protocol-related procedures, including screening evaluations

• Life expectancy ≥ 12 months

• RCC, diagnosed by radiographic imaging and histology

• Clinical stage of RCC: T1 or T2a, Cancer stage (N0M0)

• 1-2 solid (\> 80% solid) target lesions

• Patient not an ideal candidate for partial nephrectomy or thermal ablation at the time of study entry, based on the decision of the institution's multidisciplinary tumor board. Contraindications for partial nephrectomy include inability to potentially partially resect the kidney, high risk of adverse events due to medical comorbidities, or potential high risk of adverse events due to general anesthesia. Contraindications to thermal ablation include potential inability to technically place ablation probes into the tumor, central tumors which risk thermal injury to the renal collecting system

• Patient not considered a candidate for long-term active surveillance due to oncologic risk due to tumor growth and/or tumor size

• Patient not considered ideal candidates for radical nephrectomy due to surgical comorbidity and/or development of adverse health outcomes

• Measurable tumor by RECIST 1.1 criteria

• Absence of bilateral renal tumors

• Negative serum pregnancy test in females of child-bearing potential; patients who are breast-feeding cannot participate in this trial

• Hemoglobin ≥ 9.0 g/dL

• Absolute neutrophil count ≥ 1.5 x 10\^9/L

• Absolute lymphocyte count ≥ 1.0 x 10\^9/L

• Platelet count ≥ 75 x 10\^9/L

• Glomerular filtration rate (GFR) ≥ 45 mL/min/1.73 m\^2

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 at enrollment

• Screening mapping angiogram demonstrates successful localization of tumor(s), where catheter placement location(s) would allow Y90 to distribute in the intended treatment area, without venous shunting