Stereotactic radiotherapy (SRT) is a newer type of focused radiation therapy that precisely and accurately delivers high dose radiation to a tumour, while sparing much of the nearby normal organs. The use of stereotactic radiotherapy results in high rates of tumour destruction with minimal side effects which are very well tolerated. Often stereotactic radiotherapy has been used to try to cure patients who have an early stage cancer which has not spread, but there is less experience with using it in patients with cancer which has spread.

The purpose of this study is to measure how well stereotactic radiotherapy can destroy kidney cancer tumours which are no longer being controlled by Sunitinib and to measure how much longer such an approach will allow patients to stay on Sunitinib before needing to switch to another medication. Stereotactic radiotherapy will be used to treat only the growing tumours and then patients will continue with Sunitinib.

Inclusion Criteria:

1. Age ≥ 18 years.
2. Able and willing to provide written informed consent and to comply with the study procedures.
3. Karnofsky performance status of ≥ 80%.
4. Favorable or intermediate Heng prognostic group defined as having two or less of the following factors: hemoglobin < LLN; serum corrected calcium > ULN; Karnofsky performance status < 80%; time from initial diagnosis to initiation of therapy < 1 year; absolute neutrophil count > ULN; platelet count > ULN.
5. Histologic confirmation of renal cell carcinoma with a clear cell component.
6. Evidence of measurable disease according to RECIST 1.1 criteria.
7. Already receiving first-line sunitinib therapy for at least 3 months with at least one imaging compared to baseline (or a CT from one year prior to the date of registration for patients that have been on Sunitinib therapy for over one year) that shows response or stable disease per RECIST v1.1, in all metastatic lesions (Note: SD by RECIST that allows ≤ 19% increase is allowed). Prior immunotherapy including interferon, IL-2, and checkpoint inhibitors is allowed before Sutent.
8. Radiographic evidence of ≤ 5 metastatic lesions progressing. Of the 5 progressing lesions, a maximum of 3 lesions can be in soft tissue. (Ex. If no bone metastases progressing: a maximum of 3 soft tissue lesions. If bone metastases progressing: a maximum of 5 total lesions and a maximum of 3 in soft tissue.)
9. All progressing metastases are amenable to stereotactic radiotherapy.
10. Each progression metastases fulfills at least 1 of the 3 following criteria for oligo-progression: a. Progression of an individual metastasis according to RECIST 1.1 criteria (≥ 20% enlargement of the tumour vs. baseline or nadir, taking as reference the smallest diameter seen prior to starting or during first line systemic therapy and associated with a 5 mm minimum increase in size); b. Unambiguous development of a new metastatic lesion from the time of scan taken prior to starting first-line therapy with sunitinib; c. Progressive enlargement of a known metastasis on 2 consecutive imaging studies 2-3 months apart, while receiving first-line therapy with sunitinib, with a minimum 5 mm increase in size from baseline.

Exclusion Criteria:

1. Evidence of spinal cord compression.
2. Inability to safely treat all sites of progressing metastases.
3. Prior malignancy within the past 5 years, excluding non-melanoma skin cancer and in-situ cancer.
4. Concurrent administration of other anti-cancer therapy apart from first-line sunitinib.
5. Diagnosis of ataxia telangiectasia or active collagen vascular disease.
6. Other condition, illness, psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or stereotactic radiotherapy administration, or may interfere with the interpretation of study results and in the judgment of the investigator would make the patient inappropriate for entry into this study.