CD8 Minibody Repeatability Study (PRETZCEL)

Inclusion Criteria: Aged 18 years and above Patients with at least one imageable primary or metastatic lesion > 1 cm in the lymph nodes or soft tissue excluding bone on the most recent standard of care imaging done within the last 3 months Cohort 1: Patients with melanoma on single or multiple agent Immune Checkpoint Blockade (ICB) therapy with stable response status following initiation of therapy. Stable disease would be observed over at least 2 consecutive standard of care CT scans (usually done 8-12 weeks apart) after initiation of therapy. Cohort 2: Patients with untreated renal cell carcinoma on active surveillance Women of child bearing potential must not be pregnant on study entry Participants must agree to follow contraceptive advice while on study and for 30 days after ending participation. Exclusion Criteria: Patients with uncontrolled infection or other concurrent malignancies or conditions that may confound interpretation of the scans in the opinion of the investigator Patients with urinary catheters or stoma bags Patients who have received a vaccine recently can be included to the study but can only be scanned after a washout period of 2 weeks after the vaccine. If participants have a vaccine after the first PET-CT scan, the second PET-CT scan will be delayed for at least 2 weeks after the vaccine Patients on steroids except those on replacement dose of steroids for adrenal or pituitary insufficiency. Patients requiring steroids as therapy for an unresolved immune therapy related adverse event will be excluded Participants on immunosuppressive medication e.g. cyclosporine, azathioprine, janus kinase inhibitors Participant enrolled into another therapeutic intervention study Any other contraindication that makes the patient unsuitable to take part in the study in the opinion of the investigator Women who are pregnant or breast feeding Unable to provide informed consent