The purpose of this study is to see whether 68Gallium PSMA-PET/CT scans are an effective way to detect sites of cancer in people with metastatic bladder cancer or skin cancer. The study researchers want to learn if a 68Gallium PSMA PET/CT scan will work better, the same, or not as well as the PET/CT scans doctors usually use for imaging bladder cancer or skin cancer (FDG-PET/CT scan).

Inclusion Criteria for Urothelial Carcinoma Cohort:

• Patients with histologically confirmed metastatic urothelial carcinoma with extrapelvic nodal and/or visceral sites of disease (including lung, liver, bone, or soft tissue)
• At least 1 lesion assessable by FDG PET/CT according to RECIST and PERCIST guidelines where applicable, that are determined suspicious for metastasis by an MSKCC attending radiologist or nuclear medicine physician.
• Karnofsky performance status ≥50% (or ECOG/WHO ≤2)
• Participant is ≥18 years of age
• Patient must be able to understand and is willing to sign a written informed consent document

Inclusion Criteria for Melanoma Cohort:

• Patients with histologically confirmed metastatic melanoma
• At least 1 metastatic lesion assessable by CT or FDG PET/CT according to RECIST that is determined to be suspicious for metastasis by an MSKCC attending radiologist or nuclear medicine physician
• ECOG <= 2
• Participant is >= 18 years of age
• Patient must be able to understand and is willing to sign a written informed consent document

Exclusion Criteria for Urothelial Carcinoma Cohort:

• Patients with pelvic node-only metastatic disease. If the patient has lymph node only disease, at least one PET-assessable node must be located outside of the pelvis
• Patients with bone only disease
• Unable to lie flat, still, or to tolerate a PET scan
• Patient undergoing active treatment for non-urothelial malignancy, other than skin basal cell or cutaneous superficial squamous cell carcinoma that has not metastasized.
• Patients on a therapeutic clinical trial where PSMA imaging would interfere with the conduct of the trial
• Patients undergoing active surveillance with a known history of non-urothelial malignancies
• Women who are pregnant. All women of childbearing potential must have a documented negative pregnancy test.

Exclusion Criteria for Melanoma Cohort:

• Unable to lie flat, still, or tolerate PET scan.
• Patient is on another therapeutic trial where PSMA imaging would interfere with the conduct of the trial.
• Women who are pregnant. All women of childbearing potential must have a documented negative pregnancy test.

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.