A Study of 68Gallium PSMA-PET/CT Scans in People With Bladder Cancer or Skin Cancer

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.