Background: Fibroblast-activation protein (FAP) is an enzyme that appears in high numbers in cancer-associated fibroblasts of certain cancer types. \[18F\]FAPI-74 is a new PET (positron emission tomography) tracer, a substance that is injected into a person s body before an imaging scan. Researchers believe that \[18F\]FAPI-74 PET imaging may be able to visualize cancer more effectively than the approved tracers. If so, the new tracer would make it easier to find FAP-positive tumors in the body.

Objective: To see if \[18F\]FAPI-74 PET scan is as good or better than other imaging methods for detecting certain cancers.

Eligibility: People aged 18 years or older with one of these cancer types: pancreatic ductal adenocarcinoma (PDAC), cholangiocarcinoma, hepatocellular carcinoma (HCC), gastric cancer, bladder cancer, ovarian cancer, pheochromocytoma/paraganglioma (PPGL), small cell lung cancer (SCLC) or extrapulmonary neuroendocrine cancer (EP-NEC), mesothelioma or sarcoma. Participants must be scheduled or intended to receive treatment for cancer.

Design: Participants will have 2 baseline scans: an \[18F\]FAPI-74, and the approved tracer \[18F\]-FDG. The \[18F\]FAPI-74 will be infused through a needle inserted into a vein. About 1 hour later, the participant will undergo imaging. Within 1 week, participants will undergo the same scanning procedures with the approved tracer. If the baseline scan with \[18F\]FAPI-74 shows the tumor(s), scans with this tracer will be repeated when their regular treatment regimen calls for scans again. If the scan with the regular FDG also show tumors, this scan will be repeated within the same week as the repeated \[18F\]FAPI-74 scan. If \[18F\]-FAPi PET scan shows no tumor(s), scans will not be repeated. If the participant's cancer progresses within 2 years, scans may be repeated. Follow-up calls will continue for 2 years.