This study explores the clinical application of 68Ga-DOTA-CCK-FS PET/CT in detecting cholecystokinin-2 receptor (CCK-2R)-positive tumors, particularly medullary thyroid cancer (MTC) and other malignancies. Led by Prof. Luo Yaping (PUMCH Nuclear Medicine) and Prof. Liu Zhibo (Peking University, radiochemistry expert), the trial will enroll 30-40 patients to compare 68Ga-DOTA-CCK-FS imaging with standard PET/CT (e.g., 18F-FDG or 68Ga-DOTATATE). The novel tracer shows higher tumor uptake and retention in preclinical studies, potentially improving diagnosis and treatment guidance for aggressive, CCK-2R-expressing cancers. The study leverages PUMCH's Class IV Radioactive Drug License for advanced radiopharmaceutical development. Risks are minimal (diagnostic radiation dose only), and participants receive free imaging assessments. Results aim to refine precision diagnostics for MTC and related tumors.