Superior Diagnostic Performance of 99mTc-FAPI-46 SPECT/CT Versus 18FDG PET/CT in Detecting Peritoneal Involvement in Mucinous Adenocarcinoma of the Colon

A 44-year-old man with a history of ulcerative colitis and primary sclerosing cholangitis presented with fecaluria and was found to have rectal and cecal masses, later histologically confirmed as mucinous adenocarcinomas forming rectovesical fistulas. Despite chemotherapy reducing the primary tumor size, elevated serum CEA levels prompted further imaging. An 18F-FDG PET/CT scan showed no new lesions, but a subsequent 99mTc-FAPI-46 scintigraphy revealed diffuse peritoneal involvement. During surgery for bowel obstruction, multiple metastatic deposits were identified. This case underscores the superior detection capability of 99mTc-FAPI-46 SPECT/CT in assessing peritoneal carcinomatosis in colorectal mucinous adenocarcinomas compared with 18F-FDG PET/CT.