Sequential 18F-FDG and 68Ga-FAPI-04 PET/CT in Early Peritoneal Recurrence of Signet-Ring–Component Rectal Adenocarcinoma

A 60-year-old man with signet-ring-component poorly differentiated rectal adenocarcinoma (pMMR, HER2-negative, KRAS/NRAS/BRAF wild-type) received total neoadjuvant therapy, low anterior resection, and adjuvant CAPOX. During biochemical progression (CEA 7.85 → 171.0 µg/L over ∼4 mo), 18F-DG PET/CT showed postoperative/anastomotic uptake (SUVmax 8.5) without definite peritoneal-serosal disease. 68Ga-FAPI-04 PET/CT performed 38 days later demonstrated intense uptake in peritoneal-serosal and mesenteric implants (SUVmax 51.2), the anastomotic and colostomy-site regions (SUVmax 9.3), and a Couinaud segment VI hepatic lesion (SUVmax 6.3). The case illustrates the complementary value of 68Ga-FAPI-04 PET/CT in suspected stromal-rich peritoneal recurrence in a sequential imaging setting.