FAPI PET/CT in the Diagnosis of Abdominal and Pelvic Tumors

Abstract

Positron emission tomography/computed tomography (PET/CT) with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) is currently a standard imaging examination used in clinical practice, and plays an essential role in preoperative systemic evaluation and tumor staging in patients with tumors. However, ¹⁸F-FDG PET/CT has certain limitations in imaging of some tumors, like gastric mucus adenocarcinoma, highly differentiated hepatocellular carcinoma, renal cell carcinoma, and peritoneal metastasis. Therefore, to search for new tumor diagnosis methods has always been an important topic in radiographic imaging research. Fibroblast activation protein (FAP) is highly expressed in many epithelial carcinomas, and various isotope-labelled fibroblast activation protein inhibitors (FAPI) show lower uptake in the brain and abdominal tissues than in tumor, thus achieving high image contrast and good tumor delineation. In addition to primary tumors, FAPI PET/CT is better than FDG PET/CT for detecting lymph nodes and metastases. Additionally, the highly selective tumor uptake of FAPI may open up new application areas for the non-invasive characterization, staging of tumors, as well as monitoring tumor treatment efficacy. This review focuses on the recent research progress of FAPI PET/CT in the application to abdominal and pelvic tumors, with the aim of providing new insights for diagnostic strategies for tumor patients, especially those with metastases.

Keywords: 18F-FDG; 68Ga-FAPI-04; abdominal tumors; pelvic tumors; positron emission tomography computed tomography (PET/CT); radiotherapy.

Figure 1

A 65-year-old woman diagnosed with poorly differentiated gastric adenocarcinoma underwent ¹⁸F-FDG PET/CT (A) and ⁶⁸Ga-FAPI-04 PET/CT examination (B) respectively, showing a FAPI-positive, FDG-negative primary tumor (SUV_max 11.8 and 2.3, respectively) and a perigastric lymph node (SUV_max 2.3 and 0.3, respectively). With permission from Kuten et al. (49).

Figure 2

Another 78-year-old male diagnosed with poorly differentiated gastric adenocarcinoma, ⁶⁸Ga-FAPI-04 PET/CT (A, D) and ¹⁸F-FDG PET/CT scans (B, E) revealed highly uptake of FAPI in the primary tumor and peritoneal carcinomatosis compared with FDG (primary tumor SUVmax 23 and 6.8; primary tumor tumor-to-background ratio;11.5 and 3.8; peritoneal-carcinomatosis SUVmax 7.5 and 2.3, respectively). The ⁶⁸Ga-FAPI-04 PET/CT examination after 4 months of chemotherapy (C, F) showed the disease progression of this patient. The results demonstrated that ⁶⁸Ga-FAPI-04 PET/CT was superior to ¹⁸F-FDG PET/CT in detecting primary gastric adenocarcinoma and peritoneal carcinomatosis with a gastric cancer origin. With permission from Kuten et al. (49).

Figure 3

A 53-year-old male diagnosed with moderately-differentiated HCC, ¹⁸F-FDG PET/CT images did not detect intrahepatic positive findings (A), and ⁶⁸Ga-FAPI-04 PET/CT showed a strongly FAPI positive lesion in the right lobe of the liver (B). In PET/CT images of another 53-year-old male diagnosed I with recurrent HCC and extensive peritoneal diffusion, a small metastatic lesion was not detected in ¹⁸F-FDG PET/CT (C), which was found with increased uptake in ⁶⁸Ga-FAPI-04 PET/CT (D). Therefore, ⁶⁸Ga-FAPI-04 PET/CT was more sensitive than ¹⁸F-FDG PET/CT in the detection of HCC and peritoneal metastatic carcinoma. With permission from Wang et al. (45).

Figure 4

A 47-year-old woman diagnosed with ovarian cancer, ¹⁸F-FDG PET/CT showed the omental-cake-pattern of peritoneal carcinomatosis with low-to-moderate activity throughout the entire abdomen and pelvis (A). However, there were more details in ⁶⁸Ga-FAPI-04 PET/CT images, we could find that higher uptake was observed in the omental and peritoneal surfaces, especially around the liver and small bowel mesenterium (B). Subsequent pathological findings were consistent with ⁶⁸Ga-FAPI-04 PET/CT findings. ⁶⁸Ga-FAPI-04 PET/CT showed superiority over ¹⁸F-FDG PET/CT in the display of peritoneal metastatic carcinoma. With permission from Chen et al. (88).