Different Features of 18F-FAPI, 18F-FDG PET/CT and MRI in the Evaluation of Extrahepatic Metastases and Local Recurrent Hepatocellular Carcinoma (HCC): A Case Report and Review of the Literature

Abstract

Background: Recurrence and metastasis are important causes of postoperative death in most HCC patients. Conventional imaging modalities such as ¹⁸F-FDG PET/CT and enhanced MRI are still unsatisfactory in evaluating these patients in the clinical setting. PET/CT imaging with a radiolabeled fibroblast activation protein inhibitor (FAPI) has emerged as a new imaging technique for the diagnosis and radiotherapy of malignant tumors. While many studies have focused on the diagnostic accuracy of intrahepatic primary HCC, the evaluation of recurrent and metastatic HCC remains only poorly investigated.

Case presentation: A 71-year-old man with a five-year history of HCC after radical resection underwent ¹⁸F-FDG PET/CT due to further surgery for tumor recurrence, which revealed two iso-metabolic lesions in the right peritoneum and a hypo-metabolic lesion in the right liver. ¹⁸F-FAPI PET/CT was performed to further complement ¹⁸F-FDG PET/CT in the detection of these suspected metastatic lesions. Importantly, multiple diffuse intense radioactivity was shown in the hepatic capsule, suggesting metastatic lesions, but a wedge-shaped elevated ¹⁸F-FAPI uptake disorder around the FDG-unavid necrotic lesion after radiofrequency ablation (RFA) demonstrated benign stromal fibrosis.

Conclusion: This case suggested that ¹⁸F-FAPI may have an advantage over ¹⁸F-FDG in detecting peritoneal metastasis even in tiny or early hepatic capsules of HCC, but its false positives due to postoperative stromal fibrosis should be noted. Wedge- or strip-shaped FAPI-avid lesions with sharp edges may be post-treatment stromal fibrosis.

Keywords: 18F-FAPI; 18F-FDG; PET/CT; false-positive; hepatocellular carcinoma; magnetic resonance imaging; stromal fibrosis.

Figure 1

¹⁸F-FDG PET/CT revealed iso-metabolic metastases in the right peritoneum (A, yellow arrow), hypo-metabolic and low-density tumor necrosis after RFA in the right liver (A, red arrow) and low uptake in peritoneal metastases (A, white arrow). ¹⁸F-FAPI PET/CT showed increased diffuse uptake of metastases in the hepatic envelope (B, white arrow) and hepatic capsule (B, yellow arrows). ¹⁸F-FAPI PET/CT also showed a wedge-shaped high ¹⁸F-FAPI uptake disorder around the tumor necrosis lesion after RFA in the right liver (B, blue arrow). Increased diffuse ¹⁸F-FAPI uptake in the pancreas (B; white hollow arrows) indicated pancreatitis.

Figure 2

Enhanced liver MRI showed low signal intensity of hepatic capsular nodules on the arterial enhanced phase (A and B; yellow arrows) with high signal intensity on T2-phase (C, yellow arrow) and mild enhancement of the edge on the venous enhanced phase (D and E, yellow arrows). These lesions were resected, and metastases were confirmed by pathology (Figure 3). MRI on axial slices also showed a wedge-shaped high signal intensity around the tumor necrosis lesion after RFA in the right liver in the arterial and venous enhanced phases (B and D, blue allows).

Figure 3

Elastic fiber staining (A, black arrow, magnification, ×50), PAS staining (B, black arrow, magnification, ×50) and Masson staining (C, black arrow, magnification, ×50) showed a large of fibrous tissue with strong FAP expression in stromal cells in FAP staining (D, red arrow, magnification, ×50) around the tumor necrosis lesion. Hematoxylin-eosin staining (E, magnification, ×50) showed peritoneal metastatic hepatocellular carcinoma with strong FAP expression in stromal cells in FAP staining (F, red arrows, magnification, ×100).