## 1 Introduction  
The two most common malignant liver neoplasms, hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA) represent significant risks for global health. In 2022, around 757,948 people died from liver cancer, making it the third most common cause of cancer-related death and the sixth most prevalent kind of cancer to be diagnosed. Among them, intrahepatic CCA makes up 10-15% of instances in comparison to HCC, which makes up 75-85% [1]. The risk factors of these two tumors have characteristics in common, such as chronic liver diseases due to HBV, HCV, alcohol abuse, non-alcoholic fatty liver disease (NAFLD), and subsequent cirrhosis [2, 3, 4]. CCA, beyond these factors, presents a more complex etiology, of which risk factors comprise primary sclerosing cholangitis, gallstones, or biliary tract parasitosis contribute [5, 6]. CCA is characterized by a conspicuous hypovascular desmoplastic stroma. Cancer-associated fibroblasts (CAFs), extracellular matrix proteins, and growth factors make up this special setting, which is essential for tumor growth and treatment resistance [7; 8; 9]. The tumor microenvironment (TME) of HCC is also made up, in addition to tumor cells, of various elements such as immune cells, vascular cells, extracellular matrix, and CAFs, which represent more than 50% of the tumor mass. This TME, including CAFs, promotes the proliferation of neoplastic cells, as well as local and distant invasion. Fibroblast activation protein (FAP) is a membrane glycoprotein expressed on CAFs and is a marker associated with tumor invasion and neoangiogenesis. Its overexpression is therefore associated with a poor prognosis in several neoplasms, including HCC, pancreatic tumors, and colon-rectal cancer [10; 11; 12; 13; 14]. Early diagnosis of both HCC and CCA is often difficult. The diagnosis of biliary tract tumors (BTCs) is frequently delayed by nonspecific symptoms, necessitating a multimodal diagnostic approach based on imaging, specifically magnetic resonance imaging (MRI), contrast-enhanced (CE) computed tomography (CT), and ultrasound followed by biopsies. Although positron emission tomography (PET) using \({}^{18}\)Fluoro-2-deoxyglucose (\({}^{18}\)F-FDG) may be helpful in staging, a significant false-positive rate brought on by conditions including local infections and biliary stenting limits its usage [15]. \({}^{18}\)F-FDG-PET is useful for identifying metastases and recurrences in HCC, even though its sensitivity is lower for well-differentiated tumors [16; 17]. Through the last few years, innovative technologies like PET-CT with FAP-targeted radiopharmaceuticals have shown significant potential in tumor imaging. This approach can selectively identify FAP expressed by CAFs [10]. Most of the published studies are based on the use of FAP inhibitors (FAPI) radiolabelled with 68-Gallium (\({}^{68}\)Ga). A minority of published studies have been performed by using other isotopes rather than \({}^{68}\)Ga, mostly \({}^{18}\)F [18], as well as FAP binding motifs rather than FAPI inhibitors [19]. FAP-targeted radiopharmaceuticals have a physiological biodistribution and a high uptake in the salivary glands, thyroid, gallbladder, uterus, and renal pelvis. Among these tissues, the gallbladder and renal pelvis serve as the excretion route. A moderate uptake is present in the cervical, respiratory, abdominal, and pelvic organs [20]. These radiotracers have been demonstrated to be particularly effective in a variety of cancers, including HCC, CCA, and various gastrointestinal tumors such as those of the pancreas, stomach, colorectal region, esophagus, duodenum, and anus. It has also been successfully used in gynecological cancers (e.g., cervix and ovary), breast cancer, head and neck cancers (including nasopharyngeal carcinoma), non-small cell lung cancer, sarcomas, and cancers of unknown primary origin [21; 22; 23; 24].