Tumor Characterization by [68Ga]FAPI-46 PET/CT Can Improve Treatment Selection for Pancreatic Cancer Patients: An Interim Analysis of a Prospective Clinical Trial

FAPI PET Versus FDG PET/CT in Gastrointestinal Cancers: An Overview

Fibroblast activation protein (FAP) is a type II transmembrane serine protease that is highly expressed in cancer-associated fibroblasts (CAFs) but absent in quiescent fibroblasts. Its overexpression is associated with poor prognosis in various cancers and contributes to treatment resistance. In recent years, radiolabeled FAP inhibitors (FAPI) for PET imaging have shown promising clinical value across a range of cancers. Gastrointestinal (GI) malignancies, which often exhibit a desmoplastic reaction with a high density of FAP-expressing CAFs, are particularly well-suited for FAPI PET. Given the limitations of [18F]FDG PET in GI cancers, such as low sensitivity in certain histological subtypes and high physiological background uptake, FAPI PET is expected to serve as a complementary method, potentially enhancing both diagnostic accuracy and treatment guidance. This review provides a comprehensive comparison of the clinical applications of FAPI PET and [18F]FDG PET in various GI cancers, including their value in diagnosis, staging, and treatment guidance. Additionally, this review summarizes studies on the expanding role of FAPI PET, including its use in assessing treatment response and predicting prognosis, aiming to provide insights into its potential contribution to the improved management of GI malignancies.

FAPI radiopharmaceuticals in nuclear oncology and theranostics of solid tumours: are we nearer to surrounding the hallmarks of cancer?

[18F]FDG PET/CT is the most widely used PET radiopharmaceutical in oncology, but it is not exempt of diagnostic limitations. FAPI have emerged as a great tool in the management of several different solid tumours in which [18F]FDG is not able to provide enough information. The aim of this work was to evaluate the available evidence on diagnostic and therapeutic applications of PET/CT with FAPI radiopharmaceuticals. We underwent a non-systematic review focusing in the utility of FAPI radiopharmaceuticals in PET/CT diagnosis and in the treatment of several malignancies. FAPI radiopharmaceuticals present characteristics that can potentially overcome some known diagnostic limitations of [18F]FDG. FAPI radiopharmaceuticals present a high target-to-background ratio (TBR) in many solid tumours such as oesophageal cancer, gastric cancer, pancreatic cancer, hepatic cancer, colorectal cancer, breast cancer, ovarian, cervical cancer, and head and neck cancer. Available evidence suggests the high TBR improves sensitivity and specificity compared to [18F]FDG, especially for the detection of lymphadenopathies and peritoneal metastases, and may improve patient management and radiation treatment planning. Moreover, it is important to underline the potential theranostic application of FAPI radiopharmaceuticals.

The Role of FAPI PET Imaging in Pancreatic Cancer: A Meta-analysis Compared with 18F-FDG PET

PURPOSE

Fibroblast activating protein is a promising target for tumor molecular imaging and therapy. Studies showed that fibroblast activating protein inhibitor (FAPI) radioactive tracers presented superiority over 18F-FDG PET/CT in the evaluation of various cancer types, including pancreatic cancer (PC). Therefore, we conducted this meta-analysis to evaluate and analyze the differences between 68Ga/18F-FAPI and 18F-FDG in PC, in order to provide evidence for the clinical application of FAPI PET imaging.

METHODS

In the current meta-analysis, original studies published as of January 1, 2024 were analyzed using radiolabeled FAPI as a diagnostic radioactive tracer and compared to 18F-FDG for PET in PC. Databases searched included pubmed and web of science, and subject headings searched included PC and FAPI. The quality of the enrolled studies was evaluated by Quality Assessment of Diagnostic Accuracy Studies 2, and the meta-analysis was conducted using R language.

RESULTS

A total of seven studies including 322 patients compared the diagnostic performance of FAPI PET imaging and 18F-FDG PET/CT in PC. Overall, FAPI PET imaging showed higher pooled sensitivity (0.99 [95% CI: 0.97-1.00] vs. 0.84 [95% CI: 0.70-0.92]) and area under the curve (0.99 [95% CI: 0.98-1.00] vs. 0.91 [95% CI: 0.88-0.93]) than 18F-FDG PET/CT. The evidence showed that FAPI PET imaging is superior to 18F-FDG in pooled sensitivity to primary tumor, lymph node metastasis, and distant metastasis. Moreover, FAPI PET imaging improved TNM staging in 25% of PC patients and changed clinical management in 11.7% of PC patients compared to 18F-FDG.

CONCLUSION

FAPI PET imaging is superior to that of 18F-FDG in the detection of primary PC, nodal and distant metastases, TNM staging and clinical management.

Acquisition Duration Optimization Using Visual Grading Regression in [68Ga]FAPI-46 PET Imaging of Oncologic Patients

Fibroblast activation protein is a promising target for oncologic molecular imaging with radiolabeled fibroblast activation protein inhibitors (FAPI) in a large variety of cancers. However, there are yet no published recommendations on how to set up an optimal imaging protocol for FAPI PET/CT. It is important to optimize the acquisition duration and strive toward an acquisition that is sufficiently short while simultaneously providing sufficient image quality to ensure a reliable diagnosis. The aim of this study was to evaluate the feasibility of reducing the acquisition duration of [68Ga]FAPI-46 imaging while maintaining satisfactory image quality, with certainty that the radiologist's ability to make a clinical diagnosis would not be affected. Methods: [68Ga]FAPI-46 PET/CT imaging was performed on 10 patients scheduled for surgical resection of suspected pancreatic cancer, 60 min after administration of 3.6 ± 0.2 MBq/kg. The acquisition time was 4 min/bed position, and the raw PET data were statistically truncated and reconstructed to represent images with an acquisition duration of 1, 2, and 3 min/bed position, additional to the reference images of 4 min/bed position. Four image quality criteria that focused on the ability to distinguish specific anatomic details, as well as perceived image noise and overall image quality, were scored on a 4-point Likert scale and analyzed with mixed-effects ordinal logistic regression. Results: A trend toward increasing image quality scores with increasing acquisition duration was observed for all criteria. For the overall image quality, there was no significant difference between 3 and 4 min/bed position, whereas 1 and 2 min/bed position were rated significantly (P < 0.05) lower than 4 min/bed position. For the other criteria, all images with a reduced acquisition duration were rated significantly inferior to images obtained at 4 min/bed position. Conclusion: The acquisition duration can be reduced from 4 to 3 min/bed position while maintaining satisfactory image quality. Reducing the acquisition duration to 2 min/bed position or lower is not recommended since it results in inferior-quality images so noisy that clinical interpretation is significantly disrupted.

A Systematic Review on the Diagnostic Value of Fibroblast Activation Protein Inhibitor PET/CT in Genitourinary Cancers

In contemporary oncologic diagnostics, molecular imaging modalities are pivotal for precise local and metastatic staging. Recent studies identified fibroblast activation protein as a promising target for molecular imaging across various malignancies. Therefore, we aimed to systematically evaluate the current literature on the utility of fibroblast activation protein inhibitor (FAPI) PET/CT for staging patients with genitourinary malignancies. Methods: A systematic Embase and Medline search was conducted, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) process, on August 1, 2023. Relevant publications reporting on the diagnostic value of FAPI PET/CT in genitourinary malignancies were identified and included. Studies were critically reviewed using a modified version of a tool for quality appraisal of case reports. Study results were summarized using a narrative approach. Results: We included 22 retrospective studies with a cumulative total of 69 patients, focusing on prostate cancer, urothelial carcinoma of the bladder and of the upper urinary tract, renal cell carcinoma, and testicular cancer. FAPI PET/CT was able to visualize both local and metastatic disease, including challenging cases such as prostate-specific membrane antigen (PSMA)-negative prostate cancer. Compared with radiolabeled 18F-FDG and PSMA PET/CT, FAPI PET/CT showed heterogeneous performance. In selected cases, FAPI PET/CT demonstrated superior tumor visualization (i.e., better tumor-to-background ratios and visualization of small tumors or metastatic deposits visible in no other way) over 18F-FDG PET/CT in detecting local or metastatic disease, whereas comparisons with PSMA PET/CT showed both superior and inferior performances. Challenges in FAPI PET/CT arise from physiologic urinary excretion of most FAPI radiotracers, hindering primary-lesion visualization in the bladder and upper urinary tract, despite generally providing high tumor-to-background ratios. Conclusion: The current findings suggest that FAPI PET/CT may hold promise as a future tool to aid clinicians in detecting genitourinary malignancies. Given the substantial heterogeneity among the included studies and the limited number of patients, caution in interpreting these findings is warranted. Subsequent prospective and comparative investigations are anticipated to delve more deeply into this innovative imaging modality and elucidate its role in clinical practice.