Accurate preoperative staging with [68Ga]Ga-FAPI PET/CT for patients with oral squamous cell carcinoma: a comparison to 2-[18F]FDG PET/CT

Diagnostic Performances of Radiolabeled FAPI PET/CT for Lymph Node Staging in Head and Neck Cancer Patients: Comparison With 18F-FDG PET/CT

OBJECTIVE

The purpose of the current study was to compare the diagnostic performances of radiolabeled FAPI and 18F-FDG PET/CT for the detection of lymph node (LN) metastasis in head and neck cancer (HNC) patients.

METHODS

The PubMed, Cochrane database, and EMBASE database, from the earliest available date of indexing through December 31, 2024, were searched for studies comparing diagnostic performances of radiolabeled FAPI and 18F-FDG PET/CT for the detection of metastatic LN in HNC patients. We estimated pooled sensitivities and specificities across studies.

RESULTS

Across 8 studies (14 results), the pooled sensitivity of FAPI PET/CT was 0.89 and the pooled specificity was 0.93. The pooled sensitivity of 18F-FDG PET/CT was 0.91 and the pooled specificity was 0.50. On patient-based analysis, the estimated sensitivity and specificity of FAPI were 0.96 and 0.96, and those of 18F-FDG were 0.95 and 0.34, respectively. On lesion-based analysis, the estimated sensitivity and specificity of FAPI were 0.84 and 0.94, and those of 18F-FDG were 0.86 and 0.78, respectively. On neck side-based analysis, the estimated sensitivity and specificity of FAPI were 0.88 and 0.79, and those of 18F-FDG were 0.91 and 0.29, respectively.

CONCLUSIONS

Radiolabeled FAPI showed a good diagnostic performance for the detection of metastatic LN in HNC patients. Also, 18F-FDG PET/CT revealed low specificity for LN staging in HNC patients. Future large multicenter research with more patients would be necessary to provide a more comprehensive overview of the usefulness of radiolabeled FAPI for LN staging in HNC patients.

Applications of Fibroblast Activation Protein Inhibitor-PET in Interventional Oncology

Molecular imaging-guided techniques increase precision in surgical procedure and reduce postinterventional morbidity. Fibroblast activation protein (FAP) ligands may contribute to the superior preoperative assessment compared with conventional radionuclides due to its higher sensitivity and tumor delineation in epithelial malignancies. Wide spectrum of currently available FAP ligands including diagnostic and therapeutic emitters allows a flexibility regarding the optimal choice for individual need. Moreover, newly introduced hybrid tracers with fluorescence-based FAP probes enrich this spectrum by providing intraoperative FAP-targeting without radiation exposure. Thus, the use of FAP ligands in interventional oncology has great promise in improving the efficiency of local-interventional surgery.

Correlation of preoperative 18F-FDG-PET/CT tumor staging and maximum standardized uptake values with preoperative CT, postoperative tumor classification, and histopathological parameters of oral squamous cell carcinoma

INTRODUCTION

This study aimed to correlate preoperative 18F-fluorodeoxyglucose-positron emission tomography/computed tomography (18F-FDG-PET/CT) tumor staging, and maximum standardized uptake values (SUVmax) with preoperative CT data, postoperative tumor classification, and histopathological parameters of oral squamous cell carcinoma (OSCC).

MATERIAL AND METHODS

Thirty-seven OSCC patients staged via full-body 18F-FDG-PET/CT, including contrast agent CT of the head and neck in 2020 and 2021, were enclosed. Patients received tumor resection and stage-dependent neck dissection. Preoperative clinical (c) 18F-FDG-PET/CT UICC tumor stages and TNM classifications were correlated to corresponding CT and to postoperative histopathological (p) UICC tumor stages and TNM classifications. SUVmax of the primary tumor was associated with pUICC and pTNM, including extranodal extension (ENE), perineural invasion (Pn), lymphatic spread (L), vascular invasion (V), tumor grading (G), and -thickness.

RESULTS

Comparing 18F-FDG-PET/CT and CT, cUICC, cT, and cN differed in 32.3%, 16.7%, and 37.8% of the cases, respectively. For 18F-FDG-PET/CT, a moderate correlation was found between c- and pUICC (0.494; p = 0.0018) with a misestimation of c- compared to pUICC in 43.2% of the cases. Comparing c- and pTNM, misestimations concerning c- and pT were seen in 51.4% and concerning c- and pN in 37.8% of the cases. An increased SUVmax significantly correlated with increased pT- and pUICC (adjusted Odds ratio 1.103; p = 0.042 and 1.126; p = 0.021, respectively). The predictive quality of an SUVmax cutoff value for detecting cervical lymph node metastases and G was poor, as indicated by the low AUC values from the ROC analysis. No correlations were found between SUVmax and ENE, Pn-, L-, and V-status. A strong correlation was found between SUVmax and tumor thickness with an adjusted coefficient of 1.034 (p = 0.01).

CONCLUSION

The predictive value of 18F-FDG-PET/CT and SUVmax on histopathological tumor classification and parameters appears limited.

CLINICAL RELEVANCE

18F-FDG-PET/CT can not unreservedly be recommended for primary OSCC staging. There is an urgent need to specify its indications in detail further.

Dual-Time Point 68 Ga-FAPI-04 PET/CT Improves Tumor Delineation and Cervical Lymph Node Metastasis Identification in Patients With Head and Neck Squamous Cell Carcinoma

PURPOSE

The purpose of this study was to evaluate the benefit of dual-time point 68 Ga-FAPI-04 PET/CT in staging head and neck squamous cell carcinoma (HNSCC).

PATIENTS AND METHODS

Sixty-nine treatment-naive patients with HNSCC were enrolled. Each patient underwent whole-body 68 Ga-FAPI-04 PET/CT at approximately 30 minutes postinjection and a delayed scan in head and neck region at 2 hours. Radiotracer uptake (SUV max ), tumor-to-background ratio, change in SUV max (∆SUV max ), retention index, diagnostic performance, and staging were explored. Histopathology was the reference standard.

RESULTS

Primary tumors showed similar average SUV max between early (17.89) and delayed scans (17.86, P = 0.241). However, the tumor-to-background ratios of delayed imaging were all significantly higher than those of early imaging (all P < 0.001). In 38 patients who underwent neck dissection, metastatic lymph nodes showed higher mean SUV max on delayed imaging than on early imaging (early 10.53 ± 5.98 vs delayed 11.71 ± 6.36, P < 0.001), whereas nonmetastatic lymph nodes showed the opposite result (early 3.51 ± 0.51 vs delayed 2.58 ± 0.63, P = 0.002). The mean ∆SUV max and retention index of metastatic and nonmetastatic lymph nodes were 1.19 versus -0.93 and 12.79% versus -26.55%, respectively. N staging was correctly altered in 3 (3/38) patients based on delayed images.

CONCLUSIONS

Delayed 68 Ga-FAPI-04 PET/CT can effectively acquire high-contrast images, better tumor delineation, and detect hidden lesions near or within the tissues influenced by physiological uptake for HNSCC. In addition, dual-time point imaging adds diagnostic value for the differentiation of metastatic from nonmetastatic lymph nodes.

Comparing diagnostic performance of PET/CT, MRI, and CT in characterization of cN0 head and neck squamous cell carcinoma: A multicenter study

INTRODUCTION

Assessing local invasion is essential for determining stage of cN0 head and neck squamous cell carcinoma (HNSCC). We aimed to evaluate the performance of fluorine-18 fluorodeoxyglucose positron emission tomography/CT (18F-FDG PET/CT) in HNSCC characterization and compare it with conventional imaging.

METHODS

This multicentral study included 278 consecutive newly diagnosed cN0 HNSCC patients recruited from ACRIN 6685 (American College of Radiology Imaging Network) dataset. Four board-certified nuclear radiologists interpreted preoperative PET/CT, MRI, and CT examinations of patients. Imaging results were compared to pathological reference tests through area under curve (AUC), sensitivity, specificity, and receiver operating characteristic (ROC) curve using Stata 18 and Medcalc 22.017.

RESULTS

PET/CT demonstrated 23.5 %, 24.7 %, and 51.6 % upstaging, downstaging, and same staging in T staging of patients in comparison to histopathological evaluation, respectively. When evaluating N status, PET/CT showed 25.7 % upstaging, 20.3 % downstaging, and 53.9 % same staging. An optimal SUVmax cut-off value of 10.9 was determined to predict early-stage (T1, T2) and advanced-stage (T3, T4) HNSCC tumors with an AUC of 0.709 (95 % CI = 0.648-0.766). This cut-off value also predicted N0 and N+ patients with an AUC of 0.670 (95 % CI = 0.606-0.729). Sensitivity and specificity of PET/CT, MRI, and CT for bone invasion, muscle invasion, nerve invasion, cartilage invasion, superficial tissue invasion, overall invasion, and fixed vocal cord were calculated.

CONCLUSION

Our findings support the valuable accuracy of 18F-FDG PET/CT in staging HNSCC patients. Also, 18F-FDG PET/CT outperformed conventional imaging in characterization of HNSCC tumors.

IMPLICATIONS FOR PRACTICE

By offering an in-depth investigation in imaging of HNSCC tumors, this study contributes to evidence-based clinical decision-making.

[68Ga]Ga-FAPI04 Outperforms [18F]FDG PET/CT for Detecting Nodal Metastasis of Head and Neck Squamous Cell Carcinoma: A Single-Center Experience

This study assesses fibroblast activated protein inhibitor (FAPI) targeted PET/CT imaging against [18F]FDG PET/CT (FDG PET) for detecting nodal involvement in head and neck squamous cell carcinoma (HNSCC), intending to improve diagnostic precision for metastatic lymph nodes and lay the groundwork for future investigations. Methods: Patients diagnosed with HNSCC were retrospectively enrolled. All patients underwent [68Ga]Ga-FAPI04 PET/CT (FAPI PET) and FDG PET within 6 d. Primary tumor, lymph nodes, and tracer uptake were visually and quantitatively compared. The metastatic lymph nodes were evaluated using patient-and lesion-based analyses, with biopsy or postoperative histopathological examination as the reference. Results: The cohort includes 24 patients (17 men, 7 women; mean age 60 ± 11.8 years) who underwent FDG and FAPI PET for preoperative diagnostic workup or restaging due to known recurrence of HNSCC. Lesions included 24 primary tumors, 54 cervical lymph nodes, and 5 metastases. Primary tumors exhibited significant uptake on both PET modalities (median maximum standardized uptake value [SUVmax]: FDG 19.4 ± 11.6, FAPI 16.9 ± 4.6), with no statistically significant difference (p > 0.5). For lymph nodes, FAPI and FDG PET showed median SUVmax of 9.18 ± 6.77 and 9.67 ± 6.5, respectively. The patient-based analysis found FDG PET sensitivity at 88.2% and FAPI PET at 94.1%, with FAPI PET specificity significantly higher (85.7% vs. 42.8% for FDG PET). Lesion-based analysis revealed FAPI PET sensitivity and specificity at 84.2% and 93.7%, respectively, contrasting FDG PET's at 81.5% and 25%, respectively. Conclusion: This study underscores the efficacy of FAPI PET in detecting primary tumors in HNSCC. Furthermore, FAPI PET shows improved specificity over FDG PET for metastatic lymph nodes advocating further investigations for integrating FAPI PET into HNSCC clinical protocols for its enhanced precision in detecting metastatic lymph nodes.

In Vivo Detection of Lymph Nodes Metastasis of ESCC Using CXCR4-Targeted Tracer [64Cu]Cu-NOTA-CP01

PURPOSE

Esophageal squamous cell carcinoma (ESCC) frequently exhibits skip metastasis to lymph nodes; however, non-invasive imaging techniques capable of directly visualizing metastatic lymph nodes (MLN) are still lacking. Although biopsy is the clinical standard method, it is invasive and poses risks to patient health. This study aims to detect MLN in an intralymphatic tumor metastasis model of ESCC using the CXCR4-targeted tracer [64Cu]Cu-NOTA-CP01.

PROCEDURES

The CXCR4 expression in ESCC cell lines was assessed using Western blot and immunofluorescence. An intralymphatic tumor metastasis model was established and monitored using bioluminescence imaging (BLI). Small animal PET studies and biodistribution studies were performed to evaluate the specificity of [64Cu]Cu-NOTA-CP01 for MLN. Histopathology evaluation was employed to check for the presence of metastatic tumor cells and to assess CXCR4 expression levels in the metastatic lymph nodes.

RESULTS

The intralymphatic tumor metastasis model was successfully established using the EC109/Luc cell line, which exhibited high CXCR4 expression, as verified by BLI. PET/CT imaging showed that the MLN uptakes in the baseline group were significantly inhibited in the blocking group. The ratios of MLN/muscle and MLN/blood were also significantly higher in the baseline group than in the blocking group. Ex vivo PET/CT imaging of MLN corroborated the in vivo data. Biodistribution studies further supported the PET imaging studies, showing rapid clearance of the tracer from the blood and major organs, with significantly higher MLN/muscle and MLN/blood ratios in the baseline group compared to the blocking group. Histopathological staining verified positive CXCR4 expression in these lymph nodes containing metastatic tumor cells.

CONCLUSIONS

Targeting CXCR4 with [64Cu]Cu-NOTA-CP01 for PET imaging of lymph nodes metastasis represents a promising approach that warrants further investigation. These findings have the potential to enhance diagnostic and therapeutic strategies for individuals with lymph nodes metastasis of ESCC.

Unveiling the Tumor Microenvironment Through Fibroblast Activation Protein Targeting in Diagnostic Nuclear Medicine: A Didactic Review on Biological Rationales and Key Imaging Agents

The tumor microenvironment (TME) is a dynamic and complex medium that plays a central role in cancer progression, metastasis, and treatment resistance. Among the key elements of the TME, cancer-associated fibroblasts (CAFs) are particularly important for their ability to remodel the extracellular matrix, promote angiogenesis, and suppress anti-tumor immune responses. Fibroblast activation protein (FAP), predominantly expressed by CAFs, has emerged as a promising target in both cancer diagnostics and therapeutics. In nuclear medicine, targeting FAP offers new opportunities for non-invasive imaging using radiolabeled fibroblast activation protein inhibitors (FAPIs). These FAP-specific radiotracers have demonstrated excellent tumor detection properties compared to traditional radiopharmaceuticals such as [18F]FDG, especially in cancers with low metabolic activity, like liver and biliary tract tumors. The most recent FAPI derivatives not only enhance the accuracy of positron emission tomography (PET) imaging but also hold potential for theranostic applications by delivering targeted radionuclide therapies. This review examines the biological underpinnings of FAP in the TME, the design of FAPI-based imaging agents, and their evolving role in cancer diagnostics, highlighting the potential of FAP as a target for precision oncology.

Fibroblast Activation Protein Inhibitor Tracers and Their Preclinical, Translational, and Clinical Status in China

Quinoline-based fibroblast activation protein (FAP) inhibitors (FAPIs) have recently emerged as a focal point in global nuclear medicine, underscored by their promising applications in cancer theranostics and the diagnosis of various nononcological conditions. This review offers an in-depth summary of the existing literature on the evolution and use of FAPI tracers in China, tracing their journey from preclinical to clinical research. Moreover, this review also assesses the diagnostic accuracy of FAPI PET for the most common cancers in China, analyzes its impact on oncologic management paradigms, and investigates the potential of FAP-targeted radionuclide therapy in patients with advanced or metastatic cancer. This review also summarizes studies using FAPI PET for nononcologic disorders in China. Thus, this qualitative overview presents a snapshot of China's engagement with FAPI tracers, aiming to guide future research endeavors.

Head-to-head comparison of [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT for the evaluation of tonsil cancer and lymph node metastases: a single-centre retrospective study

BACKGROUND

This study aimed to compare the diagnostic value of [68 Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT imaging for primary lesions and metastatic lymph nodes in patients with tonsil cancer.

METHOD

Twenty-one tonsil cancer patients who underwent [68 Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT scans within two weeks in our centre were retrospectively enrolled. The maximum standardized uptake value (SUVmax) and tumor-to-background ratio (TBR) of the two tracers were compared by using the Mann‒Whitney U test. In addition, the sensitivity, specificity, and accuracy of the two methods for diagnosing metastatic lymph nodes were analysed.

RESULTS

In detecting primary lesions, the efficiency was higher for [68 Ga]Ga-DOTA-FAPI-04 PET/CT (20/22) than for [18F]FDG PET/CT (9/22). Although [68 Ga]Ga-DOTA-FAPI-04 uptake (SUVmax, 5.03 ± 4.06) was lower than [18F]FDG uptake (SUVmax, 7.90 ± 4.84, P = 0.006), [68 Ga]Ga-DOTA-FAPI-04 improved the distinction between the primary tumor and contralateral normal tonsillar tissue. The TBR was significantly higher for [68 Ga]Ga-DOTA-FAPI-04 PET/CT (3.19 ± 2.06) than for [18F]FDG PET/CT (1.89 ± 1.80) (p < 0.001). In lymph node analysis, SUVmax and TBR were not significantly different between [68 Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT (7.67 ± 5.88 vs. 8.36 ± 6.15, P = 0.498 and 5.56 ± 4.02 vs. 4.26 ± 3.16, P = 0.123, respectively). The specificity and accuracy of [68 Ga]Ga-DOTA-FAPI-04 PET/CT were higher than those of [18F]FDG PET/CT in diagnosing metastatic cervical lymph nodes (all P < 0.05).

CONCLUSION

The availability of [68 Ga]Ga-DOTA-FAPI-04 complements the diagnostic results of [18F]FDG by improving the detection rate of primary lesions and the diagnostic accuracy of cervical metastatic lymph nodes in tonsil cancer compared to [18F]FDG.

From basic research to clinical application: targeting fibroblast activation protein for cancer diagnosis and treatment

PURPOSE

This study aims to review the multifaceted roles of a membrane protein named Fibroblast Activation Protein (FAP) expressed in tumor tissue, including its molecular functionalities, regulatory mechanisms governing its expression, prognostic significance, and its crucial role in cancer diagnosis and treatment.

METHODS

Articles that have uncovered the regulatory role of FAP in tumor, as well as its potential utility within clinical realms, spanning diagnosis to therapeutic intervention has been screened for a comprehensive review.

RESULTS

Our review reveals that FAP plays a pivotal role in solid tumor progression by undertaking a multitude of enzymatic and nonenzymatic roles within the tumor stroma. The exclusive presence of FAP within tumor tissues highlights its potential as a diagnostic marker and therapeutic target. The review also emphasizes the prognostic significance of FAP in predicting tumor progression and patient outcomes. Furthermore, the emerging strategies involving FAPI inhibitor (FAPI) in cancer research and clinical trials for PET/CT diagnosis are discussed. And targeted therapy utilizing FAP including FAPI, chimeric antigen receptor (CAR) T cell therapy, tumor vaccine, antibody-drug conjugates, bispecific T-cell engagers, FAP cleavable prodrugs, and drug delivery system are also introduced.

CONCLUSION

FAP's intricate interactions with tumor cells and the tumor microenvironment make it a promising target for diagnosis and treatment. Promising strategies such as FAPI offer potential avenues for accurate tumor diagnosis, while multiple therapeutic strategies highlight the prospects of FAP targeting treatments which needs further clinical evaluation.

Imaging of Tumor Stroma Using 68Ga-FAPI PET/CT to Improve Diagnostic Accuracy of Primary Tumors in Head and Neck Cancer of Unknown Primary: A Comparative Imaging Trial

The low detection rate of primary tumors by current diagnostic techniques remains a major concern for patients with head and neck cancer of unknown primary (HNCUP). Therefore, in this study, we aimed to investigate the potential role of 68Ga-labeled fibroblast activation protein inhibitor (68Ga-FAPI) PET/CT compared with 18F-FDG PET/CT for the detection of primary tumors of HNCUP. Methods: In this prospective comparative imaging trial conducted at Fudan University Shanghai Cancer Center, 91 patients with negative or equivocal findings of a primary tumor by comprehensive clinical examination and conventional imaging were enrolled from June 2020 to September 2022. The presence of a primary tumor was recorded by 3 experienced nuclear medicine physicians. Primary lesions were validated by histopathologic analysis and a composite reference standard. Results: Of the 91 patients (18 women, 73 men; median age, 60 y; age range, 24-76 y), primary tumors were detected in 46 (51%) patients after a thorough diagnostic work-up. 68Ga-FAPI PET/CT detected more primary lesions than 18F-FDG PET/CT (46 vs. 17, P < 0.001) and showed better sensitivity, positive predictive value, and accuracy in locating primary tumors (51% vs. 25%, 98% vs. 43%, and 51% vs. 19%, respectively). Furthermore, 68Ga-FAPI PET/CT led to treatment changes in 22 of 91 (24%) patients compared with 18F-FDG PET/CT. The Kaplan-Meier curve illustrated that patients with unidentified primary tumors had a significantly worse prognosis than patients with identified primary tumors (hazard ratio, 5.77; 95% CI, 1.86-17.94; P = 0.0097). Conclusion: 68Ga-FAPI PET/CT outperforms 18F-FDG PET/CT in detecting primary lesions and could serve as a sensitive, reliable, and reproducible imaging modality for HNCUP patients.

Diagnostic Accuracy of 68Ga-FAPI Versus 18F-FDG PET in Patients with Various Malignancies

To assess the diagnostic accuracy of 68Ga-labeled fibroblast activation protein inhibitor (FAPI) and 18F-labeled FDG PET for the detection of various tumors, we performed a head-to-head comparison of both imaging modalities across a range of tumor entities as part of our ongoing 68Ga-FAPI PET observational trial. Methods: The study included 115 patients with 8 tumor entities who received imaging with 68Ga-FAPI for tumor staging or restaging between October 2018 and March 2022. Of those, 103 patients received concomitant imaging with 68Ga-FAPI and 18F-FDG PET and had adequate lesion validation for accuracy analysis. Each scan was evaluated for the detection of primary tumor, lymph nodes, and visceral and bone metastases. True or false positivity and negativity to detected lesions was assigned on the basis of histopathology from biopsies or surgical excision, as well as imaging validation. Results: 68Ga-FAPI PET revealed higher accuracy than 18F-FDG PET in the detection of colorectal cancer (n = 14; per-patient, 85.7% vs. 78.6%; per-region, 95.6% vs. 91.1%) and prostate cancer (n = 22; per-patient, 100% vs. 90.9%; per-region, 96.4% vs. 92.7%). 68Ga-FAPI PET and 18F-FDG PET had comparable per-patient accuracy in detecting breast cancer (n = 16, 100% for both) and head and neck cancers (n = 10, 90% for both modalities). 68Ga-FAPI PET had lower per-patient accuracy than 18F-FDG PET in cancers of the bladder (n = 12, 75% vs. 100%) and kidney (n = 10, 80% vs. 90%), as well as lymphoma (n = 9, 88.9% vs. 100%) and myeloma (n = 10, 80% vs. 90%). Conclusion: 68Ga-FAPI PET demonstrated higher diagnostic accuracy than 18F-FDG PET in the diagnosis of colorectal cancer and prostate cancer, as well as comparable diagnostic performance for cancers of the breast and head and neck. Accuracy and impact on management will be further assessed in an ongoing prospective interventional trial (NCT05160051).

Diagnostic Accuracy of [68Ga]Ga Labeled Fibroblast-Activation Protein Inhibitors in Detecting Head and Neck Cancer Lesions Using Positron Emission Tomography: A Systematic Review and a Meta-Analysis

Several studies have examined the use of positron emission tomography (PET) using [68Ga]Ga-radiolabeled fibroblast-activation protein inhibitors (FAPi) across multiple subtypes of head and neck cancer (HNC). The purpose of the present study was to evaluate the diagnostic accuracy of a newly developed molecular imaging approach in the context of HNC through a comprehensive review and meta-analysis. A thorough literature review was conducted to identify scholarly articles about the diagnostic effectiveness of FAP-targeted PET imaging. The present study incorporates original publications assessing the efficacy of this innovative molecular imaging test in both newly diagnosed and previously treated HNC patients. This systematic review examined eleven investigations, of which nine were deemed suitable for inclusion in the subsequent meta-analysis. The quantitative synthesis yielded a pooled detection rate of 99% for primary HNC lesions. Additionally, on a per patient-based analysis, the pooled sensitivity and specificity for regional lymph node metastases were found to be 90% and 84%, respectively. The analysis revealed a statistical heterogeneity among the studies for the detection rate of primary HNC lesions. The quantitative findings presented in this study indicate a favorable diagnostic performance of FAP-targeted PET imaging in detecting primary HNC tumors. In contrast, discordant results concerning the diagnostic accuracy of lymph node metastases were found. However, further multicentric trials are required to validate the efficacy of FAP-targeted PET in this specific group of patients.

Cancer-associated fibroblasts: Mediators of head and neck tumor microenvironment remodeling

Cancer-associated fibroblasts (CAFs) are involved in critical aspects of head and neck squamous cell carcinoma (HNSCC) pathogenesis, such as the formation of a tumor-permissive extracellular matrix structure, angiogenesis, or immune and metabolic reprogramming of the tumor microenvironment (TME), with implications for metastasis and resistance to radiotherapy and chemotherapy. The pleiotropic effect of CAFs in TME is likely to reflect the heterogeneity and plasticity of their population, with context-dependent effects on carcinogenesis. The specific properties of CAFs provide many targetable molecules that could play an important role in the future therapy of HNSCC. In this review article, we will focus on the role of CAFs in the TME of HNSCC tumors. We will also discuss clinically relevant agents targeting CAFs, their signals, and signaling pathways, which are activated by CAFs in cancer cells, with the potential for repurposing for HNSCC therapy.

Head-to-Head Comparison of FDG and Radiolabeled FAPI PET: A Systematic Review of the Literature

FAPI-based radiopharmaceuticals are a novel class of tracers, mainly used for PET imaging, which have demonstrated several advantages over [18F]FDG, especially in the case of low-grade or well-differentiated tumors. We conducted this systematic review to evaluate all the studies where a head-to-head comparison had been performed to explore the potential utility of FAPI tracers in clinical practice. FAPI-based radiopharmaceuticals have shown promising results globally, in particular in detecting peritoneal carcinomatosis, but studies with wider populations are needed to better understand all the advantages of these new radiopharmaceuticals.

The added value of [68Ga]Ga-DOTA-FAPI-04 PET/CT in pancreatic cancer: a comparison to [18F]F-FDG

OBJECTIVES

We aimed to compare the diagnostic and prognostic performance of [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT in pancreatic cancer.

METHODS

This single-center retrospective study enrolled 51 patients who underwent [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT. The final diagnosis on PET/CT images was verified by histopathology or 1-year follow-up. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of [18F]FDG and [68Ga]Ga-DOTA-FAPI-04 PET/CT were calculated to compare the diagnostic efficacy. Progression-free survival (PFS) was the endpoint for the survival analysis. Twenty-six patients were eligible for the Kaplan-Meier survival analysis using a log-rank test. And multivariate analysis including age, sex, stage, CA199 level, and SUVmax of [18F]FDG and [68Ga]Ga-DOTA-FAPI-04 was also performed. Two-tailed p < 0.05 was considered statistically significant.

RESULTS

[68Ga]Ga-DOTA-FAPI-04 showed a higher sensitivity than [18F]FDG for detecting primary tumor (100% vs. 95.0%), metastatic lymph nodes (96.2% vs. 61.5%), and distant metastases (100% vs. 84.0%) (p < 0.0001, respectively). For [68Ga]Ga-DOTA-FAPI-04, the tumor-to-liver background ratio (TLBR) of liver metastases was higher (5.7 ± 3.2 vs. 3.2 ± 1.3, p < 0.0001). Furthermore, SUVmax > 14.9 on [68Ga]Ga-DOTA-FAPI-04 was significantly associated with PFS rates (chi-square = 12.05, p = 0.001). The Cox regression analysis showed that SUVmax of [68Ga]Ga-DOTA-FAPI-04 was an independent prognostic factor for PFS (p = 0.001; hazard ratio, 8.877).

CONCLUSIONS

[68Ga]Ga-DOTA-FAPI-04 PET/CT showed a higher sensitivity and accuracy than [18F]FDG PET/CT in diagnosing pancreatic cancer and might have an independent prognostic value for pancreatic cancer patients.

KEY POINTS

• [68Ga]Ga-DOTA-FAPI-04 PET/CT had a higher sensitivity and accuracy in detecting primary tumors, metastatic lymph nodes, and distant metastases than [18F]FDG PET/CT. • SUVmax > 14.9 on [68Ga]Ga-DOTA-FAPI-04 PET/CT before chemotherapy was significantly associated with progress-free status rates (chi-square = 12.05, p = 0.001) in pancreatic cancer patients.

FAPI PET/CT Imaging-An Updated Review

Despite revolutionizing the field of oncological imaging, Positron Emission Tomography (PET) with [18F]Fluorodeoxyglucose (FDG) as its workhorse is limited by a lack of specificity and low sensitivity in certain tumor subtypes. Fibroblast activation protein (FAP), a type II transmembrane glycoprotein, is expressed by cancer-associated fibroblasts (CAFs) that form a major component of the tumor stroma. FAP holds the promise to be a pan-cancer target, owing to its selective over-expression in a vast majority of neoplasms, particularly epithelial cancers. Several radiolabeled FAP inhibitors (FAPI) have been developed for molecular imaging and potential theranostic applications. Preliminary data on FAPI PET/CT remains encouraging, with extensive multi-disciplinary clinical research currently underway. This review summarizes the existing literature on FAPI PET/CT imaging with an emphasis on diagnostic applications, comparison with FDG, pitfalls, and future directions.

Evaluation of the Diagnostic Accuracy of FAPI PET/CT in Oncologic Studies: Systematic Review and Metaanalysis

Fibroblast-activation protein is a promising target for oncologic molecular imaging. Studies show that fibroblast activation protein inhibitor (FAPI) radiotracers are accurate diagnostics with favorable tumor-to-background ratios across various cancers. Therefore, we performed a systematic review and metaanalysis to assess the diagnostic performance of FAPI PET/CT in comparison with [¹⁸F]FDG PET/CT, the most widely used radiotracer in oncology. Methods: We conducted a systematic search in MEDLINE, Embase, Scopus, PubMed, Cochrane Central Register of Controlled Trials, relevant trial registries, and bibliographies. The search consisted of combinations of terms for 3 topics: neoplasia, PET/CT, and FAPI. Two authors independently screened retrieved articles using predefined inclusion and exclusion criteria and extracted the data. Study quality was assessed using the criteria of QUADAS-2 (Quality Assessment of Diagnostic Accuracy Studies 2). For each study, the sensitivity, specificity, and 95% CIs were calculated to determine diagnostic accuracy for primary, nodal, and metastatic lesions. A random-effects metaanalysis was used for pooling the data, and heterogeneity was assessed (I² index). Results: Thirty-nine studies (1,259 patients) investigating the use of FAPI PET/CT were included. On a patient-based analysis, pooled sensitivity was 0.99 (95% CI, 0.97-1.0) for the detection of primary lesions. Pooled sensitivity for nodal and distant metastases was 0.91 (95% CI, 0.81-0.96) and 0.99 (95% CI, 0.96-1.0), respectively. On a paired analysis between FAPI and [¹⁸F]FDG PET/CT, FAPI had a higher sensitivity in the detection of primary, nodal, and metastatic lesions (all P < 0.001). The differences in sensitivities between FAPI and [¹⁸F]FDG were statistically significant. In terms of heterogeneity, analyses on primary lesions were moderately affected, distant metastatic lesions were highly affected, and the nodal metastatic analyses had negligible heterogeneity. Conclusion: The diagnostic performance of FAPI PET/CT is superior to that of [¹⁸F]FDG in the detection of primary, nodal, and distant metastases. However, further studies are needed to better evaluate its utility and indication in specific cancer types and clinical settings.

The performance of 68Ga-FAPI-04 PET/CT in head and neck squamous cell carcinoma: a prospective comparison with 18F-FDG PET/CT

PURPOSE

This study was designed to compare the performance of 68Ga-FAPI-04 and 18F-FDG PET/CT for initial staging and recurrence detection of head and neck squamous cell carcinoma (HNSCC).

METHODS

Prospectively, 77 patients with histologically proven or highly suspected HNSCC underwent paired 18F-FDG and 68Ga-FAPI-04 PET/CT in a week for either initial staging (n = 67) or restaging (n = 10). The diagnostic performance was compared for the two imaging approaches, especially for N staging. SUVmax, SUVmean, and target-to-background ratio (TBR) were assessed for paired positive lesions. Furthermore, change in management by 68Ga-FAPI-04 PET/CT and histopathologic FAP expression of some lesions were explored.

RESULTS

18F-FDG and 68Ga-FAPI-04 PET/CT exhibited a comparable detection efficiency for primary tumor (100%) and recurrence (62.5%). In the twenty-nine patients receiving neck dissection, 68Ga-FAPI-04 PET/CT showed greater specificity and accuracy in evaluating preoperative N staging than 18F-FDG based on patient (p = 0.031 and p = 0.070), neck side (p = 0.002 and p = 0.006), and neck level (p < 0.001 and p < 0.001). As for distant metastasis, 68Ga-FAPI-04 PET/CT detected more positive lesions than 18F-FDG (25 vs 23) and with higher SUVmax (7.99 ± 9.04 vs 3.62 ± 2.68, p = 0.002) by lesion-based analysis. The type of neck dissection in 9 cases (9/33) was altered by 68Ga-FAPI-04. Overall, clinical management was significantly changed in 10 patients (10/61). Three patients had a follow-up 68Ga-FAPI-04 PET/CT post neoadjuvant therapy: One showed complete remission, and the others showed progression. The 68Ga-FAPI-04 uptake intensity was confirmed to be consistent with FAP expression.

CONCLUSION

68Ga-FAPI-04 outperforms 18F-FDG PET/CT in evaluating preoperative N staging in patients with HNSCC. Furthermore, 68Ga-FAPI-04 PET/CT also shows the potential in clinical management and monitoring response to treatment.

Fibroblast Activation Protein Inhibitor PET Imaging in Head and Neck Cancer

⁶⁸Ga-fibroblast activation protein inhibitor (FAPI)-PET is highly promising for head and neck cancers including oral squamous cell carcinomas, hypopharynx carcinomas, adenoid cystic carcinomas, thyroid cancer, and cervical cancer of unknown primary. For oral squamous cell carcinomas, hypopharynx carcinomas, and adenoid cystic carcinomas, ⁶⁸Ga-FAPI-PET has high potential for the assessment of primary tumors with impact on radiotherapy planning. ⁶⁸Ga-FAPI-PET can be applied for staging of metastasized thyroid carcinomas. To date, the data on cervical cancer of unknown primary are sparse but highly interesting as ⁶⁸Ga-FAPI-PET may detect a significant portion of ¹⁸fluoro-deoxyglucose-PET-negative primary tumors.

Current research topics in FAPI theranostics: a bibliometric analysis

PURPOSE

The study aimed to provide a comprehensive bibliometric overview of the current scientific publications on fibroblast activation protein inhibitor (FAPI) positron emission tomography imaging and radionuclide therapy.

METHODS

A PubMed search was performed to identify all MEDLINE-indexed publications on FAPI imaging and radionuclide therapy. The last update was performed on 31 May 2022. An online database of this literature was created, and hierarchical topic-related tags were subsequently assigned to all relevant studies. Frequency analysis was used to evaluate the distribution of the following characteristics: first author's country of origin, journal of publication, study design, imaging techniques and radiopharmaceutical used, histopathological correlation, the type of cancer, and benign disease/uptake types evaluated.

RESULTS

A total of 294 relevant publications on original studies were identified, consisting of 209 (71%) case reports/series and 85 cohort studies (29%). The majority of studies focused on imaging topics, predominantly comparing uptake on FAPI-PET/CT with 2-[¹⁸F]FDG-PET/CT, anatomical imaging, and/or histopathology results. 68% of studies focused on malignancies, with gastro-intestinal cancer, hepato-pancreato-biliary cancer, mixed cancers/metastases, lung cancer, sarcoma, head and neck cancer, and breast cancer being the most frequently reported. 42% of studies focused on benign disease categories, with cardiovascular, musculoskeletal, HPB, head and neck, and IgG4-related disease as most common categories. 16/294 (5%) studies focused on radionuclide therapy, with preliminary reports of acceptable toxicity profiles, tumour activity retention, and suggestion of disease control.

CONCLUSION

FAPI research is rapidly expanding from diagnostic studies in malignancies and benign diseases to the first reports of salvage radionuclide therapy. The research activity needs to shift now from low-level-of-evidence case reports and series to prospectively designed studies in homogenous patient groups to provide evidence on how and in which clinical situations FAPI theranostics can be of added value to clinical care. We have provided an overview of current research topics to build upon.

Comparison of the Detection Performance Between FAP and FDG PET/CT in Various Cancers: A Systemic Review and Meta-analysis

PURPOSE

18F-FDG is the dominant radiotracer in oncology; however, it has limitations. Novel labeled fibroblast activation protein (FAP) radiotracers have been developed and published in several studies. Thus, this meta-analysis aimed to compare the detection rates (DRs) of FDG and FAP, based on previous studies from a systematic review.

METHODS

PubMed/MEDLINE and Cochrane library databases were used to perform a comprehensive and systematic search and are updated to April 30, 2022. The DR, relative risk, and the SUVmax were calculated between the FAP and FDG tracers. Finally, the sensitivity, specificity, diagnostic odds ratio, and summary receiver operating characteristic curve of FAP and FDG were analyzed using gold and reference standards.

RESULTS

Thirty studies (1170 patients) were included in the meta-analysis. The relative risks of FAP DR for the primary tumor, recurrent tumor, lymph node metastasis, and distant metastasis were FDG 1.06- to 3.00-fold per patient and per lesion. For the primary tumor, FAP uptake was most intense in pancreatic cancer, followed by head and neck, cervical, colorectal, lung, gastric, and hepatocellular carcinoma, and was higher than FDG except for urological system cancer. The sensitivity (0.84-0.98), diagnostic odds ratio (19.36-358.47), and summary receiver operating characteristic curve (0.94-0.99) of FAP based on patient and lesion were better for primary tumors, LN metastasis, and distant metastasis than FDG.

CONCLUSIONS

Fibroblast activation protein is an extremely potential radiotracer to replace most of the use of FDG in oncology. It is noteworthy that the FAP tracers for primary tumors had low specificity despite excellent sensitivity and had lower uptake than FDG in urological system cancer. In addition, the difference in detection between FAP and FDG for LN metastasis could not be certain in sarcoma.

Comparison of [68Ga]Ga-FAPI and [18F]FDG uptake in patients with gastric signet-ring-cell carcinoma: a multicenter retrospective study

OBJECTIVE

In this study, we investigated the role of [⁶⁸Ga]Ga-FAPI PET imaging in the detection of primary and metastatic gastric signet-ring-cell carcinoma (GSRCC) and compared with [¹⁸F]FDG PET.

METHODS

This retrospective multicenter analysis included 34 patients with histologically confirmed GSRCCs from four medical centers. The maximum standard uptake value (SUV_max), tumor-to-background ratio (TBR), and diagnostic accuracy were compared between the two modalities. [¹⁸F]FDG and [⁶⁸Ga]Ga-FAPI uptakes were compared by using the Wilcoxon signed-rank test. McNemar's test was used to compare the diagnostic accuracy between the two techniques.

RESULTS

Data were analyzed from 27 paired PET/CT and 7 paired PET/MRI scans for 34 GSRCC patients (16 men and 18 women) who had a median age of 51 years (range: 25-85 years). [⁶⁸Ga]Ga-FAPI PET showed higher SUV_max and TBR values than did [¹⁸F]FDG PET in the primary tumors (SUV_max: 5.2 vs. 2.2, p = 0.001; TBR: 7.6 vs. 1.3, p < 0.001), involved lymph nodes (SUV_max: 6.8 vs. 2.5, p < 0.001; TBR: 5.8 vs. 1.3, p < 0.001), and bone and visceral metastases (SUV_max: 6.5 vs. 2.4, p < 0.001; TBR: 6.3 vs. 1.3, p < 0.001). In diagnostic performance, [⁶⁸Ga]Ga-FAPI PET exhibited higher sensitivity than [¹⁸F]FDG PET for detecting primary tumors (73% [16/22] vs. 18% [4/22], p < 0.001), local recurrences (100% [7/7] vs. 29% [2/7], p = 0.071), lymph node metastases (77% [59/77] vs. 23% [18/77], p < 0.001), and distant metastases (93% [207/222] vs. 39% [86/222], p < 0.001).

CONCLUSION

The results from this multicenter retrospective analysis justify the clinical use of [⁶⁸Ga]Ga-FAPI tracers for GSRCC diagnosis and staging.

KEY POINTS

• [⁶⁸Ga]Ga-FAPI PET/CT is a promising imaging modality for the detection of primary and metastatic disease and has implications for TNM staging in GSRCC. • In this multicenter study of 34 patients with GSRCC, [⁶⁸Ga]Ga-FAPI PET exhibited greater radiotracer uptake, tumor-to-background ratios, and diagnostic accuracy than [¹⁸F]FDG PET for detecting primary/recurrent tumors and metastatic lesions.

[68Ga]Ga-FAPI PET/CT Improves the T Staging of Patients with Newly Diagnosed Nasopharyngeal Carcinoma: A Comparison with [18F]F-FDG

PURPOSE

This study aimed to explore the value of [⁶⁸Ga]Ga-labelled fibroblast activation protein inhibitor ([⁶⁸Ga]Ga-FAPI) positron emission tomography/computed tomography (PET/CT) in the initial staging of patients with newly diagnosed nasopharyngeal carcinoma (NPC), compared with 2-deoxy-2[¹⁸F]fluoro-D-glucose ([¹⁸F]F-FDG) PET/CT.

MATERIALS AND METHODS

Forty-seven treatment-naïve patients with newly diagnosed NPC underwent magnetic resonance imaging (MRI), [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT and [¹⁸F]F-FDG PET/CT within 1 week. The diagnostic efficiency of all imaging modalities for evaluating primary tumour extension was compared from the two aspects of soft tissue and bony structure involvement. The accuracy of two PET/CT methods for diagnosing cervical lymph node (CLN) metastases was compared, and MRI served as the standard reference. T and N stages were assessed by MRI, [⁶⁸Ga]Ga-FAPI PET/CT and [¹⁸F]F-FDG PET/CT. Immunohistochemical (IHC) staining for FAP was conducted in 22 of the patients.

RESULTS

[⁶⁸Ga]Ga-FAPI PET/CT outperformed [¹⁸F]F-FDG PET/CT in the assessment of primary tumour invasion in the cavernous sinus (10 vs. 1, p < 0.001) and bony structures (207 vs. 177, p < 0.001). Compared with MRI, [⁶⁸Ga]Ga-FAPI PET/CT upgraded and underestimated T stage in 13 and 2 patients, while [¹⁸F]F-FDG PET/CT upgraded and underestimated T stage in 5 and 13 patients. However, [⁶⁸Ga]Ga-FAPI PET/CT was inferior to [¹⁸F]F-FDG PET/CT in diagnosing positive CLNs based on the analyses of patients, neck sides, neck levels and individual nodes. [⁶⁸Ga]Ga-FAPI PET/CT changed therapeutic schedules in 8 patients because of stage group changes. The presence of FAP with high quantity and intensity in cancer-associated fibroblasts (CAFs) was confirmed in all tumour specimens.

CONCLUSION

[⁶⁸Ga]Ga-FAPI PET/CT outperformed [¹⁸F]F-FDG PET/CT in detecting the cavernous sinus and bony structure involvement of primary NPC tumours, suggesting its value in improving T staging and therapeutic regimen selection. However, the performance of [⁶⁸Ga]Ga-FAPI PET/CT is less promising for N staging because it detected fewer positive CLNs than [¹⁸F]F-FDG PET/CT.