(68)Ga-FAPI-PET/CT improves diagnostic staging and radiotherapy planning of adenoid cystic carcinomas - Imaging analysis and histological validation

Comparison of 18F-fluorodeoxyglucose PET and 68Ga-fibroblast Activation Protein Inhibitor PET in Head and Neck Cancers: A Systematic Review and Meta-analysis

OBJECTIVES

This study aimed to compare the diagnostic efficiency of 68Ga-fibroblast activation protein inhibitor (FAPI) positron emission tomography (PET) and 18F-fluorodeoxyglucose (18F-FDG) PET in patients with head and neck cancer (HNC).

DATA SOURCES

PubMed, Embase, Web of Science, and the Cochrane Library were used to perform a systemic search through June 26, 2024.

METHODS

Studies comparing the diagnostic value of 68Ga-FAPI PET and 18F-FDG PET in patients with HNC were included. We performed a bivariate meta-analysis of diagnostic data and a meta-analysis of the quantitative parameters. The summary receiver operating characteristic curve was plotted, and publication bias was evaluated via Egger's test.

RESULTS

The meta-analysis included 12 studies on 386 patients with HNC. 68Ga-FAPI PET had superior pooled sensitivity to 18F-FDG PET in detecting primary/recurrent tumors and distant metastases in both lesion-based analysis and patient-based analysis. Although the sensitivity of 18F-FDG PET for detecting lymph node metastases was greater than that of 68Ga-FAPI PET (0.93 [95% CI 0.83-0.97] vs. 0.82 [95% CI 0.63-0.93]), the specificity of 18F-FDG PET was lower than that of 68Ga-FAPI PET (0.36 [95% CI 0.01-0.96] vs. 0.97 [95% CI 0.53-1.00]). In addition, 68Ga-FAPI PET had a higher pooled mean maximum standardized uptake value for distant metastases (3.28 [95% CI 1.90-4.66]) and a higher pooled mean tumor-to-background ratio for primary/recurrent tumors (1.24 [95% CI 0.44-2.04]) than 18F-FDG PET.

CONCLUSION

Compared to 18F-FDG PET, 68Ga-FAPI PET has superior diagnostic accuracy in HNC lesions. Thus, 68Ga-FAPI PET may be a better tool for staging and restaging than 18F-FDG PET in patients with HNC.

Fluoro-2-Deoxyglucose (FDG)-Avid Adenoid Cystic Carcinoma of the Larynx: A Rare Case and Diagnostic Insight Obtained Using Positron Emission Tomography/Computed Tomography (PET/CT) Imaging

Adenoid cystic carcinoma (ACC) is a relatively uncommon tumor among head and neck cancers, with laryngeal involvement, typically subglottic, being exceptionally rare. While ACC usually originates in the salivary glands, its occurrence in the larynx is highly unusual. Laryngeal adenoid cystic carcinoma (LACC) is characterized by indolent growth, frequent perineural invasion, and a tendency for local recurrence. Moreover, distant metastasis can occur with the lungs being the most common site of metastasis. We present a case of a 28-year-old male patient with a progressively enlarging right neck mass, uncovering a rare case of infiltrative LACC invading the thyroid gland. Post thyroidectomy, the 18-fluoro-2-deoxyglucose (F-18-FDG)-positron emission tomography (PET)/computed tomography (CT) scan revealed FDG-avid right posterior subglottic residual disease with invasion into adjacent structures, including the trachea. This represents one of the few documented cases of FDG-avid LACC, contributing to the sparse literature on this rare and aggressive malignancy. This case highlights the diagnostic challenge and clinical significance of FDG-PET/CT in evaluating rare and aggressive malignancies like LACC.

Visualizing the Tumor Microenvironment: Molecular Imaging Probes Target Extracellular Matrix, Vascular Networks, and Immunosuppressive Cells

The tumor microenvironment (TME) is a critical factor in cancer progression, driving tumor growth, immune evasion, therapeutic resistance, and metastasis. Understanding the dynamic interactions within the TME is essential for advancing cancer management. Molecular imaging provides a non-invasive, real-time, and longitudinal approach to studying the TME, with techniques such as positron emission tomography (PET), magnetic resonance imaging (MRI), and fluorescence imaging offering complementary strengths, including high sensitivity, spatial resolution, and intraoperative precision. Recent advances in imaging probe development have enhanced the ability to target and monitor specific components of the TME, facilitating early cancer diagnosis, therapeutic monitoring, and deeper insights into tumor biology. By integrating these innovations, molecular imaging offers transformative potential for precision oncology, improving diagnostic accuracy and treatment outcomes through a comprehensive assessment of TME dynamics.

Recent Clinical Implications of FAPI: Imaging and Therapy

ABSTRACT

The fibroblast activation protein (FAP) is a biomarker that is selectively overexpressed on cancer-associated fibroblasts (CAFs) in various types of tumoral tissues and some nonmalignant diseases, including fibrosis, arthritis, cardiovascular, and metabolic diseases. FAP plays a critical role in tumor microenvironment through facilitating proliferation, invasion, angiogenesis, immunosuppression, and drug resistance. Recent studies reveal that FAP might be regarded as a promising target for cancer diagnosis and treatment. FAP-targeted imaging modalities, especially PET, have shown high sensitivity and specificity in detecting FAP-expressing tumors. FAP-targeted imaging can potentially enhance tumor detection, staging, and monitoring of treatment response, and facilitate the development of personalized treatment strategies. This study provides a comprehensive view of FAP and its function in the pathophysiology of cancer and nonmalignant diseases. It also will discuss the characteristics of radiolabeled FAP inhibitors, particularly those based on small molecules, their recent clinical implications in imaging and therapy, and the associated clinical challenges with them. In addition, we present the results of imaging and biodistribution radiotracer 68 Ga-FAPI-46 in patients with nonmalignant diseases, including interstitial lung disease, primary biliary cirrhosis, and myocardial infarction, who were referred to our department. Our results show that cardiac FAP-targeted imaging can provide a novel potential biomarker for managing left ventricle remodeling. Moreover, this study has been organized and presented in a manner that offers a comprehensive overview of the current status and prospects of FAPI inhibitors in the diagnosis and treatment of diseases.

The Potential Role of PSMA-Targeted PET in Salivary Gland Malignancies: An Updated Systematic Review

BACKGROUND

Recent studies have suggested using positron emission tomography/computed tomography (PET/CT) with prostate-specific membrane antigen (PSMA)-targeting radiopharmaceuticals for the detection of salivary gland malignancies (SGM), particularly adenoid-cystic carcinoma (ACC).

METHODS

The authors conducted an extensive review of the scientific literature to examine the potential diagnostic role of PET/CT using PSMA-targeting radiopharmaceuticals in salivary gland malignancies (SGMs) and adenoid cystic carcinoma (ACC). This study included newly diagnosed SGM patients and those with disease recurrence in their imaging evaluation.

RESULTS

This updated systematic review included a total of six studies that examined the diagnostic performance of PSMA-targeted PET/CT in ACC. The articles provided evidence of a high detection rate of PSMA-targeting PET/CT in ACC across all clinical contexts examined. SGMs other than ACC exhibited poorer diagnostic performance.

CONCLUSIONS

PSMA-targeted PET/CT seems promising in detecting ACC lesions; moreover, PSMA appears to be a suitable potential target for radioligand therapy. Prospective multicentric studies are warranted to strengthen the role of PSMA-targeting radiopharmaceuticals in ACC, as both diagnostic and theragnostic agents.

18F-FDG and 68Ga-FAPI-04 PET/CT Findings of a Rare Epithelial-myoepithelial Carcinoma Arising From Ex Pleomorphic Adenoma of Parotid

Epithelial-myoepithelial carcinoma (EMC) is a rare low-grade salivary gland neoplasm. Distant metastasis is rare, and 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) has been used to determine the metastatic disease in EMC. 68Ga-fibroblast activation protein inhibitors (FAPI) PET/CT is a promising imaging modality for diagnostic and theognostic purposes in various malignancies. Comparison studies with 18F-FDG have investigated the role of 68Ga-FAPI PET/CT. Herein, we present 18F-FDG and 68Ga-FAPI-04 PET/CT findings of a 51-year-old woman with metastatic EMC arising from ex-pleomorphic adenoma of the parotid.

Diagnostic Potential of Supplemental Static and Dynamic 68Ga-FAPI-46 PET for Primary 18F-FDG-Negative Pulmonary Lesions

PET using 68Ga-labeled fibroblast activation protein (FAP) inhibitors (FAPIs) holds high potential for diagnostic imaging of various malignancies, including lung cancer (LC). However, 18F-FDG PET is still the clinical gold standard for LC imaging. Several subtypes of LC, especially lepidic LC, are frequently 18F-FDG PET-negative, which markedly hampers the assessment of single pulmonary lesions suggestive of LC. Here, we evaluated the diagnostic potential of static and dynamic 68Ga-FAPI-46 PET in the 18F-FDG-negative pulmonary lesions of 19 patients who underwent surgery or biopsy for histologic diagnosis after PET imaging. For target validation, FAP expression in lepidic LC was confirmed by FAP immunohistochemistry. Methods: Hematoxylin and eosin staining and FAP immunohistochemistry of 24 tissue sections of lepidic LC from the local tissue bank were performed and analyzed visually. Clinically, 19 patients underwent static and dynamic 68Ga-FAPI-46 PET in addition to 18F-FDG PET based on individual clinical indications. Static PET data of both examinations were analyzed by determining SUVmax, SUVmean, and tumor-to-background ratio (TBR) against the blood pool, as well as relative parameters (68Ga-FAPI-46 in relation to18F-FDG), of histologically confirmed LC and benign lesions. Time-activity curves and dynamic parameters (time to peak, slope, k 1, k 2, k 3, and k 4) were extracted from dynamic 68Ga-FAPI-46 PET data. The sensitivity and specificity of all parameters were analyzed by calculating receiver-operating-characteristic curves. Results: FAP immunohistochemistry confirmed the presence of strongly FAP-positive cancer-associated fibroblasts in lepidic LC. LC showed markedly elevated 68Ga-FAPI-46 uptake, higher TBRs, and higher 68Ga-FAPI-46-to-18F-FDG ratios for all parameters than did benign pulmonary lesions. Dynamic imaging analysis revealed differential time-activity curves for LC and benign pulmonary lesions: initially increasing time-activity curves with a decent slope were typical of LC, and steadily decreasing time-activity curve indicated benign pulmonary lesions, as was reflected by a significantly increased time to peak and significantly smaller absolute values of the slope for LC. Relative 68Ga-FAPI-46-to-18F-FDG ratios regarding SUVmax and TBR showed the highest sensitivity and specificity for the discrimination of LC from benign pulmonary lesions. Conclusion: 68Ga-FAPI-46 PET is a powerful new tool for the assessment of single 18F-FDG-negative pulmonary lesions and may optimize patient stratification in this clinical setting.

Case report: [18F]FAPI-42 PET/CT visualize primary adenoid cystic carcinoma not detected by [18F]FDG

Adenoid cystic carcinoma (ACC) is a rare salivary gland cancer. Still, its growth and invasion progress is slow, and its hematogenous metastasis is ACC's most common distant metastasis. Because of the broad expression and low background uptake of fibroblast activation protein (FAP) in tumor stroma, FAPI is considered another potential tracer of ACC in addition to FDG. In this case, we report a patient who was diagnosed with metastatic ACC liver cancer by fine needle aspiration biopsy (FNAB) and underwent PET/CT examination of [18F]FDG and [18F]FAPI-42 to find the primary cancer lesion. Finally, the primary cancer lesion was found in the left submandibular gland and was pathologically confirmed as ACC after resection.

Gallium-68-Labeled Fibroblast Activation Protein Inhibitor as an Alternative Radiotracer to Fluorine 18-Fluorodeoxyglucose: A Case Report of Rare Pulmonary Colloid Adenocarcinoma Diagnosed by PET/CT

Colloid pulmonary adenocarcinoma represents a seldom encountered neoplasm in clinical practice. The diagnostic process for this rare neoplasm is complicated by its infrequency and the limited understanding of its specific molecular imaging characteristics. We report a 65-year-old male who was diagnosed with pulmonary colloid mucinous cystadenocarcinoma. Fluorine 18-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) was conducted for initial evaluation. The scan showed mild 18F-FDG expression at the primary tumor site, and several non-18F-FDG-avid mediastinal and paraesophageal lymph nodes exhibited suspicious morphologic features. Owing to the ongoing atrial fibrillation, initial histopathological confirmation of the primary tumor mass carries a sense of risk, prompting the imperative for cardiological assessment before proceeding. Instead, Gallium-68-labeled fibroblast activation protein inhibitor (68Ga-FAPI) PET/CT was performed, expecting this to be more informative in terms of malignancy potential than 18F-FDG PET in colloid mucinous histology. A scan revealed moderate 68Ga-FAPI expression at the primary tumor site but unremarkable 68Ga-FAPI expression at the questionable lymph node. Subsequently, a biopsy from a mediastinal node (left para-aortic) lymph node via endobronchial ultrasound (EUS) showed benign findings. The patient was treated with concurrent chemoradiation. This case underscores the vital role that 68Ga-FAPI PET/CT can play in specific cases of rare cancers, especially when invasive testing for tissue biopsy is not feasible.

PET/CT FAPI: Procedure and evidence review in oncology

Neoplasms are composed of malignant tumor cells, which are surrounded by other non-tumor cellular elements, in what has been defined as the microenvironment or tumor stroma. Evidence on the importance of the tumor microenvironment has not stopped growing in recent years. It plays a central role in cell proliferation, tissue invasion, angiogenesis and cell migration. The paradigm is the family of new FAPI radiopharmaceuticals that show the density of the fibroblast activation protein (FAP) which is overexpressed in the cell membrane of activated cancer-associated fibroblasts (CAF), and its presence is related to poor prognosis. This educational document includes the procedure for performing PET/CT FAPI, biodistribution and the main potentially clinical applications in oncology to date.

Immunohistochemical FAP Expression Reflects 68Ga-FAPI PET Imaging Properties of Low- and High-Grade Intraductal Papillary Mucinous Neoplasms and Pancreatic Ductal Adenocarcinoma

Pancreatic intraductal papillary mucinous neoplasms (IPMNs) are grossly visible (typically > 5 mm) intraductal epithelial neoplasms of mucin-producing cells, arising in the main pancreatic duct or its branches. According to the current 2-tiered grading scheme, these lesions are categorized as having either low-grade (LG) dysplasia, which has a benign prognosis, or high-grade (HG) dysplasia, which formally represents a carcinoma in situ and thus can transform to pancreatic ductal adenocarcinoma (PDAC). Because both entities require different treatments according to their risk of becoming malignant, a precise pretherapeutic diagnostic differentiation is inevitable for adequate patient management. Recently, our group has demonstrated that 68Ga-fibroblast activation protein (FAP) inhibitor (FAPI) PET/CT shows great potential for the differentiation of LG IPMNs, HG IPMNs, and PDAC according to marked differences in signal intensity and tracer dynamics. The purpose of this study was to biologically validate FAP as a target for PET imaging by analyzing immunohistochemical FAP expression in LG IPMNs, HG IPMNs, and PDAC and comparing with SUV and time to peak (TTP) measured in our prior study. Methods: To evaluate the correlation of the expression level of FAP and α-smooth muscle actin (αSMA) in neoplasm-associated stroma depending on the degree of dysplasia in IPMNs, 98 patients with a diagnosis of LG IPMN, HG IPMN, PDAC with associated HG IPMN, or PDAC who underwent pancreatic surgery at the University Hospital Heidelberg between 2017 and 2023 were identified using the database of the Institute of Pathology, University Hospital Heidelberg. In a reevaluation of hematoxylin- and eosin-stained tissue sections of formalin-fixed and paraffin-embedded resection material from the archive, which was originally generated for histopathologic routine diagnostics, a regrading of IPMNs was performed by a pathologist according to the current 2-tiered grading scheme, consequently eliminating the former diagnosis of "IPMN with intermediate-grade dysplasia." For each case, semithin tissue sections of 3 paraffin blocks containing neoplasm were immunohistologically stained with antibodies directed against FAP and αSMA. In a masked approach, a semiquantitative analysis of the immunohistochemically stained slides was finally performed by a pathologist by adapting the immunoreactive score (IRS) and human epidermal growth factor receptor 2 (Her2)/neu score to determine the intensity and percentage of FAP- and αSMA-positive cells. Afterward, the IRS of 14 patients who underwent 68Ga-FAPI-74 PET/CT in our previous study was compared with their SUVmax, SUVmean, and TTP for result validation. Results: From 98 patients, 294 specimens (3 replicates per patient) were immunohistochemically stained for FAP and αSMA. Twenty-three patients had LG IPMNs, 11 had HG IPMNs, 10 had HG IPMNs plus PDAC, and 54 had PDAC. The tumor stroma was in all cases variably positive for FAP. The staining intensity, percentage of FAP-positive stroma, IRS, and Her2/neu score increased with higher malignancy. αSMA expression could be shown in normal pancreatic stroma as well as within peri- and intraneoplastic desmoplastic reaction. No homogeneous increase in intensity, percentage, IRS, and Her2/neu score with higher malignancy was observed for αSMA. The comparison of the mean IRS of FAP with the mean SUVmax, SUVmean, and TTP of 68Ga-GAPI-74 PET/CT showed a matching value increasing with higher malignancy in 68Ga-FAPI-74 PET imaging and immunohistochemical FAP expression. Conclusion: The immunohistochemical staining of IPMNs and PDAC validates FAP as a biology-based stromal target for in vivo imaging. Increasing expression of FAP in lesions with a higher degree of malignancy matches the expectation of a stronger FAP expression in PDAC and HG IPMNs than in LG IPMNs and corroborates our previous findings of higher SUVs and a longer TTP in PDAC and HG IPMNs than in LG IPMNs.

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.

Impact of 68Ga-FAPI PET/CT on Staging and Oncologic Management in a Cohort of 226 Patients with Various Cancers

Since the development of fibroblast activation protein-targeted radiopharmaceuticals, 68Ga-fibroblast activation protein inhibitor (FAPI) PET/CT has been found to be suitable for detecting primary and metastatic lesions in many types of tumors. However, there is currently a lack of reliable data regarding the clinical impact of this family of probes. To address this gap, the present study aimed to analyze the clinical impact of 68Ga-FAPI PET/CT by examining a large cohort of patients with various tumors. Methods: In total, 226 patients (137 male and 89 female) were included in this retrospective analysis. Pancreatic cancer and head and neck cancers were the most common tumor types in this cohort. TNM stage and oncologic management were initially determined with gold standard imaging, and these results were compared with 68Ga-FAPI PET/CT. Changes were classified as major and minor. Results: For 42% of all patients, TNM stage was changed by 68Ga-FAPI PET/CT results. Most of these changes resulted in upstaging. A change in clinical management occurred in 117 of 226 patients. Although a major change in management occurred in only 12% of patients, there was a significant improvement in the ability to accurately plan radiation therapy. In general, the highest clinical impact of 68Ga-FAPI PET/CT imaging was found in patients with lung cancer, pancreatic cancer, and head and neck tumors. Conclusion: 68Ga-FAPI PET/CT is a promising imaging probe that has a significant impact on TNM stage and clinical management. 68Ga-FAPI PET/CT promises to be a crucial new technology that will improve on conventional radiologic imaging methods such as contrast-enhanced CT and contrast-enhanced MRI typically acquired for cancer staging.

Diagnostic Value of Preoperative uPAR-PET/CT in Regional Lymph Node Staging of Oral and Oropharyngeal Squamous Cell Carcinoma: A Prospective Phase II Trial

The detection of lymph node metastases is a major challenge in oral and oropharyngeal squamous cell carcinoma (OSCC and OPSCC). 68Ga-NOTA-AE105 is a novel positron emission tomography (PET) radioligand with high affinity to urokinase-type plasminogen activator receptor (uPAR), a receptor expressed on the surfaces of tumor cells. The aim of this study was to investigate the diagnostic value of uPAR-PET/CT (computerized tomography) in detecting regional metastatic disease in patients with OSCC and OPSCC compared to the current imaging work-up. In this phase II trial, patients with OSCC and OPSCC referred for surgical treatment were prospectively enrolled. Before surgery, 68Ga-NOTA-AE105 uPAR-PET/CT was conducted, and SUVmax values were obtained from the primary tumor and the suspected lymph nodes. Histology results from lymph nodes were used as the standard of truth of metastatic disease. The diagnostic values of 68Ga-uPAR-PET/CT were compared to conventional routine preoperative imaging results (CT and/or MRI). The uPAR expression in resected primary tumors and metastases was determined by immunohistochemistry and quantified digitally (H-score). A total of 61 patients underwent uPAR-PET/CT. Of the 25 patients with histologically verified lymph node metastases, uPAR-PET/CT correctly identified regional metastatic disease in 14 patients, with a median lymph node metastasis size of 14 mm (range 3-27 mm). A significant correlation was found between SUVmax and the product of the H-score and tumor depth (r = 0.67; p = 0.003). The sensitivity and specificity of uPAR-PET/CT in detecting regional metastatic disease were 56% and 100%, respectively. When added to CT/MRI, uPAR-PET was able to upstage 2/11 (18%) of patients with occult metastases and increase the sensitivity to 64%. The sensitivity and specificity of 68Ga-NOTA-AE105 uPAR-PET/CT were equivalent to those of CT/MRI. The significant correlation between SUVmax and uPAR expression verified the target specificity of 68Ga-NOTA-AE105. Despite the target specificity, the sensitivity of imaging is too low for nodal staging and it cannot replace neck dissection.

Role of cancer-associated fibroblasts in the progression, therapeutic resistance and targeted therapy of oesophageal squamous cell carcinoma

Oesophageal squamous cell carcinoma (ESCC) is one of the most aggressive malignant tumours with high morbidity and mortality. Although surgery, radiotherapy and chemotherapy are common treatment options available for oesophageal cancer, the 5-year survival rate remains low after treatment. On the one hand, many oesophageal cancers are are discovered at an advanced stage and, on the other hand, treatment resistance is a major obstacle to treating locally advanced ESCC. Cancer-associated fibroblasts (CAFs), the main type of stromal cell in the tumour microenvironment, enhance tumour progression and treatment resistance and have emerged as a major focus of study on targeted therapy of oesophageal cancer.With the aim of providing potential, prospective targets for improving therapeutic efficacy, this review summarises the origin and activation of CAFs and their specific role in regulating tumour progression and treatment resistance in ESCC. We also emphasize the clinical potential and emerging trends of ESCC CAFs-targeted treatments.

Utility of 68 Ga-FAPI-04 PET/CT in Adenoid Cystic Carcinoma Compared With 18 F-FDG PET/CT : Two Case Reports

ABSTRACT

68 Ga-fibroblast activation protein inhibitor (FAPI) PET/CT is an emerging imaging modality with high sensitivity and high tumor-to-background ratio in various cancers including in the head and neck regions. The authors present 2 cases of adenoid cystic carcinoma who underwent 68 Ga-FAPI-04 and 18 F-FDG PET/CT. Locoregional recurrence has been detected more precisely in the first case with 68 Ga-FAPI-04. In the second case, 68 Ga-FAPI-04 outperformed 18 F-FDG in the number of lesions and demonstrated intense FAP uptake on widespread metastases, which could provide a treatment option as a theranostic concept. These cases highlight that 68 Ga-FAPI-04 PET/CT may be useful for detecting local recurrence and metastases and help select patients for radionuclide treatments targeting cancer-associated fibroblasts.

Radiation Therapy Planning Using Fibroblast Activation Protein Inhibitor

Computed tomography (CT), MR imaging, and PET with fluorodeoxyglucose F18/CT are commonly used for radiation therapy planning; however, issues including precise nodal staging on CT or false positive results on PET/CT limit their usability. Clinical trials using fibroblast activation protein ligands for additional imaging have provided promising results regarding staging and target volume delineation-particularly suitable for sarcoma, some gastrointestinal tumors, head and neck tumors, and lung and pancreatic cancer. Although further prospective trials are necessary to identify clinical settings for its application in radiation oncology, fibroblast activation protein inhibitor PET/CT indisputably represents an excellent opportunity for assisting radiotherapy planning.

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.

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.

Evaluation of Astatine-211-Labeled Fibroblast Activation Protein Inhibitor (FAPI): Comparison of Different Linkers with Polyethylene Glycol and Piperazine

Fibroblast activation proteins (FAP) are overexpressed in the tumor stroma and have received attention as target molecules for radionuclide therapy. The FAP inhibitor (FAPI) is used as a probe to deliver nuclides to cancer tissues. In this study, we designed and synthesized four novel ²¹¹At-FAPI(s) possessing polyethylene glycol (PEG) linkers between the FAP-targeting and ²¹¹At-attaching moieties. ²¹¹At-FAPI(s) and piperazine (PIP) linker FAPI exhibited distinct FAP selectivity and uptake in FAPII-overexpressing HEK293 cells and the lung cancer cell line A549. The complexity of the PEG linker did not significantly affect selectivity. The efficiencies of both linkers were almost the same. Comparing the two nuclides, ²¹¹At was superior to ¹³¹I in tumor accumulation. In the mouse model, the antitumor effects of the PEG and PIP linkers were almost the same. Most of the currently synthesized FAPI(s) contain PIP linkers; however, in our study, we found that PEG linkers exhibit equivalent performance. If the PIP linker is inconvenient, a PEG linker is expected to be an alternative.

[Positron emission tomography in CUP syndrome]

CLINICAL/METHODOLOGICAL ISSUE

In approximately 2% of all cancers, no primary tumor can be detected and cancer of unknown primary (CUP) syndrome, a diagnosis of exclusion, is made.

STANDARD RADIOLOGICAL METHODS

In CUP syndrome, computed tomography (CT) and/or magnetic resonance imaging (MRI) do not lead to the detection of primary tumors.

METHODOLOGICAL INNOVATIONS

In the advanced diagnostic workup of CUP syndrome, ¹⁸F‑fluordeoxyglucose positron emission tomography/computed tomography (¹⁸F‑FDG PET/CT) can be used. In addition, ⁶⁸Ga-fibroblast activation protein inhibitor (FAPI) PET/CT as a novel, experimental imaging technique may be considered.

PERFORMANCE

¹⁸F‑FDG PET/CT is clinically established for the detection of primary tumors of cervical CUP syndrome. High detection rates have also been reported for ¹⁸F‑FDG-PET/CT in extracervical CUP syndrome. ⁶⁸Ga-FAPI PET/CT has not yet been clinically established, but remarkably high detection rates have been shown for ¹⁸F‑FDG-negative cervical CUP syndrome due to its low background activity.

ACHIEVEMENTS

The benefit of ¹⁸F‑FDG PET in CUP syndrome has been documented in several meta-analyses. To date, the evidence for the use of ⁶⁸Ga-FAPI PET/CT in CUP syndrome is still rudimentary.

PRACTICAL RECOMMENDATIONS

¹⁸F‑FDG PET should be applied regularly in cervical CUP syndrome and can be individually considered in extracervical CUP syndrome.

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.

The Superiority of Fibroblast Activation Protein Inhibitor (FAPI) PET/CT Versus FDG PET/CT in the Diagnosis of Various Malignancies

Cancer represents a major cause of death worldwide and is characterized by the uncontrolled proliferation of abnormal cells that escape immune regulation. It is now understood that cancer-associated fibroblasts (CAFs), which express specific fibroblast activation protein (FAP), are critical participants in tumor development and metastasis. Researchers have developed various FAP-targeted probes for imaging of different tumors from antibodies to boronic acid-based inhibitor molecules and determined that quinoline-based FAP inhibitors (FAPIs) are the most appropriate candidate as the radiopharmaceutical for FAPI PET/CT imaging. When applied clinically, FAPI PET/CT yielded satisfactory results. Over the past few years, the utility and effectiveness of tumor detection and staging of FAPI PET/CT have been compared with FDG PET/CT in various aspects, including standardized uptake values (SUVs), rate of absorbance and clearance. This review summarizes the development and clinical application of FAPI PET/CT, emphasizing the diagnosis and management of various tumor types and the future prospects of FAPI imaging.

Recent Advances, Systemic Therapy, and Molecular Targets in Adenoid Cystic Carcinoma of the Head and Neck

With an incidence of 3-4.5 cases per million, adenoid cystic carcinoma (ACC) of the head and neck is one of the most common tumors of the parotid and sublingual salivary glands. In the clinical course, ACC is shown to have an aggressive long-term behavior, which leads to the fact that radical surgical resection of the tumor with tumor-free margins remains the "gold standard" in treating ACC. Particle radiation therapy and systemic molecular biological approaches offer new treatment options. However, risk factors for the formation and prognosis of ACC have not yet been clearly identified. The aim of the present review was to investigate long-term experience of diagnosis and treatment as well as risk and prognostic factors for occurrence and outcome of ACC.

Repetitive Early 68Ga-FAPI PET Acquisition Comparing 68Ga-FAPI-02, 68Ga-FAPI-46, and 68Ga-FAPI-74: Methodologic and Diagnostic Implications for Malignant, Inflammatory/Reactive, and Degenerative Lesions

⁶⁸Ga-labeled fibroblast activation protein (FAP) inhibitor (⁶⁸Ga-FAPI) PET targets ⁶⁸Ga-FAPI-positive activated fibroblasts and is a promising imaging technique for various types of cancer and nonmalignant pathologies. However, discrimination between malignant and nonmalignant ⁶⁸Ga-FAPI-positive lesions based on static PET with a single acquisition time point can be challenging. Additionally, the optimal imaging time point for ⁶⁸Ga-FAPI PET has not been identified yet, and different ⁶⁸Ga-FAPI tracer variants are currently used. In this retrospective analysis, we evaluate the diagnostic value of repetitive early ⁶⁸Ga-FAPI PET with ⁶⁸Ga-FAPI-02, ⁶⁸Ga-FAPI-46, and ⁶⁸Ga-FAPI-74 for malignant, inflammatory/reactive, and degenerative lesions and describe the implications for future ⁶⁸Ga-FAPI imaging protocols. Methods: Whole-body PET scans of 24 cancer patients were acquired at 10, 22, 34, 46, and 58 min after the administration of 150-250 MBq of ⁶⁸Ga-FAPI tracer molecules (8 patients each for ⁶⁸Ga-FAPI-02, ⁶⁸Ga-FAPI-46, and ⁶⁸Ga-FAPI-74). Detection rates and SUVs (SUV_max and SUV_mean) for healthy tissues, cancer manifestations, and nonmalignant lesions were measured, and target-to-background ratios (TBR) versus blood and fat were calculated for all acquisition time points. Results: For most healthy tissues except fat and spinal canal, biodistribution analysis showed decreasing uptake over time. We analyzed 134 malignant, inflammatory/reactive, and degenerative lesions. Detection rates were minimally reduced for the first 2 acquisition time points and remained at a constant high level from 34 to 58 min after injection. The uptake of all 3 variants was higher in malignant and inflammatory/reactive lesions than in degenerative lesions. ⁶⁸Ga-FAPI-46 showed the highest uptake and TBRs in all pathologies. For all variants, TBRs versus blood constantly increased over time for all pathologies, and TBRs versus fat were constant or decreased slightly. Conclusion: ⁶⁸Ga-FAPI PET/CT is a promising imaging modality for malignancies and benign lesions. Repetitive early PET acquisition added diagnostic value for the discrimination of malignant from nonmalignant ⁶⁸Ga-FAPI-positive lesions. High detection rates and TBRs over time confirmed that PET acquisition earlier than 60 min after injection delivers high-contrast images. Additionally, considering clinical feasibility, acquisition at 30-40 min after injection might be a reasonable compromise. Different ⁶⁸Ga-FAPI variants show significant differences in time-dependent biodistributional behavior and should be selected carefully depending on the clinical setting.

Safety and Efficacy of 90Y-FAPI-46 Radioligand Therapy in Patients with Advanced Sarcoma and Other Cancer Entities

PURPOSE

We report efficacy and safety of 90Y-labeled FAPI-46 (90Y-FAPI-46-RLT) in patients with advanced sarcoma, pancreatic cancer, and other cancer entities.

EXPERIMENTAL DESIGN

Up to four cycles of radioligand therapy (RLT) were offered to patients with (i) progressive metastatic malignancy, (ii) exhaustion of approved therapies, and (iii) high fibroblast activation protein (FAP) expression, defined as SUVmax ≥ 10 in more than 50% of tumor. Primary endpoint was RECIST response after RLT. Secondary endpoints included PET response (PERCIST), overall survival (OS), dosimetry, and safety of FAP-RLT.

RESULTS

Among 119 screened patients, 21 (18%) were found eligible [n = 16/3/1/1 sarcoma/pancreatic cancer/prostate/gastric cancer; 38% Eastern Cooperative Oncology Group (ECOG) ≥ 2] and received 47 90Y-FAPI-46-RLT cycles; 16 of 21 (76%) patients underwent repeat RLT. By RECIST, disease control was confirmed in 8 of 21 patients [38%; 8/16 (50%) of evaluable patients). There was one partial response (PR) and seven stable diseases after RLT. Disease control was associated with prolonged OS (P = 0.013). PERCIST response was noted in 8 of 21 patients [38%; 8/15 (53%) of evaluable patients]. Dosimetry was acquired in 19 (90%) patients. Mean absorbed dose was 0.53 Gy/GBq in kidney, 0.04 Gy/GBq in bone marrow, and <0.14 Gy/GBq in liver and lung. Treatment-related grade 3 or 4 adverse events were observed in 8 (38%) patients with thrombocytopenia (n = 6) and anemia (n = 6) being most prevalent.

CONCLUSIONS

90Y-FAPI-46-RLT was safe and led to RECIST PR in one case as well as stable disease in about one third of patients with initially progressive sarcomas, pancreatic cancer, and other cancers. Discontinuation after the first cycle and a low rate of PR requires future improvement of FAP-RLT.

Head‑to‑head assessment of [68Ga]Ga-DOTA-FAPI-04 PET/CT vs [18F]FDG PET/CT in fibroblastic tumors

OBJECTIVES

We aimed to evaluate [⁶⁸Ga]Ga-DOTA-FAPI-04 versus [¹⁸F]FDG PET/CT in the application of fibroblastic tumors.

METHODS

Twenty participants with 6 subtypes of fibroblastic tumors prospectively underwent ¹⁸F-FDG and [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT examinations to evaluate the lesions. PET/CT findings were confirmed by surgical pathology of fifteen participants, puncture biopsy of two participants, or imaging follow-up of three participants. Two independent sample t tests were used to compare the uptake of [¹⁸F]FDG vs [⁶⁸Ga]Ga-DOTA-FAPI-04 in primary, recurrent and metastatic lesions. One-way ANOVA was used to compare the uptake of [¹⁸F]FDG or [⁶⁸Ga]Ga-DOTA-FAPI-04 among primary, recurrent, and metastatic lesions. The uptake of [⁶⁸Ga]Ga-DOTA-FAPI-04 vs [¹⁸F]FDG in different histopathological lesions was compared by two independent sample t tests.

RESULTS

Twenty participants were confirmed to have 38 lesions. Although there was no significant difference in the detection of lesions between [⁶⁸Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT (38 vs 36, p = 0.493), the uptake of [⁶⁸Ga]Ga-DOTA-FAPI-04 in lesions was significantly higher than that of [¹⁸F]FDG (p < 0.001), including primary (p < 0.001), recurrent (p = 0.018) and metastatic (p < 0.001) lesions. The SUVmax of [⁶⁸Ga]Ga-DOTA-FAPI-04 in primary and recurrent lesions was higher than that in metastasis (p = 0.034 and p = 0.015, respectively). The SUVmax of [⁶⁸Ga]Ga-DOTA-FAPI-04 in primary and recurrent malignant lesions was significantly higher than that of the intermediate (p < 0.001). The SUVmax of [⁶⁸Ga]Ga-DOTA-FAPI-04 in one participant of recurrent SFT with 5 lesions was significantly lower after treatment than before treatment (p = 0.016).

CONCLUSIONS

[⁶⁸Ga]Ga-DOTA-FAPI-04 outperformed [¹⁸F]FDG PET/CT in displaying the primary, recurrent and metastatic lesions of fibroblastic tumors.

[68Ga]FAPI-PET/CT for radiation therapy planning in biliary tract, pancreatic ductal adeno-, and adenoidcystic carcinomas

Biliary-tract-carcinomas (BTC), pancreatic-ductal-adenocarcinomas (PDAC) and adenoidcystic-carcinomas (AC) have in common that they are traditionally treated with large clinical-target-volumes (CTV). The aim of this study is to examine the impact of pretreatment-[⁶⁸Ga]FAPI-PET/CT on target-volume-definition and posttreatment-[⁶⁸Ga]FAPI-PET/CT-response-assessment for BTC-, PDAC- and AC-patients referred to radiation-therapy. All consecutive BTC-, PDAC-, and AC-patients who received pretreatment-[⁶⁸Ga]FAPI-PET/CT±[¹⁸F]FDG-PET/CT were included from 01.01.2020 to 01.03.2022. MTV and SUV_max were separately generated based on [⁶⁸Ga]FAPI- and [¹⁸F]FDG-PET/CT-images. A [⁶⁸Ga]FAPI- and [¹⁸F]FDG-based-CTV was defined. Treatment-plans were compared. Treatment-response was reassessed by a second [⁶⁸Ga]FAPI-PET/CT and [¹⁸F]FDG-PET/CT after treatment-completion. Intermodality comparison of lesion-to-background-ratios [SUV_{max_lesion}/SUV_{mean_background}] for individual timepoints t₁ and t₂ revealed significant higher values for [⁶⁸Ga]FAPI compared to [¹⁸F]FDG (t₁, p = 0.008; t₂, p = 0.005). Intermodality comparison of radiation-therapy-plans showed that [⁶⁸Ga]FAPI-based planning resulted in D100% = 97.2% and V95% = 98.8% for the [¹⁸F]FDG-MTV. [¹⁸F]FDG-based-planning resulted in D100% = 35.9% and V95% = 78.1% for [⁶⁸Ga]FAPI-MTV. [¹⁸F]FDG-based-planning resulted only in 2 patients in V95% > 95% for [⁶⁸Ga]FAPI-MTV, and in 1 patient in D100% > 97% for [⁶⁸Ga]FAPI-MTV. GTV-coverage in terms of V95% was 76.4% by [¹⁸F]FDG-based-planning and 99.5% by [⁶⁸Ga]FAPI-based-planning. Pretreatment [⁶⁸Ga]FAPI-PET/CT enhances radiation-treatment-planning in this particular group of patients. While perilesional and tumoral follow-up [¹⁸F]FDG-uptake behaved uniformly, perilesional and tumoral reaction may differ in follow-up [⁶⁸Ga]FAPI-imaging. Complementary [⁶⁸Ga]FAPI- and [¹⁸F]FDG-imaging enhance treatment-response-assessment.

FAP-Specific Signalling Is an Independent Diagnostic Approach in ACC and Not a Surrogate Marker of MRI Sequences

Background: Fibroblast Activation Protein (FAP) is a new target for positron emission tomography and computed tomography (PET/CT) imaging of epithelial tumours embedded in a fibrous stroma. Adenoid cystic carcinomas (ACCs) have shown elevated tracer uptake in 68Gallium (68Ga)-labelled FAPIs in previous studies. The current gold standard for ACC imaging is contrast-enhanced (ce) MRI, where intertumoural heterogeneity leads to variable appearance on T1-weighted (T1w) and T2-weighted (T2w) images. In this retrospective analysis, we correlated 68Ga-FAPI PET signalling at three time points with ceT1w and T2w MRI signals to further characterise the significance of 68Ga-FAPI uptake in ACCs. Methods: Clinical PET/CT scans of 12 ACC patients were performed at 10, 60 and 180 min post i.v. administration of 68Ga-labelled-FAPI tracer molecules. 68Ga-PET- and corresponding MRI-scans were co-registered, and 3D volumetric segmentations were performed on ceT1w and T2w lesions of co-registered MRI slides. Signal intensity values of 68Ga-FAPI PET signalling and ceT1w/T2w MRI scans were analysed for their pixelwise correlation in each patient. Pooled estimates of the correlation coefficients were calculated using the Fisher z-transformation. Results: 68Ga-FAPI PET signals showed a very weak positive correlation with ceT1w values (pooled correlation 0.114, 0.147 and 0.162 at 10, 60 and 180 min) and a weak negative correlation with T2w values (pooled correlation −0.148, −0.121 and −0.225 at 10, 60 and 180 min). Individual r-values at 60 min ranged from −0.130 to 0.434 in ceT1w and from −0.466 to 0.637 in T2w MRI scans. Conclusion: There are only slight correlations between the intensity of 68Ga-FAPI PET signals and tumour appearance in ceT1w or T2w MRI scans, which underlines that 68Ga-FAPI PET signalling is not a surrogate marker of MRI sequences but an independent signal.

86Y-Labeled Albumin-Binding Fibroblast Activation Protein Inhibitor for Late-Time-Point Cancer Diagnosis

Fibroblast activation protein inhibitor (FAPI) is a novel quinoline-based radiopharmaceutical that has theranostic potential, yet the limited tumor retention hinders late-time diagnosis and radionuclide treatment. This study synthesized four albumin-binding FAPIs (TE-FAPI-01 to 04) and evaluated their in vitro stability, binding affinity, in vivo biodistribution, and tumor uptake with ⁶⁸Ga, ⁸⁶Y, and ¹⁷⁷Lu labeling, aiming to select the best molecule that has favorable pharmacokinetics to extend the blood circulation and tumor uptake in FAP-expressing tumors. All TE-FAPIs were stable in saline and plasma and displayed high FAP-binding affinity, with IC₅₀ values ranging from 3.96 to 34.9 nmol/L. The capabilities of TE-FAPIs to be retained in circulation were higher than that of FAPI-04, and TE-FAPI-04 displayed minimum physiological uptake in major organs compared with other molecules. TE-FAPI-03 and TE-FAPI-04 exhibited persistent tumor accumulation, with tumor radioactivity 24 h after administration of 2.84 ± 1.19%ID/g and 3.86 ± 1.15%ID/g for ¹⁷⁷Lu-TE-FAPI-03 and ¹⁷⁷Lu-TE-FAPI-04, respectively, both of which outperformed ¹⁷⁷Lu-FAPI-04 (0.34 ± 0.07%ID/g). TE-FAPI-04 was recognized as the albumin-binding FAPI with the most favorable pharmacokinetics and imaging performance. The enhanced circulation half-life and tumor uptake of TE-FAPI-04 aided the theranostics of malignant tumors and warrant further clinical investigations.

Current opinions on diagnosis and treatment of adenoid cystic carcinoma

Adenoid cystic carcinoma (ACC) is a rare malignant tumor derived mainly from the salivary glands, representing approximately 1% of all headandneck carcinomasand 10% of all salivary gland neoplasms. ACC displays a paradoxical behavioral combination of an indolent growth pattern but an aggressive progression, with local recurrence and distant metastasis. The propensity of ACC of the head and neck (ACCHN) for perineural invasion and its anatomical location, especially if it extends to the nasal cavity and paranasal sinuses, facilitates tumor involvement in the surrounding structures, such as the orbit, pterygopalatine fossa, Meckel'scave, and cavernous sinus, which can lead to skull base involvement and intracranial extension. Despite advances in molecular mechanisms and diagnostic imaging, ACC treatment remainschallenging due to the lack ofconsensuson treatment patterns. In this review, we aimed toprovideanupdatedinsight intothe understanding of ACCHN by focusing on clinical behavior, imaging diagnosis, pathological features, and therapeutic strategies. We reviewed the molecular mechanisms, especially in ACCHN with perineural invasion, and elaborated on treatment options, including chemotherapy, targeted therapies, and immunotherapy, to establish a comprehensive understanding of ACC to arrive at a policy for proper diagnosis, preoperative evaluation, and therapeutic strategies.

[Radiation therapy of malignant salivary gland tumors]

Due to their rarity, histologic heterogeneity, and localization, treatment of malignant salivary gland tumors requires an interdisciplinary approach. First-line treatment includes complete tumor resection. Postoperative radiation therapy is advised in patients with risk factors, i.e., incomplete tumor resection, high-grade tumors, or perineural invasion. Definitive radiation therapy is only advised for inoperable tumors because of significantly lower local control and survival rates when compared to combined surgery and radiation therapy. In radiation oncology, modern techniques such as intensity-modulated radiation therapy (IMRT) or particle therapy with heavy ions (i.e., C12) have led to improved outcomes in the treatment of head and neck tumors, especially of adenoid cystic carcinomas. Given the biological and physical benefits of particles, particle therapy, particularly carbon ion radiation, is a promising therapeutic approach for salivary gland tumors that will be further investigated in prospective clinical studies.

Interobserver Agreement Rates on Fibroblast Activation Protein Inhibitor-Directed Molecular Imaging and Therapy

OBJECTIVES

Fibroblast activation protein (FAP) has emerged as a novel target for FAP inhibitor (FAPI)-directed molecular imaging and endoradiotherapy (ERT). We aimed to assess the interobserver agreement rates for interpretation of 68Ga-FAPI-4 PET/CT and decision for ERT.

PATIENTS AND METHODS

A random order of 68Ga-FAPI-4 PET/CTs from 49 oncology patients were independently interpreted by 4 blinded readers. Per scan, visual assessment was performed, including overall scan impression, number of organ/lymph node (LN) metastases, and number of affected organs/LN regions. Moreover, a maximum of 3 target lesions, defined as largest in size and/or most intense, per organ compartment were identified, which allowed for an additional quantitative interobserver assessment of LN and organ lesions. To investigate potential reference tissues, quantification also included unaffected liver parenchyma and blood pool. Readers also had to indicate whether FAPI-directed ERT should be considered (based on intensity of uptake and widespread disease). Interobserver agreement rates were evaluated using intraclass correlation coefficients (ICCs) and interpreted according to Cicchetti (with 0.4-0.59 indicating fair, and 0.6-0.74 good, agreement).

RESULTS

On a visual basis, the agreement rate for an overall scan impression was fair (ICC, 0.42; 95% confidence interval [CI], 0.27-0.57). The concordance rate for number of affected LN areas was also fair (ICC, 0.59; 95% CI, 0.45-0.72), whereas the number of LN metastases, number of affected organs, and number of organ metastases achieved good agreement rates (ICC, ≥0.63). In a quantitative analysis, concordance rates for LN were good (ICC, 0.70; 0.48-0.88), but only fair for organ lesions (ICC, 0.43; 0.26-0.60). In regards to background tissues, ICCs were good for unaffected liver parenchyma (0.68; 0.54-0.79) and fair for blood pool (0.43; 0.29-0.58). When readers should decide on ERT, concordance rates were also fair (ICC, 0.59; 95% CI, 0.46-0.73).

CONCLUSIONS

For FAPI-directed molecular imaging and therapy, a fair to good interobserver agreement rate was achieved, supporting the adoption of this radiotracer for clinical routine and multicenter trials.

Could Fibroblast Activation Protein (FAP)-Specific Radioligands Be Considered as Pan-Tumor Agents?

Background

Cancer-associated fibroblasts (CAFs) can strongly modulate the response to therapy of malignant tumor cells, facilitating their continuous proliferation and invading behaviors. In this context, several efforts were made in identifying the fibroblast activation protein (FAP) as a CAF recognizer and in designing FAP-specific PET radiotracers (as 68Ga-FAPI) along with FAP-specific therapeutic radioligands. Herein, we review different clinical studies using the various FAP-specific radioligands as novel theranostic agents in a wide range of oncologic and nononcologic indications.

Methods

A comprehensive systematic search was conducted on the PubMed and Scopus databases to find relevant published articles concerning the FAP-specific PET imaging as well as the FAP-specific radionuclide therapy in patients with oncologic and nononcologic indications. The enrolled studies were dichotomized into oncologic and nononcologic categories, and the required data were extracted by precisely reviewing the whole text of each eligible study. A meta-analysis was also performed comparing the detection rates of 68Ga-FAPI vs. 18F-FDG PET/CT using odds ratio (OR) and risk difference as outcome measures.

Results

Of the initial 364 relevant papers, 49 eligible articles (1479 patients) and 55 case reports were enrolled in our systematic review. These studies observed high radiolabeled FAPI avidity as early as 10 minutes after administration in primary sites of various malignant tumors. Based on the meta-analysis which was done on the reported detection rates of the 68Ga-FAPI and 18F-FDG PET/CT scans, the highest OR belonged to the primary lesion detection rate of gastrointestinal tumors (OR = 32.079, 95% CI: 4.001-257.212; p = 0.001) with low heterogeneity (I2 = 0%). The corresponding value of the nodal metastases belonged to hepatobiliary tumors (OR = 11.609, 95% CI: 1.888-71.365; p = 0.008) with low heterogeneity (I2 = 0%). For distant metastases, the highest estimated OR belonged to nasopharyngeal carcinomas (OR = 77.451, 95% CI: 7.323-819.201; p < 0.001) with low heterogeneity (I2 = 0%).

Conclusions

The outperformance of 68Ga-FAPI PET/CT over 18F-FDG PET/CT in identifying certain primary tumors as well as in detecting their metastatic lesions may open indications for evaluation of cases with inconclusive 18F-FDG PET/CT findings. What needs to be emphasized is that the false-positive results might be problematic and must be taken into account in 68Ga-FAPI PET/CT interpretation. More clarification on the role of FAPI radioligands in oncologic imaging, radionuclide therapy, and radiotherapy treatment planning is therefore required.

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

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.

Fibroblast activation protein-based theranostics in cancer research: A state-of-the-art review

In recent years, quinoline-based fibroblast activation protein (FAP) inhibitors (FAPI) have shown promising results in the diagnosis of cancer and several other diseases, making them the hotspot of much productive research. This review summarizes the literature for the state-of-the-art FAPI-PET imaging for cancer diagnosis compared with fluorodeoxyglucose (FDG)-PET. We also summarize the use of FAPI-PET for therapeutic regimen improvement and fibroblast activation protein (FAP)-targeted molecule modification strategies, as well as preliminary clinical studies regarding FAP-targeted radionuclide therapy. Our qualitative summary of the literature to date can inform future research directions, medical guidelines, and optimal clinical decision-making.

Fibroblast-Activated Protein Inhibitor PET/CT: Cancer Diagnosis and Management

Fibroblast activation protein (FAP), overexpressed on cancer-associated fibroblasts (CAFs), is a novel target for molecular imaging of various tumors. Recently, the development of several small-molecule FAP inhibitors for radiolabeling with 68Ga has resulted in the emergence of studies evaluating its clinical role in cancer imaging. Preliminary findings have demonstrated that, in contrast to radiotracers taking advantage of cancer-specific targets such as PSMA and DOTATATE, FAPs as a target are the most promising that can compete with 18FDG in terms of widespread indications. They also have the potential to overcome the shortcomings of 18FDG, particularly false-positive uptake due to inflammatory or infectious processes, low sensitivity in certain cancer types, and radiotherapy planning. In addition, the attractive theranostic properties may facilitate the treatment of many refractory cancers. This review summarizes the current FAP variants and related clinical studies, focusing on radiopharmacy, dosimetry, and diagnostic and theranostic applications.

Methylosystem for Cancer Sieging Strategy

As cancer is a genetic disease, methylation defines a biologically malignant phenotype of cancer in the association of one-carbon metabolism-dependent S-adenosylmethionine (SAM) as a methyl donor in each cell. Methylated substances are involved in intracellular metabolism, but via intercellular communication, some of these can also be secreted to affect other substances. Although metabolic analysis at the single-cell level remains challenging, studying the "methylosystem" (i.e., the intercellular and intracellular communications of upstream regulatory factors and/or downstream effectors that affect the epigenetic mechanism involving the transfer of a methyl group from SAM onto the specific positions of nucleotides or other metabolites in the tumor microenvironment) and tracking these metabolic products are important research tasks for understanding spatial heterogeneity. Here, we discuss and highlight the involvement of RNA and nicotinamide, recently emerged targets, in SAM-producing one-carbon metabolism in cancer cells, cancer-associated fibroblasts, and immune cells. Their significance and implications will contribute to the discovery of efficient methods for the diagnosis of and therapeutic approaches to human cancer.

FAP and FAPI-PET/CT in Malignant and Non-Malignant Diseases: A Perfect Symbiosis?

A fibroblast activation protein (FAP) is an atypical type II transmembrane serine protease with both endopeptidase and post-proline dipeptidyl peptidase activity. FAP is overexpressed in cancer-associated fibroblasts (CAFs), which are found in most epithelial tumors. CAFs have been implicated in promoting tumor cell invasion, angiogenesis and growth and their presence correlates with a poor prognosis. However, FAP can generally be found during the remodeling of the extracellular matrix and therefore can be detected in wound healing and benign diseases. For instance, chronic inflammation, arthritis, fibrosis and ischemic heart tissue after a myocardial infarction are FAP-positive diseases. Therefore, quinoline-based FAP inhibitors (FAPIs) bind with a high affinity not only to tumors but also to a variety of benign pathologic processes. When these inhibitors are radiolabeled with positron emitting radioisotopes, they provide new diagnostic and prognostic tools as well as insights into the role of the microenvironment in a disease. In this respect, they deliver additional information beyond what is afforded by conventional FDG PET scans that typically report on glucose uptake. Thus, FAP ligands are considered to be highly promising novel tracers that offer a new diagnostic and theranostic potential in a variety of diseases.