[68Ga]Ga-DOTA-FAPI-04 improves tumor staging and monitors early response to chemoradiotherapy in a patient with esophageal cancer

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 68Ga-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.

PET Imaging of Fibroblast Activation Protein in Various Cancers Using [18F]AlF‑NOTA‑FAPI‑04: Comparison with 18F-FDG in a Single-Center, Prospective Study

RATIONALE AND OBJECTIVES

Targeting fibroblast-activation protein is a newer diagnostic approach for the visualization of tumor stroma, and a novel aluminum-[18F] fluoride (Al18F)-labeled fibroblast-activation protein inhibitor-4 (FAPI-04), hereafter [18F] AlF-NOTA-FAPI-04, presents a promising alternative to gallium 68 (68Ga)-labeled FAPI owing to its relatively longer half-life. This study sought to evaluate the clinical usefulness of [18F] AlF-NOTA-FAPI-04 PET/CT for the diagnosis of various types of cancer, compared to [18F] FDG PET/CT.

MATERIALS AND METHODS

In this prospective study conducted from October 2021 to January 2024, a total of 148 patients with 16 different tumor entities underwent contemporaneous 18F-FDG and 18F-FAPI-04 PET/CT either for an initial assessment or for recurrence detection. Uptake of 18F-FDG and 18F-FAPI-04 was quantified by the maximum standard uptake value (SUV max). Diagnostic sensitivity, specificity, and accuracy were compared by using the McNemar test between these two imaging agents.

RESULTS

18F-FAPI-04 PET/CT could clearly depict 16 different types of cancer with excellent image contrast, thereby leading to a higher detection rate of primary tumors than did 18F-FDG PET/CT (98.06% vs. 81.55%, P＜0.001). In per-lymph node analysis, the sensitivity, specificity, and accuracy in the diagnosis of metastatic lymph nodes were 92.44%, 90.44%, and 91.56%, respectively, which was much higher than that 18F-FDG PET/CT (80.23%, 79.41%, and 79.87%, respectively). Meanwhile, 18F-FAPI-04 PET/CT outperformed 18F-FDG PET/CT in identifying more suspected distant metastases (86.57% vs. 74.13%, P＜0.001). Furthermore, 18F-FAPI-04 PET/CT upgraded tumor staging in 36/101 patients (35.6%), and detected tumor recurrence or metastases in 43/47 patients (91.49%).

CONCLUSION

Our findings demonstrated that primary and metastatic lesions in patients with various types of malignant tumors are well-visualized on 18F-FAPI-04 PET/CT, which exhibited a superior diagnostic performance than 18F-FDG PET/CT. Moreover, 18F-FAPI-04 PET/CT is a promising tool for tumor staging and follow-up of various malignancies.

Value of [18F]AlF-NOTA-FAPI-04 PET/CT for differential diagnosis of malignant and various inflammatory lung lesions: comparison with [18F]FDG PET/CT

OBJECTIVE

Uptake of the imaging tracers [18F]AlF-NOTA-FAPI-04 and [18F]FDG varies in some inflammatory lesions, which may result in false-positive findings for malignancy on PET/CT. Our aim was to compare the [18F]AlF-NOTA-FAPI-04 and [18F]FDG PET/CT imaging features of malignant and various inflammatory lung lesions and to analyze their value for differential diagnosis.

METHODS

We retrospectively analyzed [18F]AlF-NOTA-FAPI-04 PET/CT scans from 67 cancer patients taken between December 2020 and January 2022, as well as the scans of 32 patients who also underwent [18F]FDG PET/CT imaging. The maximum and mean standardized uptake values (SUVmax and SUVmean, respectively) and lesion-to-background ratio (LBR) were calculated. The predictive capabilities of semiquantitative PET/CT parameters were analyzed by receiver operating characteristic curve analysis.

RESULTS

A total of 70 inflammatory and 37 malignant lung lesions were evaluated by [18F]AlF‑NOTA‑FAPI‑04 PET/CT, and 33 inflammatory and 26 malignant lung lesions also were evaluated by [18F]FDG PET/CT. Inflammatory lesions exhibited lower [18F]AlF-NOTA-FAPI-04 and [18F]FDG uptake compared to malignant lesions, with statistically significant differences in SUVmax, SUVmean, and LBR (all p < 0.001). [18F]AlF-NOTA-FAPI-04 uptake also varied among different types of inflammatory lesions (SUVmax, p = 0.005; SUVmean, p = 0.008; LBR, p < 0.001), with the highest uptake observed in bronchiectasis with infection, followed by postobstructive pneumonia, and the lowest in pneumonia. [18F]FDG uptake was higher in postobstructive pneumonia than in pneumonia (SUVmax, p = 0.009; SUVmean, p = 0.016; LBR, p = 0.004).

CONCLUSION

[18F]AlF-NOTA-FAPI-04/[18F]FDG PET/CT showed significantly lower uptake in inflammatory lesions than malignancies as well as variation in different types of inflammatory lesions, and thus, may be valuable for distinguishing malignant and various inflammatory findings.

CLINICAL RELEVANCE STATEMENT

Our study confirmed that the uptake of [18F]AlF-NOTA-FAPI-04/[18F]FDG PET/CT in inflammatory and malignant lung lesions is different, which is beneficial to distinguish inflammatory and malignant lung lesions in clinic.

KEY POINTS

• Malignant and different inflammatory lung lesions showed varying degrees of uptake of [18F]AlF-NOTA-FAPI-04 and [18F]FDG. • Inflammatory lung lesions showed significantly less uptake than malignancies, and uptake varied among different types of inflammatory lesions. • Both types of PET/CT could differentiate malignant and various inflammatory lung findings.

Head-to-Head Comparison of 18 F-FDG and 68 Ga-FAPI-04 PET/CT for Radiological Evaluation of Cervical Cancer

PURPOSE

We aimed to compare the potential value of 68 Ga-FAPI-04 and 18 F-FDG PET/CT in primary cervical cancer and lymph node metastases.

METHODS

Patients with cervical cancer underwent both 68 Ga-FAPI-04 and 18 F-FDG PET/CT. Histopathology and follow-up CT or MRI results (at least 3 months of follow-up) were used as reference criteria. Paired-sample t test was used to compare the SUV max of 18 F-FDG and 68 Ga-FAPI-04 PET/CT for cervical cancer primary lesions and metastatic lymph nodes.

RESULTS

A total of 35 patients with a mean age of 53 ± 11 years (range, 30-76 years) were included. The detection rate of both tracers for primary tumors was 100%. There was no significant correlation between 18 F-FDG and 68 Ga-FAPI-04 for SUV max (14.5 ± 5.7 vs 15.1 ± 6.2; P = 0.645). In addition, the detection rates of 68 Ga-FAPI-04 and 18 F-FDG for lymph node metastasis were 100% and 98%, respectively. No significant difference was found in SUV max between 18 F-FDG and 68 Ga-FAPI-04 groups (7.6 ± 4.0 vs 7.0 ± 3.5; P = 0.572). Twelve false-positive lymph nodes were detected in 8 patients with 18 F-FDG PET/CT, none of which were developed on 68 Ga-FAPI-04 PET/CT.

CONCLUSION

68 Ga-FAPI-04 PET/CT has a high tracer rate for the diagnosis of primary cervical cancer and lymph node metastases. Moreover, 68 Ga-FAPI-04 PET/CT also showed good results in distinguishing metastatic lymph nodes from reactive lymph nodes of cervical cancer.

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 Role of 68 Ga-FAPI PET/CT in Breast Cancer Response Assessment and Follow-Up

PURPOSE

68 Ga-fibroblast activation protein inhibitor (FAPI), a new PET/CT radiotracer targeting cancer-associated fibroblasts in tumor microenvironment, can detect many types of cancer. We aimed to assess whether it can also be used for response assessment and follow-up.

METHODS

We followed up patients with FAPI-avid invasive lobular breast cancer (ILC) before and after treatment changes and correlated qualitative maximal intensity projection images and quantitative tumor volume with CT results and blood tumor biomarkers.

RESULTS

Six consenting ILC breast cancer patients (53 ± 8 years old) underwent a total of 24 scans (baseline for each patient and 2-4 follow-up scans). We found a strong correlation between 68 Ga-FAPI tumor volume and blood biomarkers ( r = 0.7, P < 0.01), but weak correlation between CT and 68 Ga-FAPI maximal intensity projection-based qualitative response assessment.

CONCLUSIONS

We found a strong correlation between ILC progression and regression (as assessed by blood biomarkers) and 68 Ga-FAPI tumor volume. 68 Ga-FAPI PET/CT could possibly be used for disease response assessment and follow-up.

The additional role of [68Ga]Ga-FAPI-04 PET/CT in patients with unknown primary lesion with a negative or equivocal [18F]FDG

PURPOSE

[18F]FDG PET/CT to detect unknown primary lesions is essential for clinical management but still has limitations. [68Ga]Ga-FAPI is a tumor-stromal imaging agent that provides a promising alternative to [18F]FDG for the assessment of malignancies. We aimed to investigate whether [68Ga]Ga-FAPI PET/CT has an additional role in identifying unknown primary lesions with negative or equivocal [18F] FDG PET/CT results.

METHODS

This single-center prospective clinical study was conducted between March 2020 and March 2022 at Southwest Medical University Hospital. Patients underwent [18F]FDG PET/CT for the identification of unknown primary lesions. They underwent repeat [68Ga]Ga-FAPI PET/CT when [18F]FDG PET/CT results were negative or equivocal. Histopathological examination, surgery, or clinical follow-up (at least 3 months) for FAPI-positive lesions. The diagnostic efficacy of [68Ga]Ga-FAPI in identifying unknown primary lesions was evaluated.

RESULTS

A total of 44 participants (median age, 57 ± 12 [SD]; 22 [50%] men) were evaluated. Thirteen of the 44 patients had equivocal [18F]FDG PET/CT findings, while the diagnosis was clear on [68Ga]Ga-FAPI PET/CT. [68Ga]Ga-FAPI PET/CT also revealed primary lesions in additional 17 patients with negative [18F]FDG PET/CT findings. In fourteen of 44 patients, no primary lesion was detected by either tracer. On this basis, we analyzed 94 lymph node metastatic lesions. The mean SUVmax of lymph node metastases on [68Ga] Ga-FAPI PET/CT and [18F]FDG PET/CT were 9.2 ± 5.1, 7.9 ± 4.8 (p = 0.03) and the mean TBR were 9.1 ± 5.2, 4.9 ± 3.1 (p < 0.01), respectively.

CONCLUSION

[68Ga]Ga-FAPI PET/CT showed great potential for identifying unknown primary lesions and has the potential to improve the detection rate of unknown primary lesions with negative or equivocal for [18F]FDG findings.

TRIAL REGISTRATION

ClinicalTrial.gov. Identifier: ChiCTR2100044131.

Role of 68Ga-FAPI-04 PET/CT in the Initial Staging of Esophageal Cancer

AIM

To investigate the clinical value of 68Ga-FAPI-04 PET/CT for initial staging of esophageal cancer.

METHODS

A total of 44 newly diagnosed patients with esophageal cancer were included in the analysis on the basis of postoperative pathology or clinical and radiologic follow-up.

RESULTS

All primary lesions showed increased 68Ga-FAPI-04 uptake, with an SUVmax of 14.92 ± 6.91. A total of 561 lymph nodes were verified by surgery (507) or clinical and radiologic follow-up (54), of which 92 lymph nodes were diagnosed as showing metastases. Seventy-five lymph nodes with metastases showed positive findings on 68Ga-FAPI-04, with a diameter of about 1.06 ± 0.53 cm and SUVmax of 8.10±4.71. The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value for lymph node metastasis detection were 81.5%, 99.3%, 96.6%, 96.2%, and 96.5%, respectively.

CONCLUSION

68Ga-FAPI-04 PET/CT showed good diagnostic performance in detecting lymph node metastases of esophageal cancer.

FAPI PET/CT in Diagnostic and Treatment Management of Colorectal Cancer: Review of Current Research Status

FAPI PET/CT is a novel imaging tool targeting fibroblast activation protein (FAP), with high tumor uptake rate and low background noise. Therefore, the appearance of FAPI PET/CT provides a good tumor-to-background ratio between tumor and non-tumor tissues, which is beneficial to staging, tumor description and detection. Colorectal cancer has the biological characteristics of high expression of FAP, which provides the foundation for targeted FAP imaging. FAPI PET/CT may have a potential role in changing the staging and re-staging of colorectal cancer, monitoring recurrence and treatment management, and improving the prognosis of patients. This review will summarize the application status of FAPI PET/CT in colorectal cancer and provide directions for further application research.

Prognostic value of fibroblast activation protein expressing tumor volume calculated from [68 Ga]Ga-FAPI PET/CT in patients with esophageal squamous cell carcinoma

BACKGROUND

This study aimed to investigate the prognostic value of semiquantitative parameters derived from [68 Ga]Ga-fibroblast activation protein inhibitor (FAPI) PET/CT for patients with esophageal squamous cell carcinoma (ESCC) treated with definitive chemoradiotherapy.

METHODS

We conducted a retrospective analysis on patients from a prospective parent study (NCT04416165). A total of 45 patients with locally advanced ESCC who underwent [68 Ga]Ga-FAPI from December 2019 to March 2021 were included. The maximum standard uptake value (SUVmax), gross tumor volume (GTV), and total lesion-FAPI (TL-FAPI) of the primary tumor were calculated from the corresponding PET/CT image. Unpaired parameters were compared using Student's t test or the Mann-Whitney U test. Paired parameters were compared using the paired t test or the Wilcoxon matched-pairs signed-rank test. Kaplan-Meier curves were generated to calculate progression-free survival (PFS) and overall survival (OS) rates, and Cox regression analysis was performed to determine which PET/CT parameters were prognostic factors for PFS and/or OS.

RESULTS

Thirty-four of the 45 patients met the criteria, and the median follow-up time was 24 months (16-29 months). SUVmax-FAPI, GTVFAPI, and TL-FAPI in patients with stage T4 tumors were significantly higher than those in patients with stage T2/T3 tumors (all P < 0.01). In the univariate Cox regression analysis, T stage, N stage, GTVFAPI, and TL-FAPI were associated with PFS, and T stage, GTVFAPI, and TL-FAPI were associated with OS. Upon multivariable analysis, GTVFAPI was an independent prognostic factor for both PFS (hazard ratio (HR), 5.76; 95% confidence interval (CI), 2.13-15.57, P = 0.001) and OS (HR, 4.96; 95% CI, 2.55-18.79, P = 0.001).

CONCLUSION

This pilot study revealed that [68 Ga]Ga-FAPI PET/CT may have prognostic value for patients with ESCC treated with definitive chemoradiotherapy. It may aid in personalized patient management by steering treatment modifications before therapy. Prospective studies with larger samples and longer observation periods are needed.

Case Report: 68Ga-FAPI PET/CT, a more advantageous detection mean of gastric, peritoneal, and ovarian metastases from breast cancer

Breast cancer is the most common malignant tumor in adult women. Its common metastatic sites are lymph nodes, bones, lungs, the liver, and the brain. It is so rare for a patient with breast cancer to have metastases of the gastrointestinal tract, peritoneum, and ovary at the same time that the clinical reporting rate is low. We present a case of a 61-year-old woman who underwent right mastectomy and chemoradiotherapy 3 years ago because of mixed invasive ductal-lobular breast cancer. This time, she came to the hospital due to the symptom of stomach discomfort for 2 weeks. The gastroscopy biopsy result showed gastric metastasis from breast cancer. Then, ¹⁸F-FDG imaging and ⁶⁸Ga-FAPI PET/CT imaging were performed for further diagnosis; ⁶⁸Ga-FAPI PET/CT demonstrated a significantly elevated FAPI activity in the thickened gastric wall, peritoneum, and bilateral adnexal areas, which was superior to that of ¹⁸F-FDG. Finally, a biopsy of suspicious lesions was taken for pathological and histochemical examination, which confirmed that, in addition to the gastric metastasis, the peritoneum and bilateral ovaries were all consistent with metastatic breast cancer.

Automated Radiosynthesis, Preliminary In Vitro/In Vivo Characterization of OncoFAP-Based Radiopharmaceuticals for Cancer Imaging and Therapy

FAP-targeted radiopharmaceuticals represent a breakthrough in cancer imaging and a viable option for therapeutic applications. OncoFAP is an ultra-high-affinity ligand of FAP with a dissociation constant of 680 pM. OncoFAP has been recently discovered and clinically validated for PET imaging procedures in patients with solid malignancies. While more and more clinical validation is becoming available, the need for scalable and robust procedures for the preparation of this new class of radiopharmaceuticals continues to increase. In this article, we present the development of automated radiolabeling procedures for the preparation of OncoFAP-based radiopharmaceuticals for cancer imaging and therapy. A new series of [⁶⁸Ga]Ga-OncoFAP, [¹⁷⁷Lu]Lu-OncoFAP and [¹⁸F]AlF-OncoFAP was produced with high radiochemical yields. Chemical and biochemical characterization after radiolabeling confirmed its excellent stability, retention of high affinity for FAP and absence of radiolysis by-products. The in vivo biodistribution of [¹⁸F]AlF-NOTA-OncoFAP, a candidate for PET imaging procedures in patients, was assessed in mice bearing FAP-positive solid tumors. The product showed rapid accumulation in solid tumors, with an average of 6.6% ID/g one hour after systemic administration and excellent tumor-to-healthy organs ratio. We have developed simple, quick, safe and robust synthetic procedures for the preparation of theranostic OncoFAP-compounds based on Gallium-68, Lutetium-177 and Fluorine-18 using the commercially available FASTlab synthesis module.

Comparison of 68Ga-FAPI and 18F-FDG PET/CT in the Evaluation of Patients With Newly Diagnosed Non-Small Cell Lung Cancer

Purpose

Several studies have demonstrated that ⁶⁸Ga-FAPI PET/CT shows high intratumoral tracer uptake and low normal tissue uptake, allowing for excellent visualization of cancer. The purpose of this study was to compare the ability of ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT for the evaluation of newly diagnosed NSCLC.

Materials and Methods

A prospective analysis of 28 individuals with histopathologically newly confirmed NSCLC that underwent ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT was conducted. The performance of two imaging modalities was compared based upon visual assessment, rates of cancer detection, and semi-quantitative parameters (target-to-background ratio [TBR], maximum standard uptake value [SUVmax]) for both primary tumors and metastases.

Results

In total, this study enrolled 28 participants (13 male, 15 female; median age: 60.5 years, range: 34 – 78 years. <u>For primary tumors, ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT have similar detection performance (28 vs. 27). However, ⁶⁸Ga-FAPI PET/CT was found to more effectively evaluate most metastases as compared to ¹⁸F-FDG PET/CT. ⁶⁸Ga-FAPI PET/CT detecting more metastases present within the lymph nodes (53 vs. 49), pleura (8 vs. 7), liver (4 vs. 1), and bone (41 vs. 35).</u> The SUVmax and TBR values for ⁶⁸Ga-FAPI were substantially superior to those for ¹⁸F-FDG in lymph node, pleural, and bone metastases. While the SUVmax for these two imaging approaches was comparable for hepatic metastases, ⁶⁸Ga-FAPI exhibited a significantly higher TBR in relation to that of ¹⁸F-FDG. In addition, ⁶⁸Ga-FAPI PET/CT demonstrates excellent N (80% [8/10]) and M (92.9% [26/28]) staging accuracy in NSCLC patients.

Conclusions

⁶⁸Ga-FAPI PET/CT as an examination modality is excellent for evaluation of newly diagnosed NSCLC. ⁶⁸Ga-FAPI PET/CT improves the detection rates of most metastases and facilitating the superior staging of patients with newly diagnosed NSCLC, relative to that achieved by ¹⁸F-FDG PET/CT.

Comparison of [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG Uptake in Esophageal Cancer

Purpose

Accurate clinical staging is crucial to managing esophageal cancer. [⁶⁸Ga]Ga-DOTA-FAPI-04 exhibits good diagnostic performance in various tumors, showing a promising alternative to [¹⁸F]FDG. Here, we investigated the diagnostic performance of [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT and [¹⁸F]FDG PET/CT in the diagnosis of primary and metastatic lesions of esophageal cancer.

Methods

Patients with esophageal cancer who underwent concurrent [⁶⁸Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT between January 2020 and June 2021 were retrospectively analyzed. [⁶⁸Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT uptakes were compared by using the paired samples t test. The McNemar test was used to compare the diagnostic performance between the two techniques.

Results

Thirty-five patients (ranging from 44-83 years old with a median age of 63.5 years) were evaluated in our study. In treatment-naive patients (n=25), [⁶⁸Ga]Ga-DOTA-FAPI-04 PET could detect all esophageal cancers, whereas 1 patient with superficial esophageal cancer was negative in FDG but positive in [⁶⁸Ga]Ga-DOTA-FAPI-04 (T1). [⁶⁸Ga]Ga-DOTA-FAPI-04 uptake was higher than [¹⁸F]FDG in primary lesions (13.8 ± 6.9 vs 10.9 ± 6.8, respectively, P=0.004), involved lymph nodes (9.3 ± 5.2 vs 6.4 ± 5.9, respectively, P=0.002), and bone and visceral metastases (10.4 ± 6.0 vs 6.1 ± 7.5, respectively, P=0.001). In addition, [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT has a higher detection sensitivity than [¹⁸F]FDG PET/CT for primary tumors [100% (25/25) vs. 96.0% (24/25), respectively], lymph nodes [95.0% (57/60) vs 75.0% (45/60), P<0.001], and bone and visceral metastases [100% (25/25) vs 72% (18/25), respectively; P= 0.008].

Conclusion

[⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT has higher tracer uptake value and is superior to [¹⁸F]FDG PET/CT in detecting primary and metastatic lesions in patients with esophageal cancer.

68Ga-FAPI PET/CT Distinguishes the Reactive Lymph Nodes From Tumor Metastatic Lymph Nodes in a Patient With Nasopharyngeal Carcinoma

ABSTRACT

Accurate clinical staging is crucial for managing nasopharyngeal carcinoma (NPC). However, false-positive finding of cervical lymph nodes in 18F-FDG PET/CT and MRI may lead to unnecessary chemotherapy for NPC. In this case, we report a patient with NPC who was incorrectly diagnosed with bilateral cervical lymph node metastases by both 18F-FDG PET/CT and MRI. Subsequent pathology results showed proliferating lymphoid cells with no signs of tumor metastasis. Interestingly, no abnormal 68Ga-FAPI uptake was observed in the nonmetastatic reactive lymph nodes. Therefore, 68Ga-FAPI PET/CT may be useful for assessment of pretreatment lymph node status in NPC.

Increased 68Ga-FAPI Uptake in the Pulmonary Cryptococcus and the Postradiotherapy Inflammation

ABSTRACT

A 51-year-old man, previously treated for nasopharyngeal carcinoma, presented a newly detected pulmonary nodule. This patient underwent 18F-FDG and 68Ga-FAPI PET/CT to detect tumor recurrence under the prospective study NCT04416165. Both 18F-FDG and 68Ga-FAPI PET/CT showed intense uptake in a pulmonary nodule in the right lower lobe. In addition, 68Ga-FAPI PET/CT demonstrated increased uptake in the nasopharynx that previously treated with radiotherapy. The pulmonary nodule was found to be cryptococcus by the CT-guided biopsy, and the increased 68Ga-FAPI uptake in the nasopharynx was interpreted as false-positive finding due to a stable Epstein-Barr virus level and no progression on follow-up imaging.

FAPI PET/CT research progress in digestive system tumours

¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography has been used in clinical practice for many years. This modality is of great value for tumour diagnosis, staging, and efficacy evaluations, but it has many limitations in the diagnosis and treatment of digestive system tumours. Fibroblast activation protein is highly expressed in gastrointestinal tumours. Various isotope-labelled fibroblast activation protein inhibitors are widely used in clinical research. These inhibitors have low background uptake in the brain, liver and oral/pharyngeal mucosa and show good contrast between the tumour and background, which makes up for the lack of fluorodeoxyglucose in the diagnosis of digestive system tumours. It better displays the primary tumours, metastases and regional lymph nodes of digestive system tumours, such as oesophageal cancer, gastric cancer and liver cancer, and also provides a new method for treating these tumours. Based on this background, this article introduces the current research status of fibroblast activation protein inhibitor positron emission tomography/computed tomography in various types of digestive system malignant tumours to provide more valuable information for diagnosing and treating digestive system tumours.

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.

Comparison of 18F-FDG PET/CT and 68Ga-FAPI-04 PET/CT in the staging and restaging of gastric adenocarcinoma

OBJECTIVE

In this study, we aimed to evaluate the diagnostic sensitivities of 68Ga-FAPI-04 PET/CT and 18F-FDG PET/CT in the primary tumor, and nodal, peritoneal and distant organ metastases of primary and recurrent gastric adenocarcinoma (GAc) with patient and lesion-based comparison.

MATERIALS AND METHOD

Twenty-one patients with histopathologically proven newly diagnosed or recurrent GAc who underwent 18F-FDG and 68Ga-FAPI-04 imaging were included in the study. Both imaging techniques were evaluated visually according to the intensity of organ-based uptake. SUVmax and tumor-to-background ratio (TBR) values obtained from primary tumor/relapse and metastatic organs were compared statistically.

RESULTS

68Ga-FAPI-04 uptake was positive in all 15 newly diagnosed patients, while two patients among them who had mucinous and signet ring cell carcinoma did not exhibit 18F-FDG uptake. The sensitivity and specificity of 68Ga-FAPI-04 PET/CT in detecting primary gastric were 100%, while the sensitivity and specificity of 18F-FDG were 86.6 and 100%, respectively. 68Ga-FAPI-04 imaging revealed diffuse stomach uptake in seven patients, while 18F-FDG could only show two of them. The sensitivity and specificity of in-patient-based detection of lymph node metastases were 100 and 95.2%, respectively, while these values were 71.4 and 93.7%, respectively, for 18F-FDG. For peritoneal involvement 68Ga-FAPI-04 had a sensitivity and specificity of 100%, whereas 18F-FDG had a sensitivity of 40% and a specificity of 100%.

CONCLUSION

68Ga-FAPI-04 PET/CT is an imaging modality with the potential of yielding more sensitive and specific findings 18F-FDG PET/CT. This modality may help avoid invasive diagnostic procedures that may be frequently required in GAc.

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.

Head-to-head comparison of [68Ga]Ga-FAPI-04 and [18F]-FDG PET/CT in evaluating the extent of disease in gastric adenocarcinoma

BACKGROUND

[¹⁸F]-Fluorodeoxyglucose (FDG) positron emission tomography-computed tomography (PET/CT) may sometimes be suboptimal for imaging gastric adenocarcinoma. The recently introduced [⁶⁸Ga]Ga-FAPI-04 (FAPI) PET/CT targets tumor stroma and has shown considerable potential in evaluating the extent of disease in a variety of tumors.

METHODS

We performed a head-to-head prospective comparison of FAPI and FDG PET/CT in the same group of 13 patients with gastric adenocarcinoma who presented for either initial staging (n = 10) or restaging (n = 3) of disease. Lesion detection and maximum standardized uptake value (SUV_max) were compared between the two types of radiotracers.

RESULTS

All ten primary gastric tumors were FAPI-positive (100% detection rate), whereas only five were also FDG-positive (50%). SUV_max was not significantly different, but the tumor-to-background ratio was higher for FAPI (mean, median, and range of 4.5, 3.2, and 0.8-9.7 for FDG and 12.9, 11.9, and 2.2-23.9 for FAPI, P = 0.007). The level of detection of regional lymph node involvement was comparable. FAPI showed a superior detection rate for peritoneal carcinomatosis (100% vs. none). Two patients with widespread peritoneal carcinomatosis underwent a follow-up FAPI scan after chemotherapy: one showed partial remission and the other showed progressive disease.

CONCLUSIONS

The findings of this pilot study suggest that FAPI PET/CT outperforms FDG PET/CT in detecting both primary gastric adenocarcinoma and peritoneal carcinomatosis from gastric cancer. FAPI PET/CT also shows promise for monitoring response to treatment in patients with peritoneal carcinomatosis from gastric cancer; however, larger trials are needed to validate these preliminary findings.

The Role of Fibroblast Activation Protein Ligands in Oncologic PET Imaging

Fibroblast activation protein inhibitor emerges as a novel and highly promising agent for diagnostic and possibly theranostic application in various malignant and non-malignant diseases. FAPI impresses with its selective expression in several pathologies, ligand induced internalization, and presence in a large variety of malignancies. Current studies indicate that FAPI is equal or even superior to the current standard oncological tracer fluorodeoxyglucose in several oncological diseases. It seems to present lower background activity, stronger uptake in tumorous lesions and thus sharper contrasts. For improved comprehension of fibroblast activation, protein expression and clinicopathologic conditions, further studies are of essence.

68Ga-DOTA-FAPI-04 PET/CT in Erdheim-Chester Disease

ABSTRACT

A 56-year-old woman presented with edema in the upper part of the body, enlarged axillary lymph nodes, and progressive bone pain for 1 year. Malignant disease was initially suspected. This patient underwent 68Ga-DOTA-FAPI-04 PET/CT for detection of the primary lesion and evaluation of disease extent. 68Ga-DOTA-FAPI-04 PET/CT revealed multiple abnormal foci at the skeleton, muscle, lymph nodes, and subcutaneous fat. The imaging findings, highly suggestive of Erdheim-Chester disease, have been later confirmed by histopathology. This case highlights that 68Ga-DOTA-FAPI-04 PET/CT would be useful for evaluating the disease extent, guiding biopsies, and gauging the treatment response in patients with Erdheim-Chester disease.

68Ga-fibroblast activation protein inhibitor PET/CT on gross tumour volume delineation for radiotherapy planning of oesophageal cancer

BACKGROUND AND PURPOSE

To compare ⁶⁸Ga-fibroblast activation protein inhibitor (FAPI) and ¹⁸F-FDG PET/CT in imaging locally advanced oesophageal cancer, and evaluate the potential usefulness of ⁶⁸Ga-FAPI PET/CT on gross target volume (GTV) delineation aimed at radiotherapy planning for oesophageal cancer as compared with contrast-enhanced CT (CE-CT) and ¹⁸F-FDG PET/CT.

MATERIALS AND METHODS

Twenty-one patients with newly diagnosed oesophageal cancer who underwent both ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT scans were selected. GTVs of the primary tumours based on CE-CT (GTV_CT), PET/CT, and CE-CT plus PET/CT were delineated. Gross tumour lengths were measured by GTVs and endoscopy and recorded.

RESULTS

The ⁶⁸Ga-FAPI PET showed significantly higher radiotracer uptake than ¹⁸F-FDG PET (median SUVmax 16.71 vs. 11.23; P = 0.002) in the primary tumours. SUV thresholds of FAPI ×20%, 30%, 40%, and FDG ×40% showed similar lesion lengths compared with that in endoscopic examination (P > 0.05). GTV_CT demonstrated the largest volume (median: 48.80 mm³, range: 14.83-162.23 mm³) than PET-based GTVs. For PET/CT-guided complementary contouring of GTV_CT, four patients (19%) were increased by FAPI ×20% and 30%, two patients (9.5%) were increased by FAPI ×40%, and only one patient was increased by FDG ×40%. Furthermore, the volume of GTV based on CE-CT plus FAPI ×20%, 30%, and 40% showed no significant difference with GTV_CT and planning target volume based CE-CT plus FAPI-PET and meets the organ at risk standard.

CONCLUSION

The ⁶⁸Ga-FAPI PET/CT methodology showed favourable tumour-to-background contrast in oesophageal cancer and might provide additional information for target volume delineation and help avoid tumour geographic misses.

Comparison of 68Ga-FAPI and 18F-FDG Uptake in Gastric, Duodenal, and Colorectal Cancers

Background Accurate clinical staging is crucial to managing gastrointestinal cancer, but fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG) PET/CT has limitations. Targeting fibroblast-activation protein is a newer diagnostic approach for the visualization of tumor stroma, and gallium 68 (⁶⁸Ga)-labeled fibroblast-activation protein inhibitors (FAPIs), hereafter ⁶⁸Ga-FAPIs, present a promising alternative to ¹⁸F-FDG. Purpose To compare the diagnostic efficacy of ⁶⁸Ga-FAPI PET/CT in primary and metastatic lesions of gastrointestinal malignancies with that of ¹⁸F-FDG PET/CT. Materials and Methods Images from patients with gastric, duodenal, and colorectal cancers who underwent contemporaneous ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT between October 2019 through June 2020 were retrospectively analyzed. ¹⁸F-FDG and ⁶⁸Ga-FAPI uptakes were compared by using the Wilcoxon signed-rank test. The McNemar test was used to compare the diagnostic performance between the two techniques. Results Thirty-five patients (median age, 64 years [interquartile range, 53-68 years]; 18 men) were evaluated. In treatment-naive patients (n = 19), ⁶⁸Ga-FAPI PET/CT led to upstaging of the clinical TNM stage in four (21%) patients compared with ¹⁸F-FDG PET/CT. Tracer uptake was higher with ⁶⁸Ga-FAPI PET/CT than with ¹⁸F-FDG PET/CT in primary lesions (gastric cancer: 12.7 vs 3.7, respectively, P = .003; colorectal cancer: 15.9 vs 7.9, P = .03), involved lymph nodes (6.7 vs 2.4, P < .001), and bone and visceral metastases (liver metastases: 9.7 vs 5.2, P < .001; peritoneal metastases: 8.4 vs 3.6, P < .001; bone metastases: 4.3 vs 2.2, P < .001; lung metastases: 4.4 vs 1.9, P = .01). In addition, the sensitivity of ⁶⁸Ga-FAPI PET/CT was higher than that of ¹⁸F-FDG PET/CT in the detection of primary tumors (100% [19 of 19] vs 53% [10 of 19], respectively; P = .004), lymph nodes (79% [22 of 28] vs 54% [15 of 28], P < .001), and bone and visceral metastases (89% [31 of 35] vs 57% [20 of 35], P < .001). Conclusion Gallium 68 fibroblast-activation protein inhibitor PET/CT was superior to fluorine 18 fluorodeoxyglucose PET/CT in the detection of primary and metastatic lesions in gastric, duodenal, and colorectal cancers, with higher tracer uptake in most primary and metastatic lesions. Published under a CC BY 4.0 license.

Usefulness of [68Ga]Ga-DOTA-FAPI-04 PET/CT in patients presenting with inconclusive [18F]FDG PET/CT findings

PURPOSE

This prospective study aimed to evaluate the potential usefulness of [⁶⁸Ga]Ga-DOTA-FAPI-04 positron emission tomography/computed tomography (PET/CT) in the oncological evaluation of patients presenting with inconclusive [¹⁸F]FDG PET/CT findings.

METHODS

[⁶⁸Ga]Ga-DOTA-FAPI-04 was performed in patients presenting with inconclusive [¹⁸F]FDG PET/CT findings. Tumour uptake was quantified by the maximum standard uptake value (SUV). Histopathology or follow-up imaging served as the standard for the final diagnosis.

RESULTS

A total of 68 patients with inconclusive [¹⁸F]FDG PET/CT findings underwent additional [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT. Of them, 18 (26.5%) were for discrimination of mass lesions detected on conventional imaging, 6 (8.8%) for detection of the unknown primary site in biopsy-proven metastatic malignancy, 21 (30.9%) for the staging of cancer, and the other 23 (33.8%) for evaluation of suspected disease recurrence. Most of the primary and metastatic lesions demonstrated higher uptake of [⁶⁸Ga]Ga-DOTA-FAPI-04 than did [¹⁸F]FDG, which resulted in favourable tumour-to-background contrast in various types of cancer. As a result, [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT identified suspicious mass lesions with an accuracy of 12/18 (66.7%), detected the primary site in 4/6 patients (66.7%) with unknown malignancy, upgraded tumour staging in 7/21 patients (33.3%), and detected disease recurrence in 20/23 patients (87.0%).

CONCLUSIONS

In patients undergoing oncological evaluation with inconclusive [¹⁸F]FDG PET/CT findings, [⁶⁸Ga]Ga-DOTA-FAPI-04 may have a complementary role in discriminating mass lesions on conventional imaging, locating the primary site of unknown malignancy, modifying tumour staging, and detecting suspected disease recurrence. Nevertheless, careful attention should be paid when reading the [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT images in tumours complicated with inflammation.