[68Ga]Ga-DOTA-FAPI-04 and [18F] FDG PET/CT for the diagnosis of primary and metastatic lesions in patients with hepatic cancer

Physiological Uptake of 68Ga-FAPI-04 in Female Reproductive System

PURPOSE

Since the avid uptake of 68Ga-labeled fibroblast activation protein inhibitor (68Ga-FAPI-04) observed in female reproductive organs, our objectives were to investigate the physiological uptake characteristics and provide preliminary reference ranges for clinical use.

PROCEDURES

We reviewed the findings of female patients who underwent 68Ga-FAPI-04 PET/CT at our institution between April 2022 and June 2023. The standard uptake value (SUV) of reproductive organs and menstrual information were collected. All patients were categorized into reproductive period group, perimenopause group, and postmenopause group. We analysed the uptake levels among the three groups, and their association with age and menstrual cycle.

RESULTS

A total of 109 patients were included in this study. Higher ovarian SUVs were detected in reproductive patients (SUVright: 2.75 ± 0.84, IQR: 1.39-5.26; SUVleft: 2.72, IQR: 2.34-3.20; p = 0.315) than in postmenopausal patients (SUVright: 2.27, IQR: 2.01-2.75; SUVleft: 2.38 ± 0.55, IQR: 1.35-3.60; p = 0.767), as well as uterine 68Ga-FAPI-04 accumulations. The SUVs of uterine fundus and corpus were approximately three times higher than that of the cervix. In reproductive period group, higher SUVs were observed in bilateral ovaries around the ovulatory phase to the early luteal phase, and higher uterine SUVs were noted in the menstrual and proliferative phases. The SUVs in all reproductive organs (except the ovaries) showed significant negative correlations with age in all patients (all p < 0.01).

CONCLUSIONS

68Ga-FAPI-04 SUVs in reproductive organs are higher in premenopausal patients than in postmenopausal patients. The 68Ga-FAPI-04 accumulation in reproductive organs might be associated with menstrual cycle. Including more patients from different menstrual phases could contribute to investigating the uptake characteristics and their underlying mechanisms.

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.

Diagnostic and Therapeutic Application of Fibroblast Activation Protein Inhibitors in Oncologic and Nononcologic Diseases

ABSTRACT

Fibroblast activation protein inhibitor positron emission tomography (PET) has gained interest for its ability to demonstrate uptake in a diverse range of tumors. Its molecular target, fibroblast activation protein, is expressed in cancer-associated fibroblasts, a major cell type in tumor microenvironment that surrounds various types of cancers. Although existing literature on FAPI PET is largely from single-center studies and case reports, initial findings show promise for some cancer types demonstrating improved imaging when compared with the widely used 18F-fludeoxyglucose PET for oncologic imaging. As we expand our knowledge of the utility of FAPI PET, accurate understanding of noncancerous uptake seen on FAPI PET is crucial for accurate evaluation. In this review, we summarize potential diagnostic and therapeutic applications of radiolabeled FAP inhibitors in oncological and nononcological disease processes.

Increased 68 Ga-FAPI Activity in Hepatic Inflammatory Myofibroblastoma

ABSTRACT

Hepatic inflammatory myofibroblastoma is an uncommon borderline tumor. We present 68 Ga-FAPI PET/CT findings of hepatic inflammatory myofibroblastoma in a 58-year-old man. The rare hepatic inflammatory myofibroblastoma demonstrated intense tracer uptake on 68 Ga-FAPI PET/CT. This case demonstrates the potential value of 68 Ga-FAPI PET/CT for the evaluation of hepatic inflammatory myofibroblastoma.

Diagnosis of Seronegative Rheumatoid Arthritis by 68 Ga-FAPI PET/CT

Early diagnosis of rheumatoid arthritis with the initiation of disease-modifying antirheumatic drugs is important to prevent future disability. Seronegative rheumatoid arthritis lacks the classical immunological markers, thus imposing clinical diagnostic difficulty. In this case, we reported 68 Ga-FAPI PET/CT findings of seronegative rheumatoid arthritis in a 60-year-old lady. This case illustrates how 68 Ga-FAPI PET/CT aids in the diagnosis of seronegative rheumatoid arthritis.

Non-oncologic incidental uptake on FAPI PET/CT imaging

Fibroblast-activation protein (FAP) is a serine protease classified in the dipeptidyl peptidase 4 (DPP4) family. FAP is predominantly expressed in activated fibroblasts such as the cancer-associated fibroblasts (CAFs). FAP expression in CAFs is associated with tumor progression and poor prognosis in solid cancers. Recently, radiolabeled FAP inhibitors (FAPI) has been developed, which enables positron emission tomography (PET) imaging of FAP. FAPI PET/CT can provide a higher tumor-to-background ratio (TBR) than 18F-fludeoxyglucose PET/CT in various cancers, and thus has attracted substantial attention. As studies on FAPI PET grow in number and size, incidental findings related to non-oncologic conditions have been increasingly reported. FAPI PET uptake has been reported in various conditions such as benign tumors, fibrotic, granulomatosis, scarring/wound, degenerative diseases, and inflammatory diseases.The knowledge of physiological and non-oncologic causes of FAPI uptake is indispensable for accurate FAPI PET/CT interpretation and can help appropriate management of incidental findings on FAPI PET/CT in patients referred for cancer staging indications. In this review article, we describe for each organ system (Brain, Oral mucosa, Salivary Glands, Thyroid, Lung, Myocardium, Breast, Esophagus, Stomach, Intestine, Liver, Gallbladder, Pancreas, Spleen, Kidney, , Uterus, Bone marrow, Joints, Muscle, Vessels, Lymph nodes), the patterns of physiological FAPI uptake and the main causes of non-oncological uptake reported from the literature with FAPI-02, FAPI-04 and FAPI-46. We also illustrate some examples from our institutional database at UCLA.

18 F-FDG and 68 Ga-FAPI-04 in the Evaluation of Aggressive Perivascular Epithelioid Cell Tumor

ABSTRACT

A 10-year-old girl with a right renal mass underwent 18 F-FDG PET/CT and 68 Ga-FAPI-04 PET/MRI for presurgical assessment of tumor invasion and malignant potential. The mass showed low 18 F-FDG uptake and intense 68 Ga-FAPI-04 uptake. Nephrectomy was performed, and the histopathologic diagnosis was aggressive PEComa (perivascular epithelioid cell tumor). This case showed that 68 Ga-FAPI-04 PET outperformed 18 F-FDG PET in detecting aggressive PEComa.

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.

Uterine Uptake of 68Ga-FAPI-04 in Uterine Pathology and Physiology

PURPOSE

68Ga-labeled fibroblast activation protein inhibitor (68Ga-FAPI-04) has been useful in the imaging of desmoplastic reaction in different tumors. As we have found that most female patients showed avid uterine uptake of 68Ga-FAPI-04, we sought to further investigate the pathological and physiological uptake of 68Ga-FAPI-04 characteristics in the uterus.

PATIENTS AND METHODS

We reviewed the image data of female patients who had undergone 68Ga-FAPI-04 PET/MRI at our institute between May 22, 2020, and June 21, 2021. The characteristics of uterine uptake and clinical information were collected. The uterus with and without malignancy were compared. We further analyzed the relationship of age, uterus size, gynecological history, and 18F-FDG uptake (if performed) with 68Ga-FAPI-04 uptake.

RESULTS

Seventy-seven patients were included in this study. Much higher cervical 68Ga-FAPI-04 accumulation was noticed in cervical cancer patients than in normal cases, and 37 more metastases were found in 68Ga-FAPI-04 PET than that in 18F-FDG. Uterine body malignancies displayed different uptake features. Two cases with the metastases to uterine body showed relative lower 68Ga-FAPI-04 activity compared with their normal uteri. Of 67 patients without malignancy, lower 68Ga-FAPI-04 uptake was noted in postmenopausal women than in reproductive and perimenopausal patients. The invasive operation or hysteromyoma may increase 68Ga-FAPI-04 uptake.

CONCLUSIONS

68Ga-FAPI-04 PET might be a promising method in cervical cancers. However, physiological uptake may limit its diagnostic value in uterine body malignancy. It should be noticed that the metastatic lesion in the uterus may show relative lower uptake of 68Ga-FAPI-04 compared with the rest of the uterus. Age, fibroids, and uterine volume may influence 68Ga-FAPI-04 uptake in the uterus. More patients with various uterine diseases could be involved to provide more differential diagnostic information.

68Ga-DOTA-FAPI-04 PET/MR in the Evaluation of Gastric Carcinomas: Comparison with 18F-FDG PET/CT

We sought to evaluate the performance of 68Ga-DOTA-FAPI-04 ( 68 Ga-FAPI) PET/MR for the diagnosis of primary tumor and metastatic lesions in patients with gastric carcinomas and to compare the results with those of 18F-FDG PET/CT. Methods: Twenty patients with histologically proven gastric carcinomas were recruited, and each patient underwent both 18F-FDG PET/CT and 68 Ga-FAPI PET/MR. A visual scoring system was established to compare the detectability of primary tumors and metastases in different organs or regions (the peritoneum, abdominal lymph nodes, supradiaphragmatic lymph nodes, liver, ovary, bone, and other tissues). The original SUVmax and normalized SUVmax (calculated by dividing a lesion's original SUVmax with the SUVmean of the descending aorta) of selected lesions on both 18F-FDG PET/CT and 68Ga-FAPI PET/MR were measured. Original/normalized SUVmax-FAPI and SUVmax-FDG were compared for patient-based (including a single lesion with the highest activity uptake in each organ/region) and lesion-based (including all lesions [≤5] or the 5 lesions with highest activity [>5]) analyses, respectively. Results: The 20 recruited patients (median age: 56.0 y; range: 29-70 y) included 9 men and 11 women, 14 patients for initial staging and 6 for recurrence detection. 68Ga-FAPI PET was superior to 18F-FDG PET for primary tumor detection (100.00% [14/14] vs. 71.43% [10/14]; P = 0.034), and the former had higher tracer uptake levels (P < 0.05). 68Ga-FAPI PET was superior to 18F-FDG PET in both patient-based and lesion-based evaluation except for the metastatic lesions in supradiaphragmatic lymph nodes and ovaries. Additionally, multiple sequences of MR images were beneficial for the interpretation of hepatic metastases in 3 patients, uterine and rectal metastases in 1 patient, ovarian lesions in 7 patients, and osseous metastases in 2 patients. Conclusion:68Ga-FAPI PET/MR outperformed 18F-FDG PET/CT in visualizing the primary and most metastatic lesions of gastric cancer and might be a promising method, with the potential of replacing 18F-FDG PET/CT.

68Ga-FAPI-04 vs. 18F-FDG in a longitudinal preclinical PET imaging of metastatic breast cancer

PURPOSE

This longitudinal study aims to evaluate the performance of 68 Ga-FAPI-04 and 18F-FDG and to profile the dynamic process of tumor metastasis in a preclinical 4T1 breast cancer model. Although both of these two radioligands are wildly used in clinic, no study was reported on their performance in the longitudinal monitoring of tumor metastasis. Also, no correlation between the expression level of fibroblast activation protein (FAP) and the development of tumor metastasis has been elucidated previously. In this study, we evaluated the performance of 68 Ga-FAPI-04 and 18F-FDG PET during the entire process of tumor metastasis, and their potential for the early diagnosis of tumor metastasis. We also clarified the correlation of uptakes as well as the signal-to-background (S/B) ratios between these two probes at different stages of tumor metastasis.

METHODS

Forty 4T1 metastatic breast cancer murine models were established using female BALB/c mice, followed by the longitudinal imaging with 68 Ga-FAPI-04 and 18F-FDG once a week for up to 6 weeks. In vitro hematoxylin and eosin (H&E) and immunochemistry (IHE) staining were performed to evaluate FAP expression on the metastatic lesions. Further statistical analysis was performed to evaluate the correlation of 68 Ga-FAPI-04 and 18F-FDG uptake (%ID/cc) at different stages of the metastasis.

RESULTS

68 Ga-FPAI-04 holds an advantage over 18F-FDG with higher sensitivity at the early stage of tumor metastasis. However, with the progress of tumor metastasis, uptake of 68 Ga-FAPI-04 decreases and becomes less sensitive than 18F-FDG. There is also no direct correlation between uptake or S/B ratios of 68 Ga-FAPI-04 and 18F-FDG during this dynamic process.

CONCLUSION

68 Ga-FAPI-04 is more sensitive than 18F-FDG in detecting the early stage of tumor metastasis, but becomes less sensitive than 18F-FDG at the late stage of tumor metastasis. We envision this result would be meaningful for the explanation of the 68 Ga-FAPI-04 and 18F-FDG imaging both in the future clinic and preclinic studies.

Equivalent tumor detection for early and late FAPI-46 PET acquisition

INTRODUCTION

Positron emission tomography (PET) using small ligands of the fibroblast activation protein (FAP) was recently introduced. However, optimal uptake time has not been defined yet. Here, we systematically compare early (~ 10 min p.i.) and late (~ 60 min p.i.) FAPI-46 imaging in patients with various types of cancer.

METHODS

This is a retrospective single-institutional study. Imaging was performed at the Essen University Hospital, Germany. A total of 69 patients who underwent dual time-point imaging for either restaging (n = 52, 75%) or staging (n = 17, 25%) of cancer were included. Patients underwent PET with two acquisitions: early (mean 11 min, SD 4) and late (mean 66 min, SD 9). Mean injected activity was 148 MBq (SD 33).

RESULTS

In total, 400 lesions were detected in 69 patients. Two of 400 (0.5%) lesions were only seen in early time-point imaging but not in late time-point imaging. On a per-patient level, there was no significant difference between SUV_max of hottest tumor lesions (Wilcoxon: P = 0.73). Organ uptake demonstrated significant early to late decrease in SUVmean (average ∆SUVmean: - 0.48, - 0.14, - 0.27 for gluteus, liver, and mediastinum, respectively; Wilcoxon: P < 0.001). On a per-lesion basis, a slight increase of SUV_max was observed (average ∆SUV_max: + 0.4, Wilcoxon: P = 0.03).

CONCLUSION

In conclusion, early (~ 10 min p.i.) versus late (~ 60 min p.i.) FAPI-46 imaging resulted in equivalent lesion uptake and tumor detection. For improved feasibility and scan volume, we implement early FAPI-46 PET in future clinical and research protocols.

Clinical results of fibroblast activation protein (FAP) specific PET for non-malignant indications: systematic review

PURPOSE

Small molecules targeting fibroblast activation protein (FAP) have emerged as a new group of tracers for positron emission tomography (PET) in 2018. While most of the existing literature has been focussed on the application of FAP-specific PET in various kinds of cancers, some researchers have, both intentionally or unintentionally, used FAP-specific PET in patients with non-cancerous diseases. The purpose of this systematic review is therefore to summarize the available evidence of FAP-specific PET for non-malignant indications.

METHODS

The MEDLINE database was searched for studies presenting the clinical use of FAP-specific PET, the records were screened according to PRISMA guidelines and articles containing patients suffering from non-malignant diseases were included.

RESULTS

Sixteen studies with 303 patients were included. FAP-specific PET has been used in cardiac imaging, IgG₄-related disease, benign tumors as well as various kinds of inflammation. Two prospective studies on FAP-specific PET for IgG₄-related disease show its potential to differentiate inflammatory from fibrotic lesions, which could be used to determine the management of these patients.

CONCLUSION

While publications on FAP-specific PET for non-malignant indications are mostly limited to case reports and incidental findings, the first retrospective and prospective studies present promising results for IgG₄-related as well as cardiovascular disease that warrant further research. Several currently recruiting trials will add to the body evidence in the next few years.

Clinical Results of Fibroblast Activation Protein (FAP) Specific PET and Implications for Radiotherapy Planning: Systematic Review

Small molecules targeting fibroblast activation protein (FAP) have emerged as a new group of tracers for positron emission tomography (PET) in 2018. The purpose of this systematic review is therefore to summarize the evidence that has been gathered to date in patients and to discuss its possible implications for radiotherapy planning. The MEDLINE database was searched for the use of FAP-specific PET in cancer patients and the records were screened according to PRISMA guidelines. Nineteen studies were included. While dedicated analyses of FAP-specific PET for radiotherapy planning were available for glioblastoma, head and neck cancers, lung cancer, and tumors of the lower gastrointestinal tract, there is still very limited data for several epidemiologically significant cancers. In conclusion, FAP-specific PET represents a promising imaging modality for radiotherapy planning that warrants further research.