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

68Ga-FAPI PET/CT Detected Non-FDG-Avid Gastric Stromal Tumor

ABSTRACT

We presented a case of 67-year-old man with epigastric discomfort and anorexia for more than 2 months. No abnormal 18F-FDG uptake was found throughout the gastrointestinal tract on 18F-FDG PET/CT. The patient was enrolled in the 68Ga-FAPI PET/CT clinical trial, and a 68Ga-FAPI PET/CT was performed to help detect primary lesion. This examination demonstrated a nodular thickening of the gastric wall with an increased 68Ga-FAPI uptake in the greater curvature of the stomach. Finally, the pathological result confirmed the diagnosis of gastric stromal tumor. This case highlights that 68Ga-FAPI PET/CT might outperform 18F-FDG PET/CT in helping identify gastrointestinal stromal tumors.

Positron emission tomography and computed tomography with [68Ga]Ga-fibroblast activation protein inhibitors improves tumor detection and staging in patients with pancreatic cancer

PURPOSE

This study aimed to investigate the diagnostic performance of [68Ga]Ga-FAPI PET/CT for primary and metastatic pancreatic carcinoma lesions and compare the results with those of [18F]-fluorodeoxyglucose ([18F]FDG) PET/CT.

METHODS

Patients with suspected or diagnosed pancreatic malignancy, who underwent contemporaneous [18F]FDG and [68Ga]Ga-FAPI PET/CT between June 2020 and January 2021, were retrospectively analyzed. Routine contrast-enhanced CT (CE-CT) is performed in all patients as standardized care. Findings were confirmed by histopathology or radiographic follow-up. We compared radiotracer uptake, diagnostic performance, and TNM (tumor-node-metastasis) classifications.

RESULTS

We evaluated 36 participants (25/36 men; median age, 60 years), including 26 patients with pancreatic malignancies and ten patients with pancreatic benign lesions. [68Ga]Ga-FAPI PET/CT showed higher radiotracer uptake and higher sensitivity than [18F]FDG PET/CT in evaluating primary tumors (SUVmax, 21.4 vs. 4.8; sensitivity, 100% vs. 73.1%), involved lymph nodes (SUVmax, 8.6 vs. 2.7; sensitivity, 81.8% vs. 59.1%), and metastases (SUVmax, 7.9 vs. 3.5; sensitivity, 91.5% vs. 44.0%); Compared with [18F]FDG, [68Ga]Ga-FAPI PET/CT upstaged six patients' TNM staging (6/23, 26.1%) and changed two patients' clinical management (2/23, 8.7%). Compared with CE-CT, [68Ga]Ga-FAPI PET/CT upgraded TNM staging in five patients (5/23, 21.7%) and changed the therapeutic regimen in only one patient (1/23, 4.3%). Intense [68Ga]Ga-FAPI uptake was observed throughout the pancreas in 12/26 pancreatic malignancies; dual-time point [68Ga]Ga-FAPI PET/CT may differentiate pancreatitis from malignancy.

CONCLUSIONS

Compared with [18F]FDG PET/CT, [68Ga]Ga-FAPI PET/CT shows higher sensitivity in detecting primary pancreatic tumors, involved lymph nodes, and metastases and is superior in terms of TNM staging. Prospective trials with larger patient population are needed to evaluate whether [68Ga]Ga-FAPI PET/CT could elicit treatment modification in pancreatic cancer when compared with standard of care imaging.

Intraoperative fluorescence molecular imaging accelerates the coming of precision surgery in China

Purpose

China has the largest cancer population globally. Surgery is the main choice for most solid cancer patients. Intraoperative fluorescence molecular imaging (FMI) has shown its great potential in assisting surgeons in achieving precise resection. We summarized the typical applications of intraoperative FMI and several new trends to promote the development of precision surgery.

Methods

The academic database and NIH clinical trial platform were systematically evaluated. We focused on the clinical application of intraoperative FMI in China. Special emphasis was placed on a series of typical studies with new technologies or high-level evidence. The emerging strategy of combining FMI with other modalities was also discussed.

Results

The clinical applications of clinically approved indocyanine green (ICG), methylene blue (MB), or fluorescein are on the rise in different surgical departments. Intraoperative FMI has achieved precise lesion detection, sentinel lymph node mapping, and lymphangiography for many cancers. Nerve imaging is also exploring to reduce iatrogenic injuries. Through different administration routes, these fluorescent imaging agents provided encouraging results in surgical navigation. Meanwhile, designing new cancer-specific fluorescent tracers is expected to be a promising trend to further improve the surgical outcome.

Conclusions

Intraoperative FMI is in a rapid development in China. In-depth understanding of cancer-related molecular mechanisms is necessary to achieve precision surgery. Molecular-targeted fluorescent agents and multi-modal imaging techniques might play crucial roles in the era of precision surgery.

Non-18F-FDG-Avid Intrahepatic Metastasis of Breast Cancer Revealed by 68Ga-FAPI PET/CT

ABSTRACT

A patient with intrahepatic breast cancer metastasis underwent both 18F-FDG and 68Ga-FAPI PET/CT. However, the lesions are only 68Ga-FAPI avid. Our case illustrates that 68Ga-FAPI PET/CT can be more sensitive in detecting intrahepatic metastasis of breast cancer in some cases.

Fibroblast Activation Protein Inhibitor Imaging and Therapy in a Patient With Multiple Endocrine Neoplasia Type 2A Syndrome

ABSTRACT

A patient with multiple endocrine neoplasia type 2A syndrome who had exhausted several surgeries and radiotherapy was referred to nuclear medicine department for theranostic approaches. [68Ga]-DOTATATE PET/CT and [131I]I-mIBG SPECT/CT were performed, but the degree of uptake was insufficient for using the treatment companion of these tracers. Finally, 1 year later, [68Ga]-FAPI-46 PET/CT showed progressive disease with metastases to the lung, liver, bone, and lymph nodes with intense [68Ga]-FAPI-46 uptake. Treatment with [177Lu]Lu-FAPI-46 was done, and the patient tolerated treatment and showed evidence of clinical improvement following therapy.

177Lu-FAPI Therapy in a Patient With End-Stage Metastatic Pancreatic Adenocarcinoma

ABSTRACT

A 52-year-old woman with metastatic pancreatic adenocarcinoma underwent imaging with 18F-FDG PET/CT and 68Ga-FAPI-46 PET/CT, which demonstrated malignancy recurrence in the surgical bed with multiple metastatic lesions, more extensive on 68Ga-FAPI-46 PET/CT. The patient was a candidate for therapy with 177Lu-FAPI-46 due to high uptake of lesions in 68Ga-FAPI-46 images and no other available therapeutic option. Posttreatment 177Lu-FAPI-46 scans showed rather rapid washout of the radiopharmaceutical from tumoral lesions. This case report suggests that, although 68Ga-FAPI-46 is a promising agent for tumor imaging, 177Lu-FAPI-46 may not be an optimal compound for theranostic applications.

Contribution of positron emission tomography/magnetic resonance imaging in musculoskeletal malignancies

Positron emission tomography/computed tomography (PET/CT) is a promising hybrid imaging technique for evaluating musculoskeletal malignancies. Both technologies, independently are useful for evaluating this type of tumors. PET/MR has great potential combining metabolic and functional imaging PET with soft tissue contrast and multiparametric sequences of MR. In this paper we review the existing literature and discuss the different protocols, new available radiotracers to conclude with the scarce evidence available the most useful/probable indications of the PET MR for the for musculoskeletal malignancies.

Fibroblast activation protein alpha: Comprehensive detection methods for drug target and tumor marker

Fibroblast activation protein alpha (FAP-α, EC3.4.2. B28), a type II transmembrane proteolytic enzyme for the serine protease peptidase family. It is underexpressed in normal tissues but increased significantly in disease states, especially in neoplasm, which is a potential biomarker to turmor diagnosis. The inhibition of FAP-α activity will retard tumor formation, which is expected to be a promising tumor therapeutic target. At present, although the FAP-α expression detection methods has diversification, a superlative detection means is necessary for the clinical diagnosis. This review covers the discovery and the latest advances in FAP-α, as well as the future research prospects. The tissue distribution, structural characteristics, small-molecule ligands and structure-activity relationship of major inhibitors of FAP-α were summarized in this review. Furthermore, a variety of detection methods including traditional detection methods and emerging probes detection were classified and compared, and the design strategy and kinetic parameters of these FAP-α probe substrates were summarized. In addition, these comprehensive information provides a series of practical and reliable assays for the optimal design principles of FAP-α probes, promoting the application of FAP-α as a disease marker in diagnosis, and a drug target in drug design.

Imaging Cancer-Associated Fibroblasts (CAFs) with FAPi PET

The tumor microenvironment (TME) surrounding tumor cells is a complex and highly dynamic system that promotes tumorigenesis. Cancer-associated fibroblasts (CAFs) are key elements in TME playing a pivotal role in cancer cells’ proliferation and metastatic spreading. Considering the high expression of the fibroblast activation protein (FAP) on the cell membrane, CAFs emerged as appealing TME targets, namely for molecular imaging, leading to a pan-tumoral approach. Therefore, FAP inhibitors (FAPis) have recently been developed for PET imaging and radioligand therapy, exploring the clinical application in different tumor sub-types. The present review aimed to describe recent developments regarding radiolabeled FAP inhibitors and evaluate the possible translation of this pan-tumoral approach in clinical practice. At present, the application of FAPi-PET has been explored mainly in single-center studies, generally performed in small and heterogeneous cohorts of oncological patients. However, preliminary results were promising, in particular in low FDG-avid tumors, such as primary liver and gastro-entero-pancreatic cancer, or in regions with an unfavorable tumor-to-background ratio at FDG-PET/CT (i.e., brain), and in radiotherapy planning of head and neck tumors. Further promising results have been obtained in the detection of peritoneal carcinomatosis, especially in ovarian and gastric cancer. Data regarding the theranostics approach are still limited at present, and definitive conclusions about its efficacy cannot be drawn at present. Nevertheless, the use of FAPi-based radio-ligand to treat the TME has been evaluated in first-in-human studies and appears feasible. Although the pan-tumoral approach in molecular imaging showed promising results, its real impact in day-to-day clinical practice has yet to be confirmed, and multi-center prospective studies powered for efficacy are needed.

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.

Development and Validation of a Novel Model to Predict Regional Lymph Node Metastasis in Patients With Hepatocellular Carcinoma

Background

The evaluation of the nodal status of hepatocellular carcinoma (HCC) is a classic but controversial topic. This study aimed to investigate the incidence of lymph node metastasis (LNM), explore the role of lymph node dissection (LND), and develop and validate a novel model to predict LNM in patients with HCC, not other specified (NOS).

Methods

The study cohort was taken from the Surveillance, Epidemiology, and End Results database. The annual percent change (APC) was calculated using the Joinpoint regression. Survival analyses adopted the competing risk model. The nomogram was constructed based on the least absolute shrinkage and selection operator (LASSO) logistic regression algorithm and validated by calibration curves. The area under the receiver operating characteristic curve (AUROC) was obtained to compare prognostic performance. Decision curve and clinical impact curve analyses were introduced to examine the clinical value of the models.

Results

A total of 8,829 patients were finally enrolled in this study, and 1,346 (15.2%) patients received LND. The LND rate showed no noticeable fluctuation in the last decade, with an APC of 0.5% (P=0.593). LNM was identified in 56 (4.2%) patients and confirmed an independent prognostic factor of HCC patients (P=0.005). There were 2,497 lymph nodes retrieved, and 93 (3.7%) of them were positive. After propensity score matching, LND indicated no direct oncologic benefit and did not worsen competing risks. Moreover, an increased number of lymph nodes retrieved could not improve prognoses. 1,346 patients with LND were further randomly divided into the training and validation sets with the ratio of 1:1. Race, tumor size, clinical T stage, extrahepatic bile duct invasion, and tumor grade were independent risk factors for LNM. The constructed model was well calibrated and showed good discrimination power and net benefits in clinical practice.

Conclusion

LNM is an independent prognostic factor in HCC, but routine LND seems to be unnecessary in HCC patients. The constructed model could predict the presence of LNM in HCC patients with good performance, which is meaningful to patient stratification and individual treatment strategies optimization.

Influences on PET Quantification and Interpretation

Various factors have been identified that influence quantitative accuracy and image interpretation in positron emission tomography (PET). Through the continuous introduction of new PET technology-both imaging hardware and reconstruction software-into clinical care, we now find ourselves in a transition period in which traditional and new technologies coexist. The effects on the clinical value of PET imaging and its interpretation in routine clinical practice require careful reevaluation. In this review, we provide a comprehensive summary of important factors influencing quantification and interpretation with a focus on recent developments in PET technology. Finally, we discuss the relationship between quantitative accuracy and subjective image interpretation.

Positive FAPI PET/CT in a Bilateral Mammary Angiosarcoma Patient With Less Impressive FDG PET/CT Images

ABSTRACT

Bilateral primary angiosarcoma of the breast is an extremely rare disease. We describe a case with bilateral angiosarcoma of the breast well visualized on 68Ga-FAPI PET/CT in a 30-year-old woman with a history of right breast-conserving surgery. However, the lesions are less impressive on 18F-FDG PET/CT. No additional findings including lymph node and distant metastases were noted. The patient underwent bilateral mastectomy, and histopathology revealed well-differentiated angiosarcoma that involved bilateral breast parenchyma and the left nipple. Our case illustrates that 68Ga-FAPI PET/CT can be more sensitive in detecting well-differentiated angiosarcoma of the breast.

Head-to-head evaluation of [18F]FDG and [68 Ga]Ga-DOTA-FAPI-04 PET/CT in recurrent soft tissue sarcoma

Purpose

We aimed to evaluate the value of [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT for the diagnosis of recurrent soft tissue sarcoma (STS), compared with [¹⁸F]FDG PET/CT.

Methods

A total of 45 patients (21 females and 24 males; median age, 46 years; range, 18–71 years) with 13 subtypes of STS underwent [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT examination within 1 week for assessment local relapse or distant metastasis. Positive lesions on PET/CT images were verified by biopsy or 3-month follow-up. Wilcoxon matched-pairs signed-rank test was used to compare the semiquantitative values (SUV_max and TBR) of [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 in tumor lesions, and McNemar test was applied to test for differences of both tracers.

Results

Among the 45 patients, 282 local relapses and distant metastases were identified. Compared to [¹⁸F]FDG, [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT detected more lesions (275 vs. 186) and outperformed in sensitivity, specificity, PPV, NPV, and accuracy for the diagnosis of recurrent lesions (P < 0.001). [⁶⁸ Ga]Ga-DOTA-FAPI-04 demonstrated significantly higher values of SUV_max and TBR than [¹⁸F]FDG PET/CT in liposarcoma (P = 0.011 and P < 0.001, respectively), malignant solitary fibrous tumor (MSFT) (P < 0.001 and P < 0.001, respectively), and interdigitating dendritic cell sarcoma (IDCS) (P < 0.001and P < 0.001, respectively). While mean SUV_max and TBR presented favorable uptake of [¹⁸F]FDG over [⁶⁸ Ga]Ga-DOTA-FAPI-04 in undifferentiated pleomorphic sarcoma (UPS) (P = 0.003 and P < 0.001, respectively) and rhabdomyosarcoma (RMS) (P < 0.001 and P < 0.001, respectively).

Conclusion

[⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT is a promising new imaging modality for recurrent surveillance of STS, and compares favorably with [¹⁸F]FDG for identifying recurrent lesions of liposarcoma, MSFT, and IDCS.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-022-05700-4.

Preclinical evaluation and pilot clinical study of [18F]AlF-labeled FAPI-tracer for PET imaging of cancer associated fibroblasts

In recent years, fibroblast activation protein (FAP) has emerged as an attractive target for the diagnosis and radiotherapy of cancers using FAP-specific radioligands. Herein, we aimed to design a novel ¹⁸F-labeled FAP tracer ([¹⁸F]AlF-P-FAPI) for FAP imaging and evaluated its potential for clinical application. The [¹⁸F]AlF-P-FAPI novel tracer was prepared in an automated manner within 42 min with a non-decay corrected radiochemical yield of 32 ± 6% (n = 8). Among A549-FAP cells, [¹⁸F]AlF-P-FAPI demonstrated specific uptake, rapid internalization, and low cellular efflux. Compared to the patent tracer [¹⁸F]FAPI-42, [¹⁸F]AlF-P-FAPI exhibited lower levels of cellular efflux in the A549-FAP cells and higher stability in vivo. Micro-PET imaging in the A549-FAP tumor model indicated higher specific tumor uptake of [¹⁸F]AlF-P-FAPI (7.0 ± 1.0% ID/g) compared to patent tracers [¹⁸F]FAPI-42 (3.2 ± 0.6% ID/g) and [⁶⁸Ga]Ga-FAPI-04 (2.7 ± 0.5% ID/g). Furthermore, in an initial diagnostic application in a patient with nasopharyngeal cancer, [¹⁸F]AlF-P-FAPI and [¹⁸F]FDG PET/CT showed comparable results for both primary tumors and lymph node metastases. These results suggest that [¹⁸F]AlF-P-FAPI can be conveniently prepared, with promising characteristics in the preclinical evaluation. The feasibility of FAP imaging was demonstrated using PET studies.

Bone Marrow Metastases From Gastric Adenocarcinoma on 68Ga-FAPI-04 PET/CT

ABSTRACT

A 37-year-old woman complained of fatigue and dizziness for 4 days. Bone marrow biopsy findings were suggestive of digestive tract-derived metastases. 18F-FDG PET/CT revealed mild FDG avidity in the stomach and bone marrow, whereas 68Ga-FAPI-04 PET/CT showed strong FAPI uptake in the FDG-avid lesions. The patient was diagnosed with bone marrow metastases from gastric cancer by a bone marrow biopsy.

Comparison of [68Ga]-FAPI PET/CT and [18F]-FDG PET/CT in Multiple Myeloma: Clinical Experience

Objective: In this study, we aimed to compare [68Ga]FAPI PET/CT and [18F]FDG PET/CT imaging to detect lesions in multiple myeloma. Methods: A total of 14 patients with multiple myeloma who underwent [68Ga]FAPI PET/CT and [18F]FDG PET/CT imaging were included in this retrospective study. SUVmax values of [68Ga]FAPI and [18F]FDG were compared according to lesion locations. Also, lesion localization ability of both imaging methods was compared on the patient basis. Results: In 4 of 14 patients, [68Ga]FAPI PET/CT and [18F]FDG PET/CT have not detected any bone lesions. In 8 of the remaining 10 patients [18F]FDG PET/CT detected bone lesions but in this group, 6 patients showed more higher SUVmax values than [18F]FDG PET/CT in [68Ga]FAPI PET/CT.In contrast, 2 of 8 patients showed more higher SUVmax values than [68Ga]FAPI PET/CT in [18F]FDG PET/CT. Moreover, [68Ga]FAPI PET/CT detected bone lesions in two patients, which werenot detected by [18F]FDG PET/CT. Also, in five patients, [68Ga]FAPI PET/CT showed more bone lesions in comparison with[18F]FDG PET/CT. Only one patient, [18F]FDG PET/CT showed more bone lesions. Three extramedullary involvements were observed in the following locations: lung, presacral lymph node, and soft tissue mass lateral to the right maxillary sinus. Among these involvements, higher SUVmax values were observed in the lung and presacral lymph node with [68Ga]FAPI compared to [18F]FDG. However, the soft tissue mass showed a higher SUVmax value in [18F]FDG than [68Ga]FAPI. Conclusions: No significant superiority was observed in [68Ga]FAPI PET/CT over [18F]FDG PET/CT in patients with MM. However, [68Ga]FAPI PET/CT can be utilized as a complementary imaging method to [18F]FDG PET/CT in some settings, especially in low-[18F]FDG affinity and inconclusive cases. Considering the favorable aspects of [68Ga]FAPI PET/CT in MM, such as low background activity, absence of non-specific bone marrow, and physiological brain involvement, further studies with a larger sample size should be conducted.

68Ga-FAPI-04 PET/CT Versus 18F-FDG PET/CT in Malignant Peritoneal Mesothelioma

ABSTRACT

An 83-year-old man presented with abdominal pain, vomiting, and bloating. His abdominal CT revealed heterogeneous peritoneal thickening in the lower quadrants, which was more prominent in the pelvic region. 18F-FDG PET/CT performed to locate the primary tumor showed irregular peritoneal and omental thickening with low to moderate FDG uptake. 68Ga-FAPI (fibroblast activation protein-specific inhibitor)-04 PET/CT of the patient, whose primary tumor could not be detected using FDG, showed high radiopharmaceutical uptake in the areas of heterogeneous thickening on peritoneal and serosal surfaces. Peritoneal Tru-Cut biopsy revealed malignant tumoral infiltration indicating mesothelioma. This case showed that 68Ga-FAPI-04 PET/CT could be a promising radiopharmaceutical in the evaluation of peritoneal mesothelioma.

Dermatomyositis on 68Ga-FAPI PET/CT in a Patient With Nasopharyngeal Carcinoma

ABSTRACT

Dermatomyositis is an idiopathic inflammatory myopathy often associated with malignancies. 68Ga-FAPI PET/CT was performed on a 58-year-old man with newly diagnosed dermatomyositis. 68Ga-FAPI PET/CT showed multiple increased FAPI activity in whole-body muscles and nasopharyngeal lesion. A biopsy of the nasopharyngeal lesion confirmed nasopharyngeal carcinoma. 68Ga-FAPI PET/CT can provide a "1-stop" imaging method for patients with dermatomyositis.

In vitro and in vivo evaluation of 211At-labeled fibroblast activation protein inhibitor for glioma treatment

Glioma is the most common primary intracranial tumor without effective treatment. Positron emission tomography tracers labeled with 68Ga targeting fibroblast activation protein (FAP) have shown favorable characteristics in the diagnosis of glioma. However, to the best of our knowledge, FAP-targeted endoradiotherapy has never been explored in glioma. Hence, in this study, we investigated the therapeutic effect of 211At-labeled fibroblast activation protein inhibitor (FAPI) for glioma in vitro and in vivo. By astatodestannylation reaction, we prepared 211At-FAPI-04 with a radiochemical yield of 45 ± 6.7% and radiochemical purity of 98%. With good stability in vitro, 211At-FAPI-04 showed fast and specific binding to FAP-positive U87MG cells, and could significantly reduce the cell viability, arrested cell cycle at G2/M phase and suppressed cell proliferative efficacy. Biodistribution studies revealed that 6-fold higher accumulation in tumor sites was achieved by intratumoral injection in comparison with intravenous injection. In U87MG xenografts, 211At-FAPI-04 obviously suppressed the tumor growth and prolonged the median survival in a dose-dependent manner without obvious toxicity to normal organs. In addition, reduced proliferation and increased apoptosis were also observed after 211At-FAPI-04 treatment. All these results suggest that targeted alpha-particle therapy (TAT) mediated by 211At-FAPI-04 can provide an effective and promising strategy for the treatment of glioma.

PET/CT Imaging of Activated Cancer-Associated Fibroblasts Predict Response to PD-1 Blockade in Gastric Cancer Patients

Background

Promising development in immune checkpoint blockade (ICB) therapy has shown remarkable results in the treatment of gastric cancer (GC). However, the objective response rate in GC remains unsatisfactory. Noninvasive imaging to predict responses to ICB therapy via tumor microenvironment (TME) assessment is needed. Accordingly, this study aimed to evaluate the role of ⁶⁸Ga-FAPI-04 PET/CT in the assessment of the immunosuppressive TME in GC and to cross-correlate imaging findings with responses to ICB therapy.

Methods

The correlation between fibroblast-activation-protein (FAP) expression and immunosuppressive cell infiltration was analyzed using The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO) database, and GC tissue microarrays. To characterize the TME, TMEscores were calculated based on RNA-seq data from four GC patients. A total of 21 patients with GC underwent ⁶⁸Ga-FAPI-04 PET/CT before ICB treatment, and two of them were imaged after ICB therapy.

Results

FAP expression was found to be closely correlated with poor prognosis and infiltration of immunosuppressive cells, including myeloid-derived suppressor cells (MDSCs), exhausted T cells, and regulatory T cells (Tregs) in GC. We also found a strong relationship (R² = 0.9678, p = 0.0162) between ⁶⁸Ga-FAPI-04 uptake and TMEscore. Further analyses indicated that high ⁶⁸Ga-FAPI-04 uptake was correlated with reduced therapeutic benefits from ICB therapy.

Conclusions

⁶⁸Ga-FAPI-04 PET/CT may be used to noninvasively image the cancer-associated fibroblasts immunosuppressive TME in vivo and also potentially serve as a predictive biomarker of survival and antitumor immune response among patients who received ICB therapies.

Zwitterion-Coated Ultrasmall MnO Nanoparticles Enable Highly Sensitive T1-Weighted Contrast-Enhanced Brain Imaging

Manganese oxide nanoparticles (NPs) have attracted increasing attention recently as contrast agents (CAs) for magnetic resonance imaging (MRI). However, the clinical translation and popularization of conventional MnO NPs are hampered by their relatively poor imaging performance. Herein, we report the construction of ultrasmall MnO NPs (USMnO) via a one-pot synthetic approach that show a much better capability of T1-weighted contrast enhancement for MRI (r1 = 15.6 ± 0.4 mM-1 s-1 at 0.5 T) than MnCl2 and conventional large-sized MnO NPs (MnO-22). These USMnO are further coated with zwitterionic dopamine sulfonate (ZDS) molecules, which improves their biocompatibility and prevents nonspecific binding of serum albumins. Interestingly, USMnO@ZDS are capable of passing through the blood-brain barrier (BBB), which enables the acquisition of clear images showing brain anatomic structures with T1-weighted contrast-enhanced MRI. Therefore, our USMnO@ZDS could be used as a promising MRI CA for the flexible and accurate diagnosis of brain diseases, which is also instructive for the construction of manganese-based CA with a high MRI performance.

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.

Preparation and Bioevaluation of 99mTc-Labeled FAP Inhibitors as Tumor Radiotracers to Target the Fibroblast Activation Protein

Fibroblast activation protein (FAP) is overexpressed in cancer-associated fibroblasts (CAFs) in a majority of human epithelial cancers. With low expression in normal organs, FAP has become a promising molecular target for tumor theranostics. To develop a lower cost and more widely available alternative to positron emission tomography (PET), two isocyanide-containing FAP inhibitors (CN-C₅-FAPI and CN-PEG₄-FAPI) were synthesized and radiolabeled with ^99mTc to obtain [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺ and [^99mTc][Tc-(CN-PEG₄-FAPI)₆]⁺ in high yields (>95%). They showed good stability in saline and mouse serum. The partition coefficient (log P) values of [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺ and [^99mTc^][Tc-(CN-PEG₄-FAPI)₆]⁺ were -0.86 ± 0.03 and -2.38 ± 0.07, respectively, indicating that they were good hydrophilic complexes. The low nanomolar IC₅₀ values of CN-C₅-FAPI and CN-PEG₄-FAPI indicated that they had specificity to FAP. In vitro cellular uptake and blocking experiments implied a FAP-targeted uptake mechanism. The nanomolar K_d values from the saturation binding assay indicated that they had significantly high target affinity to FAP. The biodistribution and blocking study in BALB/c nude mice bearing U87MG tumors showed that both exhibited specific tumor uptake. [^99mTc][Tc-(CN-PEG₄-FAPI)₆]⁺ showed a higher tumor uptake and a higher tumor/nontarget ratio than [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺. The results of micro-single-photon emission computed tomography (SPECT) imaging studies of [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺ and [^99mTc^][Tc-(CN-PEG₄-FAPI)₆]⁺ were in accordance with the biodistribution results, suggesting that [^99mTc][Tc-(CN-PEG₄-FAPI)₆]⁺ is a promising tumor imaging agent for targeting FAP.

68Ga-DOTA-FAPI-04 PET/CT in Neuroendocrine Carcinoma of the Liver With Elevated AFP Level: Comparison With 18F-FDG PET/CT

ABSTRACT

A 68-year-old woman with a mass lesion in right hepatic lobe had an elevated α-fetoprotein level (>54,000 ng/mL). The lesion showed higher 68Ga-DOTA-FAPI-04 uptake than 18F-FDG uptake in consecutive PET/CT images. The histopathological examination revealed neuroendocrine carcinoma with Ki-67 proliferation index of 80%.

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.

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-FAPI as a Diagnostic Tool in Sarcoma: Data from the 68Ga-FAPI PET Prospective Observational Trial

Bone and soft-tissue sarcomas express fibroblast activation protein (FAP) on tumor cells and associated fibroblasts. Therefore, FAP is a promising therapeutic and diagnostic target. Novel radiolabeled FAP inhibitors (e.g., 68Ga-FAPI-46) have shown high tumor uptake on PET in sarcoma patients. Here, we report the endpoints of the 68Ga-FAPI PET prospective observational trial. Methods: Forty-seven patients with bone or soft-tissue sarcomas undergoing clinical 68Ga-FAPI PET were eligible for enrollment into the 68Ga-FAPI PET observational trial. Of these patients, 43 also underwent 18F-FDG PET. The primary study endpoint was the association between 68Ga-FAPI PET uptake intensity and histopathologic FAP expression analyzed with Spearman r correlation. Secondary endpoints were detection rate, positive predictive value (PPV), interreader reproducibility, and change in management. Datasets were interpreted by 2 masked readers. Results: The primary endpoint was met, and the association between 68Ga-FAPI PET uptake intensity and histopathologic FAP expression was significant (Spearman r = 0.43; P = 0.03). By histopathologic validation, PPV was 1.00 (95% CI, 0.87-1.00) on a per-patient and 0.97 (95% CI, 0.84-1.00) on a per-region basis. In cases with histopathologic validation, 27 of 28 (96%) confirmed patients and 32 of 34 (94%) confirmed regions were PET-positive, resulting in an SE of 0.96 (95% CI, 0.82-1.00) on a per-patient and 0.94 (95% CI, 0.80-0.99) on a per-region basis. The detection rate on a per-patient basis in 68Ga-FAPI and 18F-FDG PET was 76.6% and 81.4%, respectively. In 8 (18.6%) patients, 68Ga-FAPI PET resulted in an upstaging compared with 18F-FDG PET. 68Ga-FAPI PET readers showed substantial to almost perfect agreement for the defined regions (Fleiss κ: primary κ = 0.78, local nodal κ = 0.54, distant nodal κ = 0.91, lung κ = 0.86, bone κ = 0.69, and other κ = 0.65). Clinical management changed in 13 (30%) patients after 68Ga-FAPI PET. Conclusion: We confirm an association between tumoral 68Ga-FAPI PET uptake intensity and histopathologic FAP expression in sarcoma patients. Further, with masked readings and independent histopathologic validation, 68Ga-FAPI PET had a high PPV and sensitivity for sarcoma staging.

Head-to-Head Comparison of [68 Ga]Ga-FAPI-46-PET/CT and [18F]F-FDG-PET/CT for Radiotherapy Planning in Head and Neck Cancer

INTRODUCTION

In head and neck cancers (HNCs), fibroblast activation protein (FAP) is expressed by cancer-associated fibroblasts (CAFs) in the tumor microenvironment. Preliminary evidence suggests that detection and staging is feasible with positron emission tomography (PET/CT) imaging using [⁶⁸ Ga]-radiolabeled inhibitors of FAP ([⁶⁸ Ga]Ga-FAPI-46) in HNCs. This study aims to compare [⁶⁸ Ga]Ga-FAPI-46 PET/CT and [¹⁸F]-fluorodeoxy-D-glucose ([¹⁸F]F-FDG) PET/CT with a focus on improved target volume definition and radiotherapy planning in patients with HNC referred for chemoradiation.

METHODS

A total of 15 patients with HNCs received both [⁶⁸ Ga]Ga-FAPI-46 PET/CT and [¹⁸F]F-FDG PET/CT with a thermoplastic mask, in addition to initial tumor staging by conventional imaging with contrast-enhanced CT and/or MRI. Mean intervals between FAPI/FDG and FAPI/conventional imaging were 4 ± 20 and 17 ± 18 days, respectively. Location and number of suspicious lesions revealed by the different procedures were recorded. Subsequently, expert-generated gross tumor volumes (GTVs) based on conventional imaging were compared to those based on [¹⁸F]F-FDG and [⁶⁸ Ga]Ga-FAPI-46 PET/CT to measure the impact on subsequent radiation planning.

RESULTS

All patients had focal FAPI uptake above background in tumor lesions. Compared to FDG, tumor uptake (median SUVmax 10.2 vs. 7.3, p = 0.008) and tumor-to-background ratios were significantly higher with FAPI than with FDG (SUVmean liver: 9.3 vs. 3.2, p < 0.001; SUVmean bloodpool: 6.9 vs. 4.0, p < 0.001). A total of 49 lesions were recorded. Of these, 40 (82%) were FDG⁺ and 41 (84%) were FAP⁺. There were 5 (10%) FAP⁺/FDG^- lesions and 4 (8%) FAP^-/FDG⁺ lesions. Volumetrically, a significant difference was found between the GTVs (median 57.9 ml in the FAPI-GTV, 42.5 ml in the FDG-GTV, compared to 39.2 ml in the conventional-GTV). Disease stage identified by FAPI PET/CT was mostly concordant with FDG PET/CT. Compared to conventional imaging, five patients (33%) were upstaged following imaging with FAPI and FDG PET/CT.

CONCLUSION

We demonstrate that [⁶⁸ Ga]Ga-FAPI-46 -PET/CT is useful for detecting tumor lesions in patients with HNCs. There is now a need for prospective randomized studies to confirm the role of [⁶⁸ Ga]Ga-FAPI-46 PET/CT in relation to [¹⁸F]F-FDG PET/CT in HNCs and to evaluate its impact on clinical outcome.

68Ga-FAPI PET/CT Imaging of Multiple Muscle Metastases of Pancreatic Cancer

ABSTRACT

A 67-year-old man presented with generalized muscle pain for 2 months. A potential malignancy was suspected. The patient was subsequently enrolled in the clinical trial of 68Ga-FAPI. Increased activity of FAPI and FDG was observed in muscle lesions. Moreover, 68Ga-FAPI PET/CT showed intense FAPI uptake in the pancreas. Finally, pancreas and left iliopsoas muscle biopsies confirmed the diagnosis of pancreatic cancer with multiple muscle 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.

Design and Characterization of Fibroblast Activation Protein Targeted Pan-Cancer Imaging Agent for Fluorescence-Guided Surgery of Solid Tumors

Tumor-targeted fluorescent dyes have been shown to significantly improve a surgeon's ability to locate and resect occult malignant lesions, thereby enhancing a patient’s chances of long term survival. Although several...

Fibroblast Activation Protein-Specific PET/CT Imaging in Fibrotic Interstitial Lung Diseases and Lung Cancer: A Translational Exploratory Study

Interstitial lung diseases (ILDs) comprise over 200 parenchymal lung disorders. Among them, fibrosing ILDs, especially idiopathic pulmonary fibrosis, are associated with a poor prognosis, whereas some other ILDs, such as sarcoidosis, have a much better prognosis. A high proportion manifests as fibrotic ILD (fILD). Lung cancer (LC) is a frequent complication of fILD. Activated fibroblasts are crucial for fibrotic processes in fILD. The aim of this exploratory study was to evaluate the imaging properties of static and dynamic fibroblast activation protein (FAP) inhibitor (FAPI) PET/CT in various types of fILD and to confirm FAP expression in fILD lesions by FAP immunohistochemistry of human fILD biopsy samples and of lung sections of genetically engineered (Nedd4-2-/- ) mice with an idiopathic pulmonary fibrosislike lung disease. Methods: PET scans of 15 patients with fILD and suspected LC were acquired 10, 60, and 180 min after the administration of 150-250 MBq of a 68Ga-labeled FAPI tracer (FAPI-46). In 3 patients, dynamic scans over 40 min were performed instead of imaging after 10 min. The SUVmax and SUVmean of fibrotic lesions and LC were measured and CT-density-corrected. Target-to-background ratios (TBRs) were calculated. PET imaging was correlated with CT-based fibrosis scores. Time-activity curves derived from dynamic imaging were analyzed. FAP immunohistochemistry of 4 human fILD biopsy samples and of fibrotic lungs of Nedd4-2-/- mice was performed. Results: fILD lesions as well as LC showed markedly elevated 68Ga-FAPI uptake (density-corrected SUVmax and SUVmean 60 min after injection: 11.12 ± 6.71 and 4.29 ± 1.61, respectively, for fILD lesions and 16.69 ± 9.35 and 6.44 ± 3.29, respectively, for LC) and high TBR (TBR of density-corrected SUVmax and SUVmean 60 min after injection: 2.30 ± 1.47 and 1.67 ± 0.79, respectively, for fILD and 3.90 ± 2.36 and 2.37 ± 1.14, respectively, for LC). SUVmax and SUVmean decreased over time, with a stable TBR for fILD and a trend toward an increasing TBR in LC. Dynamic imaging showed differing time-activity curves for fILD and LC. 68Ga-FAPI uptake showed a positive correlation with the CT-based fibrosis index. Immunohistochemistry of human biopsy samples and the lungs of Nedd4-2-/- mice showed a patchy expression of FAP in fibrotic lesions, preferentially in the transition zone to healthy lung parenchyma. Conclusion:68Ga-FAPI PET/CT imaging is a promising new imaging modality for fILD and LC. Its potential clinical value for monitoring and therapy evaluation of fILD should be investigated in future studies.

Characterizing Concomitant Follicular Lymphoma and Gastric Carcinoma Using 68Ga-FAPI-04 and 18F-FDG PET/CT

ABSTRACT

We herein report a case involving a 67-year-old man with concomitant progressive follicular lymphoma and gastric carcinoma. Baseline 18F-FDG PET/CT showed high metabolic activity in multiple nodal stations and a thickened gastric antrum wall, whereas 68Ga-FAPI-04 PET/CT depicted very intense tracer uptake in the gastric lesion but mild uptake in the nodes. After the treatment, complete remission from lymphadenopathy was achieved, whereas the gastric lesion accumulated more radiotracers compared with baseline levels. Despite our incorrect initial assumption of B-cell transformation, molecular imaging was able to profile the characteristics of these 2 diseases.

Brain metastases: the role of clinical imaging

Imaging of brain metastases (BMs) has advanced greatly over the past decade. In this review, we discuss the main challenges that BMs pose in clinical practice and describe the role of imaging.Firstly, we describe the increased incidence of BMs of different primary tumours and the rationale for screening. A challenge lies in selecting the right patients for screening: not all cancer patients develop BMs in their disease course.Secondly, we discuss the imaging techniques to detect BMs. A three-dimensional (3D) T1W MRI sequence is the golden standard for BM detection, but additional anatomical (susceptibility weighted imaging, diffusion weighted imaging), functional (perfusion MRI) and metabolic (MR spectroscopy, positron emission tomography) information can help to differentiate BMs from other intracranial aetiologies.Thirdly, we describe the role of imaging before, during and after treatment of BMs. For surgical resection, imaging is used to select surgical patients, but also to assist intraoperatively (neuronavigation, fluorescence-guided surgery, ultrasound). For treatment planning of stereotactic radiosurgery, MRI is combined with CT. For surveillance after both local and systemic therapies, conventional MRI is used. However, advanced imaging is increasingly performed to distinguish true tumour progression from pseudoprogression.FInally, future perspectives are discussed, including radiomics, new biomarkers, new endogenous contrast agents and theranostics.

[18F]FAPI-42 PET imaging in cancer patients: optimal acquisition time, biodistribution, and comparison with [68Ga]Ga-FAPI-04

PURPOSE

[¹⁸F]FAPI-42 is a new fibroblast activation protein (FAP)-specific tracer used for cancer imaging. Here, we describe the optimal acquisition time and in vivo evaluation of [¹⁸F]FAPI-42 and compared intra-individual biodistribution, tumor uptake, and detection ability to [⁶⁸Ga]Ga-FAPI-04.

METHODS

A total of 22 patients with various types of cancer received [¹⁸F]FAPI-42 whole-body positron emission tomography/computed tomography (PET/CT). Among them, 4 patients underwent PET/CT scans, including an early dynamic 20-min, static 1-h, and static 2-h scans. The in vivo biodistribution in normal organs and tumor uptake were semiquantitatively evaluated using the standardized uptake value (SUV) and tumor-to-background ratio (TBR). Furthermore, both [¹⁸F]FAPI-42 and [⁶⁸Ga]Ga-FAPI-04 PET/CT were performed in 12 patients to compare biodistribution, tumor uptake, and tumor detection ability.

RESULTS

[¹⁸F]FAPI-42 uptake in the tumors was rapid and reached a high level with an average SUVmax of 15.8 at 18 min, which stayed at a similarly high level to 2 h. The optimal image acquisition time for [¹⁸F]FAPI-42 was determined to be 1 h postinjection. For tumor detection, [¹⁸F]FAPI-42 had a high uptake and could be clearly visualized in the lesions. Compared to [⁶⁸Ga]Ga-FAPI-04, [¹⁸F]FAPI-42 had the same detectability for 144 positive lesions. In addition, [¹⁸F]FAPI-42 showed a higher SUVmax in liver and bone lesions (P < 0.05) and higher TBRs in liver, bone, lymph node, pleura, and peritoneal lesions (all P < 0.05).

CONCLUSION

The present study demonstrates that the optimal image acquisition time of [¹⁸F]FAPI-42 is 1 h postinjection and that [¹⁸F]FAPI-42 exhibits comparable lesion detectability to [⁶⁸Ga]Ga-FAPI-04.

TRIAL REGISTRATION

Chinese Clinical Trial Registry (ChiCTR2100045757).

Clinical Significance of Diffusely Increased Uptake of 68Ga-FAPI in Thyroid Gland

Purpose: To determine the clinical significance of diffuse uptake of ⁶⁸Ga-FAPI in the thyroid.

Methods: From January 2020 to September 2021, all subjects with diffuse thyroid uptake in ⁶⁸Ga-FAPI PET/CT were investigated in our hospital, and compared with the age and sex matched control group. The ⁶⁸Ga-FAPI uptake in the thyroid gland was analyzed semi-quantitatively using the maximum standardized uptake value (SUVmax), and regression analysis was used to analyze the correlation between available serum thyroid stimulating hormone (TSH) and thyroid peroxidase antibody (TPOAb).

Results: Among 815 subjects, 39 subjects were found diffuse FAPI uptake in thyroid gland; 11 subjects refused further examination; a total of 28 subjects were included in the analysis, and 27 subjects were diagnosed with chronic thyroiditis (including 20 subjects with Hashimoto's thyroiditis), 3 subjects with Grave's disease, 3 subjects with only serum TSH elevated, and 1 subject with malignant of thyroid and thyroiditis. The SUVmax of 27 subjects with thyroiditis was 5.75 ± 5.45. No significant correlation was found between the SUVmax and the level of serum TSH (P = 0.389) or TPOAb (P = 0.426).

Conclusion: The incidentally discovered diffusely increased ⁶⁸Ga-FAPI uptake in the thyroid gland is mostly related to chronic lymphocytic (Hashimoto's) thyroiditis. ⁶⁸Ga-FAPI uptake level correlated neither with the degree of hypothyroidism nor with the titer of TPOAb. In addition, immune-related thyroiditis with immune checkpoint inhibitors may be accidentally found on ⁶⁸Ga-FAPI, which may be helpful in facilitate timely intervention.

FAPI-04 Uptake in Healthy Tissues of Cancer Patients in 68Ga-FAPI-04 PET/CT Imaging

Objective

The aim of this study is to investigate the uptake of ⁶⁸Ga-FAPI-04 in normal tissues and calculate standardized uptake values (SUVs) for various organs in the body.

Methods

A total of 49 patients who underwent ⁶⁸Ga-FAPI-04 PET/CT were included in our study. The following organs were identified on CT images: brain, parotid, and submandibular glands, palatine tonsils, thyroid, lymph nodes (if present), breasts, lungs, thymus, left ventricle walls, mediastinal blood pool, vertebral bone marrow, liver, spleen, pancreas, stomach, small and large intestines, adrenal glands, kidneys, uterus, testes, and prostate. Median, minimum, and maximum values (max) and average (avg) values of standard uptake value (SUV) of tissues and organs were calculated.

Results

The accumulation of ⁶⁸Ga-FAPI in normal organs showed variations. The cerebral/cerebellar cortex exhibited no ⁶⁸Ga-FAPI uptake, while the scalp showed low uptake. Low uptake was also observed in the lung parenchyma, esophagus, left ventricle walls, nipple, and glandular breast tissue. In the abdominopelvic area, the pancreas exhibited low uptake, which was higher in the tail region. Low uptake was observed in the renal cortex. Intense ⁶⁸Ga-FAPI uptake was observed throughout the uterus, which was higher in the corpus. There was no uptake of ⁶⁸Ga-FAPI in the bone cortex and medulla.

Conclusion

We determined the physiological uptake and SUVmax of FAPI-04 in different tissues and organs and created a guide for researchers.

Molecular Targeted Positron Emission Tomography Imaging and Radionuclide Therapy of Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) has an inauspicious prognosis, mainly due to difficulty in early detection of the disease by the current imaging modalities. The upcoming development of tumour-specific tracers provides an alternative solution for more accurate diagnostic imaging techniques for staging and therapy response monitoring. The future goal to strive for, in a patient with PDAC, should definitely be first to receive a diagnostic dose of an antibody labelled with a radionuclide and to subsequently receive a therapeutic dose of the same labelled antibody with curative intent. In the first part of this paper, we summarise the available evidence on tumour-targeted diagnostic tracers for molecular positron emission tomography (PET) imaging that have been tested in humans, together with their clinical indications. Tracers such as radiolabelled prostate-specific membrane antigen (PSMA)-in particular, 18F-labelled PSMA-already validated and successfully implemented in clinical practice for prostate cancer, also seem promising for PDAC. In the second part, we discuss the theranostic applications of these tumour-specific tracers. Although targeted radionuclide therapy is still in its infancy, lessons can already be learned from early publications focusing on dose fractioning and adding a radiosensitiser, such as gemcitabine.

[18F]AlF-NOTA-FAPI-04 PET/CT uptake in metastatic lesions on PET/CT imaging might distinguish different pathological types of lung cancer

Purpose

Heterogeneity is found in the tumor microenvironment among different pathological types of tumors. Radionuclide-labeled fibroblast-activation-protein inhibitor (FAPI), as an important tracer for non-invasive imaging of the tumor microenvironment, can be used to evaluate the expression of FAP in cancer-associated fibroblasts, macrophages, and tumor cells. Our aim was to explore the ability of [¹⁸F]AlF-NOTA-FAPI-04 positron emission tomography (PET)/computed tomography (CT) to distinguish different types of lung cancer by evaluating the uptake of this tracer in primary and metastatic lesions.

Methods

We prospectively enrolled 61 patients with histopathologically proven primary lung cancer with metastases. PET/CT scanning was performed before any antitumor therapy and 1 h after injection of 235.10 ± 3.89 MBq of [¹⁸F]AlF-NOTA-FAPI-04. Maximum standard uptake values (SUV_max) were calculated for comparison among primary and metastatic lesions. Immunohistochemical staining for FAP was performed on tumor specimens.

Results

Sixty-one patients with adenocarcinoma (ADC, n = 30), squamous cell carcinoma (SCC, n = 17), and small cell lung cancer (SCLC, n = 14) were enrolled in this study, and 61 primary tumors and 199 metastases were evaluated. No difference in [¹⁸F]AlF-NOTA-FAPI-04 uptake was observed among primary ADC, SCC, and SCLC tumors (P = 0.198). Additionally, no difference in uptake was found between primary and metastatic lesions of lung cancer with the same pathological type (P > 0.05). However, uptake did differ among metastases of differing pathological types (P < 0.001). The SUV_max of metastatic lymph nodes was highest for SCC, followed by ADC and then SCLC (P < 0.001). The SUV_max of bone metastases also was highest for SCC, followed by ADC and SCLC (P < 0.05), but no difference was observed between ADC and SCLC. The SUV_max of metastases in other organs was higher in SCC cases than in ADC cases but did not differ between SCC and SCLC or ADC and SCLC. Bone metastases exhibited higher uptake than those of lymph nodes and other organs in SCC and ADC (P < 0.05) but not in SCLC. Positive correlations were found between FAPI uptake and FAP expression in surgical plus biopsy specimens (r = 0.439, P = 0.012) and surgical specimens (r = 0.938, P = 0.005).

Conclusion

[¹⁸F]AlF-NOTA-FAPI-04 PET/CT imaging revealed differences in FAP expression in metastases of lung cancer, with the highest expression specifically in bone metastases, and thus, may be valuable for distinguishing different pathological types of lung cancer.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05638-z.

Head-to-head comparison of 68Ga-FAPI-46 and 18F-FDG PET/CT for evaluation of head and neck squamous cell carcinoma: a single-center exploratory study

⁶⁸Ga-conjugated fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) has become an attractive agent for positron emission tomography (PET). This study aimed to compare ⁶⁸Ga-FAPI-46 PET/computed tomography (CT) with ¹⁸F-fluorodeoxy-D-glucose (¹⁸F-FDG) PET/CT for detecting primary cancer and metastatic lesions in patients with head and neck squamous cell carcinoma (HNSCC). Methods: Twelve patients and twenty-eight patients with HNSCC underwent ⁶⁸Ga-FAPI-46 and ¹⁸F-FDG PET/CT for initial staging and recurrence detection, respectively. Concordance and diagnostic accuracy of both tracers were analyzed. Semiquantitative parameters, including the maximum and mean of standardized uptake value (SUV_max and SUVmean) and tumor-to-background ratio (T/B) were compared. FAP expression tumor volume (FTV) and total lesion FAP expression (TLF) of ⁶⁸Ga-FAPI-46 were compared with metabolic tumor volume (MTV) and total lesion glycolysis (TLG) of ¹⁸F-FDG, respectively. Differences between semiquantitative parameters were analyzed using paired t-tests. Results: ⁶⁸Ga-FAPI -46 PET/CT was 83.3% and 96.4% concordant with ¹⁸F-FDG PET/CT for initial staging and recurrence detection, respectively. Eighteen lesions had histopathological validation and both tracers displayed 100% sensitivity, 50% specificity, and 94.4% accuracy for lesion-based analysis. FTV was greater than MTV (P < 0.05), but no significant differences were observed for the other parameters. Conclusion: ⁶⁸Ga-FAPI-46 PET/CT showed good concordance and comparable diagnostic performance compared with ¹⁸F-FDG PET/CT for initial staging and recurrence detection in patients with HNSCC.

Mesenteric Inflammatory Myofibroblastic Tumor on 68Ga-FAPI PET/CT

ABSTRACT

Inflammatory myofibroblastic tumor is an uncommon mesenchymal neoplasm, which can be seen in any anatomic region from the central nervous system to the gastrointestinal tract. Herein, we report the 68Ga-FAPI PET/CT findings in a 49-year-old man with mesenteric inflammatory myofibroblastic tumor.

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.

Increased 68Ga-FAPI Uptake in Neurofibromatosis in a Patient With Pleomorphic Rhabdomyosarcoma

ABSTRACT

A 48-year-old man presented with a painless enlarging mass on his right thigh for 2 months. He had a history of neurofibromatosis. MRI showed a mass in his right thigh. Malignant tumor was suspected. He was enrolled in the clinical trial of 68Ga-FAPI in solid tumors. Multiple increased FAPI uptake was observed in the right thigh mass, lung, and mediastinal lymph nodes, which were confirmed being pleomorphic rhabdomyosarcoma by biopsy. In addition, widespread metabolic activity was also observed in his cutaneous fibroma nodules. This case showed that benign neurofibromatosis can cause FAPI uptake.