Increased uptake of 68Ga-DOTA-FAPI-04 in bones and joints: metastases and beyond

Comparative Evaluation of 68Ga-FAPI-04 PET for Initial N and M Staging in Gastric Cancer: A Study Against Histopathology and Contrast-Enhanced CT

PURPOSE

To evaluate the efficiency of 68Ga-FAPI-04 PET (PET/MRI or PET/CT) for N and M staging in gastric carcinoma and compare outcomes with histopathology and contrast-enhanced computed tomography (CECT).

PATIENTS AND METHODS

Patients with gastric carcinoma who had undergone 68Ga-FAPI-04 PET/MRI or PET/CT before treatment were retrospectively enrolled. Histopathology post lymphadenectomy was the gold standard for N staging, while histopathology and follow-up data were the reference for overall outcomes. The diagnostic efficiency of 68Ga-FAPI-04 PET for detecting regional lymph node involvement and distant metastases was compared to that of CECT.

RESULTS

Sixty-two patients were enrolled. In 18 patients who underwent 68Ga-FAPI-04 PET/MRI and lymphadenectomy, 532 lymph nodes were dissected. 68Ga-FAPI-04 PET/MRI showed similar sensitivity, specificity, and accuracy compared to CECT (28.3% vs. 23.2%, 99.8% vs. 99.3%, and 86.5% vs. 85.2%, all P > 0.05). Fifty-five patients had regional lymph node metastasis, 68Ga-FAPI-04 PET exhibited comparable diagnostic efficiency to CECT, with sensitivity of 83.6% versus 87.3%, specificity of 100% versus 85.7%, accuracy of 85.5% versus 87.1% (all P > 0.05). Excluding 3 patients with only abdominal CECT, 32 out of 59 patients had distant metastasis, with no significant differences in sensitivity, specificity, and accuracy between 68Ga-FAPI-04 PET and CECT based on patient (100% vs. 87.5%, 92.6% vs. 96.3%, and 96.6% vs. 91.5%, all P >0.05). Notably, 68Ga-FAPI-04 PET outperformed CECT in detecting peritoneal, distant lymph nodes, bone, liver, and ovarian metastases by visualizing more lesions or greater lesion extent.

CONCLUSIONS

68Ga-FAPI-04 PET exhibits comparable diagnostic performance to CECT for patient-based N staging and M staging of gastric cancer. However, it surpasses CECT in visualizing distant metastases.

Comparison of 18F-FDG and 68Ga-DOTA-IBA in detecting bone metastases: a lesion-basis study

Gallium 68 (68Ga)-labeled DOTA-conjugate ibandronic acid (DOTA-IBA) has been successfully synthesized and utilized for bone metastasis imaging. This study compares the diagnostic efficacy between 68Ga-DOTA-IBA and fluorine 18 (18F)-labeled fluorodeoxyglucose (FDG) in detecting bone metastases. This prospective study, conducted from October 2022 to September 2023, analyzed images from participants who underwent 68Ga-DOTA-IBA PET/CT and 18F-FDG PET/CT scans. Lesions were classified into five groups based on anatomical location (limbs, vertebrae, pelvis, ribs, and skull). Morphological bone changes were categorized as osteolytic, osteoblastic, or mixed. The semi-quantified radiotracer uptake, measured by the maximum standardized uptake value (SUVmax), was compared using a paired t-test. Detection rates between the two scans were analyzed using the McNemar test. A total of 46 participants (median age: 64 years [interquartile range: 53-68 years], 28 men) were evaluated. 68Ga-DOTA-IBA demonstrated higher diagnostic efficacy than 18F-FDG in detecting bone metastases in the limbs (73.2% vs. 64.1%), vertebras (78.1% vs. 67.4%), ribs (86.6% vs. 62.2%), pelvis (78.6% vs. 68.9%), and skulls (80.0% vs. 38%). For osteoblastic lesions, the detection rate for 68Ga-DOTA-IBA and 18F-FDG was 83.3% and 51.5% respectively (P < 0.001). The SUVmax of 68Ga-DOTA-IBA was 7.88 (95% CI 7.09-8.66), which was higher than that of 18F-FDG at 3.96 (95% CI 3.57-4.35) (P < 0.001). In participants with prostate cancer, the detection rate of 68Ga-DOTA-IBA and 18F-FDG was 84.7% and 55.0% respectively (P < 0.001). The SUVmax of 68Ga-DOTA-IBA was 10.44 (95% CI 8.57-12.30), which was higher than that of 18F-FDG 4.29 (95% CI 3.51-5.07) (P < 0.001). 68Ga-DOTA-IBA PET/CT demonstrates superior diagnostic performance over 18F-FDG PET/CT in detecting bone metastases, particularly in osteoblastic lesions and prostate cancer cases.

Comparison of [68Ga]Ga-Fibroblast Activation Protein Inhibitor-04 and [18F]FDG PET Imaging for Solitary Fibrous Tumor and Preliminary Application of FAP-Targeted Radiopharmaceutical Therapy

Solitary fibrous tumor (SFT) is a rare sarcoma of mesenchymal origin. Although generally benign, SFTs carry the risk of recurrence and metastasis, with limited effective treatment options. The aims of this study are to compare the performance of fibroblast activation protein inhibitor (FAPI), [68Ga]Ga-DOTA-FAPI-04 (denoted as [68Ga]Ga-FAPI-04), and conventional [18F]FDG PET/CT in patients with recurrent or metastatic SFTs head to head and to preliminarily explore the value of FAP-targeted radiopharmaceutical therapy with 177Lu for SFT patients. Methods: Thirty-one participants (21 men, 44 ± 13 y) with suspected recurrent or metastatic SFTs underwent both [18F]FDG and [68Ga]Ga-FAPI-04 PET/CT within 1 wk. The positive-lesion rates of the 2 PET/CT scans in the different organs involved and the uptake values (SUVmax) were compared. Four patients with high [68Ga]Ga-FAPI-04 uptake received single-cycle therapy of 2.22 GBq of a [177Lu]Lu-labeled, FAP-targeted radiopharmaceutical, [177Lu]Lu-Evans blue-FAPI, and were followed up for 4 mo. Results: In 522 local recurrences and distant metastases in the 31 patients, [68Ga]Ga-FAPI-04 PET detected significantly more lesions than did [18F]FDG (87.0% vs. 45.4%, P < 0.001). In terms of lesion uptake values, [68Ga]Ga-FAPI-04 PET showed a mean SUVmax higher than that of [18F]FDG in most recurrence or metastatic organs (bone, lung, central nervous system, pancreas, and pleura, P < 0.001; kidney and abdominopelvic cavity, P = 0.001; muscle and pericardium, P < 0.05). Four patients tolerated [177Lu]Lu-Evans blue-FAPI well. The total-body absorbed dose and the effective dose were 4.02E-01 ± 3.54E-02 Gy and 4.01E+02 ± 4.18E+01 mSv, respectively. Subsequent follow-up with [68Ga]Ga-FAPI-04 PET showed that these patients were in stable condition. Conclusion: [68Ga]Ga-FAPI-04 may be a promising PET agent for the assessment of SFTs. Given the lack of effective treatments for advanced SFTs, high FAP expression in this type of tumor is expected to become a potential treatment target.

Modification of Asp-Peptide Adapters: Giving the FAP-Targeted Radioligand a "Squirrel Tail"

Fibroblast activation protein (FAP) is a promising target for cancer theranostics, but most FAP-targeted radioprobes showed relatively insufficient tumor uptake and retention, which seriously hampered their further application. Inspired by the squirrel tail, this study developed a novel FAP-targeted molecule, FSND3, which is modified with three Asp-peptide adapters to enable both 68Ga ([68Ga]Ga-FSND3) and 18F ([18F]AlF-FSND3) PET imaging. Compared to [68Ga]Ga-FAPI-04, [68Ga]Ga-FAPI-42, and [18F]AlF-FAPI-42, [18F]AlF-FSND3 and [68Ga]Ga-FSND3 showed enhanced tumor uptake and prolonged residence in HT-1080-FAP and pancreatic tumor models, demonstrating the effectiveness of Asp-peptide adapters in pharmacomodulating FAP-targeted radioligands. The first-in-human pilot study revealed that [18F]AlF- and [68Ga]Ga-FSND3 exhibited comparable uptake in the primary lesion, higher-contrast images, and higher uptake in some metastases like in bone and brain, to 2-[18F] FDG PET/CT imaging. As a proof of concept, these results offer a significant enhancement to the diversity of the FAP-targeted tracer arsenal.

Assessment of fully quantitative and simplified methods for analysis of [68Ga]Ga-FAPI-46 uptake in patients with pancreatobiliary cancer using LAFOV PET/CT

PURPOSE

The aim of this study was to validate simplified methods for quantifying [68Ga]Ga-FAPI-46 uptake against full pharmacokinetic modeling.

METHODS

Ten patients with pancreatobiliary cancer underwent a 90-min dynamic PET/CT scan using a long axial field of view system. Arterial blood samples were used to establish calibrated plasma-input function from both continuous arterial sampling and image-derived input function (IDIF). Lesional [68Ga]Ga-FAPI-46 kinetics were described using conventional non-linear plasma-input tissue-compartment models. Logan plots using 30-90 min and 30-60 min post-injection (p.i), image-based target-to-whole blood ratio (TBR), mean standardized uptake values (SUVmean) normalized to body weight, lean body mass, and body surface area, at 20-30 min, 60-70 min and 80-90 min p.i were assessed.

RESULTS

One patient was excluded due to discontinued scan acquisition and missing arterial sampling. Thirteen tumoral lesions and 11 non-tumoral lesions were included. A reversible 2-tissue-compartment model showed most preferrable fits for all types of [68Ga]Ga-FAPI-46 positive lesions. The distribution volume (VT) results obtained using arterial sampling plasma-input function and those using plasma-IDIF (VT_plasma_IDIF) showed an excellent correlation (Spearman rank correlation coefficient (rs) = 0.949). Logan VT using both time intervals were highly correlated with VT_plasma_IDIF (rs ≥ 0.938). The correlation values with VT_plasma_IDIF for image-based TBR and SUVmean parameters were higher at 80-90 min (rs ≥ 0.839) and at 60-70 min (rs ≥ 0.835) p.i than at 20-30 min p.i (rs ≤ 0.774).

CONCLUSION

Image-based TBR and SUVmean at 60-70 min p.i are suitable for quantifying [68Ga]Ga-FAPI-46 uptake.

TRIAL REGISTRATION

EudraCT, EudraCT 2022-001867-29. Registered 02 November 2022.

Senescent Fibroblasts Drive FAP/OLN Imbalance Through mTOR Signaling to Exacerbate Inflammation and Bone Resorption in Periodontitis

Fibroblast activation protein (FAP), predominantly expressed in activated fibroblasts, plays a key role in inflammatory bone diseases, but its role in periodontitis remains unclear. Accordingly, this study identified a positive association between FAP levels and periodontitis susceptibility using Mendelian randomization analysis. Human and mouse periodontitis tissues show elevated FAP and reduced osteolectin (OLN), an endogenous FAP inhibitor, indicating a FAP/OLN imbalance. Single-cell RNA sequencing revealed gingival fibroblasts (GFs) as the primary FAP and OLN source, with periodontitis-associated GFs showing increased reactive oxygen species, cellular senescence, and mTOR pathway activation. Rapamycin treatment restored the FAP/OLN balance in GFs. Recombinant FAP increased pro-inflammatory cytokine secretion and osteoclast differentiation in macrophages, exacerbating periodontal damage, whereas FAP inhibition reduced macrophage inflammation, collagen degradation, and bone resorption in experimental periodontitis. Therefore, senescent fibroblasts drive the FAP/OLN imbalance through mTOR activation, contributing to periodontitis progression. Consequently, targeting FAP may offer a promising therapeutic strategy for periodontitis.

Comparative assessment of the diagnostic efficacy of [18F]AlF-NOTA-FAPI-04 and [18F]FDG PET/CT imaging for detecting postoperative recurrence in gastric cancer patients: a pilot study

Purpose

This study aimed to compare the efficacy of [18F]AlF-NOTA-FAPI-04 PET/CT with that of [18F]FDG PET/CT for detecting postoperative recurrence in patients with gastric cancer.

Methods

This single-center retrospective clinical study was performed at Hunan Cancer Hospital between December 2020 and June 2022. The participants underwent both [18F]AlF-NOTA-FAPI-04 and [18F]FDG within 14 days. Histopathologic examination, morphological imaging, and/or follow-up imaging were used as a reference for the final diagnosis. We recorded the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy of [18F]AlF-NOTA-FAPI-04 and [18F]FDG PET/CT for detecting local recurrence, lymph node metastasis and distant metastasis. The SUVmax and background ratio (TBR) of local recurrence and metastases between [18F]FDG and [18F]AlF-NOTA-FAPI-04 PET/CT were compared using paired-sample t tests.

Results

Forty-seven patients (27 males, aged 25-68 years) with gastric cancer after curative resection (27 with adenocarcinoma, 17 with signet ring cell carcinoma and 4 with mucinous adenocarcinoma) were included in the study. [18F]AlF-NOTA-FAPI-04 accumulation was significantly greater than that of [18F]FDG in terms of local recurrence (SUVmax, 11.65 vs 3.48, p< 0.0001; TBR, 12.93 vs 2.94, p< 0.0001), lymph node metastasis (SUVmax, 13.45 vs 3.05, p=0.003875; TBR, 12.43 vs 2.21, p=0.001661), and distant metastasis (SUVmax, 11.89 vs 2.96, p < 0.0001; TBR, 13.32 vs 2.32, p< 0.0001). Despite no statistical comparison was made with [18F]FDG, [18F]AlF-NOTA-FAPI-04 imaging exhibited high levels of sensitivity, specificity, PPV, NPV, and accuracy for detecting postoperative local recurrence, lymph node metastasis, and distant metastasis in patients with gastric cancer.

Conclusion

[18F]AlF-NOTA-FAPI-04 has demonstrated potential for more accurate tumor re-evaluation in GC, thus enhancing treatment decision-making.

A head-to-head comparison of [68Ga]Ga-DOTATATE and [68Ga]Ga-FAPI PET/CT in patients with nasopharyngeal carcinoma: a single-center, prospective study

PURPOSE

We aimed to compare the staging efficiency of [68Ga]Ga-DOTATATE and [68Ga]Ga-FAPI PET/CT in nasopharyngeal carcinoma (NPC) patients.

METHODS

Thirty-nine patients with pathologically confirmed NPC were enrolled in this prospective study. Each patient underwent paired [68Ga]Ga-DOTATATE and [68Ga]Ga-FAPI PET/CT on 2 successive days. The accuracy of two PET/CT for assessing T, N, and M stages was compared by using head-and-neck MRI, histopathologic diagnosis and follow-up results as reference standards. The radiotracer uptake derived from two PETs was also compared.

RESULTS

For treatment-naïve patients, [68Ga]Ga-DOTATATE PET/CT showed identical sensitivity for the primary tumours but clearer tumor delineation induced by higher tumour-to-background (TBR) ratio (19.1 ± 8.7 vs. 12.4 ± 7.7, P = 0.003), compared with [68Ga]Ga-FAPI PET/CT. Regarding cervical lymph node (CLN) metastases, [68Ga]Ga-DOTATATE PET had significantly better sensitivity and accuracy based on neck sides (98% vs. 82%, P < 0.001; 99% vs. 88% P = 0.008), neck levels (98% vs. 78%, 99% vs. 97%; both P < 0.001) and individual nodes (89% vs. 56%, 91% vs. 76%; both P < 0.001), and higher TBR (8.1 ± 4.1 vs. 6.3 ± 3.7, P < 0.001). Additionally, [68Ga]Ga-DOTATATE PET/CT revealed higher sensitivity and accuracy for distant metastases (96% vs. 53%, 95% vs. 52%; both P < 0.001), particularly in bone metastases (99% vs. 49%, 97% vs. 49%; both P < 0.001). For post-treatment patients, [68Ga]Ga-DOTATATE PET/CT identified one more true-negative case than [68Ga]Ga-FAPI PET/CT.

CONCLUSION

[68Ga]Ga-DOTATATE PET/CT performed better than [68Ga]Ga-FAPI PET/CT in visualizing the primary tumours, detecting the metastatic lesions and identifying the local recurrence, suggesting [68Ga]Ga-DOTATATE PET/CT may be superior to [68Ga]Ga-FAPI PET/CT for NPC staging.

The comparative utility of FAPI-based PET radiotracers over [18F]FDG in the assessment of malignancies

Fibroblast activation protein (FAP) is a type II transmembrane serine protease overexpressed in cancer-associated fibroblasts (CAFs) and has been associated with poor prognosis. PET/CT imaging with radiolabeled FAP inhibitors (FAPI) is currently being studied for various malignancies. This review identifies the uses and limitations of FAPI PET/CT in malignancies and compares the advantages and disadvantages of FAPI and 18F-fluorodeoxyglucose ([18F]FDG). Due to high uptake, rapid clearance from the circulation, and limited uptake in normal tissue, FAPI tumor-to-background contrast ratios are equivalent to or better than [18F]FDG in most applications. In several settings, FAPI has shown greater uptake specificity than [18F]FDG and improved sensitivity in detecting lymph node, bone, and visceral tissue metastases. Therefore, FAPI PET/CT may be complementary in distinguishing pathological lesions with conventional imaging, determining the primary site of malignancy, improving tumor staging, and detecting disease recurrence, especially in patients with inconclusive [18F]FDG PET/CT findings. Nevertheless, FAPI has limitations, including certain settings with non-specific uptake, modified uptake with age and menopause status, challenges with clinical access, and limited clinical evidence.

The Role of Fibroblast Activation Protein Inhibitor Positron Emission Tomography in Inflammatory and Infectious Diseases: An Updated Systematic Review

The role of fibroblast activation protein inhibitor (FAPI) positron emission tomography/computed tomography (PET/CT) is emerging for the assessment of non-oncological diseases, such as inflammatory and infectious diseases, even if the evidence in the literature is still in its initial phases. We conducted a systematic search of Scopus, PubMed/MEDLINE, Embase, and Cochrane library databases for studies published before 31 December 2023 reporting infectious and inflammatory disease imaging with FAPI PET/CT. We included twenty-one studies for a total of 1046 patients. The most frequent disease studied was lung interstitial disease, investigated in six studies for a total of 200 patients, followed by bone and joint diseases in two studies and 185 patients, IgG4-related disease in 53 patients, and Crohn's disease in 30 patients. Despite the heterogeneity of studies in terms of study design and technical features, FAPI PET/CT showed a high detection rate and diagnostic role. Moreover, when compared with 2-[18F]FDG PET/CT (n = 7 studies), FAPI PET/CT seems to have better diagnostic performances. The presence of chronic inflammation and tissue remodeling, typical of immune-mediated inflammatory conditions, may be the underlying mechanism of FAPI uptake.

Nonmalignant 68 Ga-FAPI-46 Uptake in 2 Cases of TENIS Syndrome : Comparison With 18 F-FDG

ABSTRACT

The cancer specificity of fibroblast activation protein inhibitor (FAPI) uptake remains understudied topic, and several cases of false-positive FAPI PET/CT findings have been reported. We present 2 patients of differentiated thyroid cancer with thyroglobulin elevation and negative iodine scintigraphy syndrome who underwent 68 Ga-FAPI PET/CT and 18 F-FDG PET/CT for evaluation of any possible sites for metastatic disease. Although no focus of metastatic disease was found in these patients, remarkable findings were noticed instead. Nonmalignant FAPI uptake was evident in the gallbladder, uterus, and degenerative changes, whereas these uptake were discordant or partially concordant with those in FDG's, also CT images showed no underlying abnormality.

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.

How to Perform FAPI PET? An Expedited Systematic Review Providing a Recommendation for FAPI PET Imaging With Different FAPI Tracers

This expedited systematic review aims to provide the first overview of the different Fibroblast activation protein inhibitor (FAPI) PET scan procedures in the literature and discuss how to efficiently obtain optimal FAPI PET images based on the best available evidence. The PubMed, Embase, Cochrane Library, and Web of Science databases were systematically searched in April 2023. Peer-reviewed cohort studies published in English and used FAPI tracers were included. Articles were excluded if critical scan procedure information was missing, or the article was not retrievable from a university library within 30 days. Data were grouped according to the FAPI tracer applied. Meta-analysis with proper statistics was deemed not feasible based on a pilot study. A total of 946 records were identified. After screening, 159 studies were included. [68Ga]Ga-FAPI-04 was applied in 98 studies (61%), followed by [68Ga]Ga-FAPI-46 in 19 studies (12%). Most studies did not report specific patient preparation. A mean/median administered activity of 80-200 MBq was most common; however, wide ranges were seen in [68Ga]Ga-FAPI-04 PET studies (56-370 MBq). An injection-to-scan-time of 60 minutes was dominant for all FAPI PET studies. A possible trend toward shorter injection-to-scan times was observed for [68Ga]Ga-FAPI-46. Three studies evaluated [68Ga]Ga-FAPI-46 PET acquisition at multiple time points in more than 593 cancer lesions, all yielding equivalent tumor detection at 10 minutes vs later time points despite slightly lower tumor-to-background Ratios. Despite the wide ranges, most institutions administer an average of 80-200 MBq [68Ga]Ga-FAPI-04/46 and scan patients at 60 minutes postinjection. For [68Ga]Ga-FAPI-46, the present evidence consistently supports the feasibility of image acquisition earlier than 30 minutes. Currently, data on the optimal FAPI PET scan procedure are limited, and more studies are encouraged. The current review can serve as a temporary guideline for institutions planning FAPI PET studies.

Impact of FAPI-46/dual-tracer PET/CT imaging on radiotherapeutic management in esophageal cancer

BACKGROUND

Fibroblast activation protein (FAP) is expressed in the tumor microenvironment (TME) of various cancers. In our analysis, we describe the impact of dual-tracer imaging with Gallium-68-radiolabeled inhibitors of FAP (FAPI-46-PET/CT) and fluorodeoxy-D-glucose (FDG-PET/CT) on the radiotherapeutic management of primary esophageal cancer (EC).

METHODS

32 patients with EC, who are scheduled for chemoradiation, received FDG and FAPI-46 PET/CT on the same day (dual-tracer protocol, 71%) or on two separate days (29%) We compared functional tumor volumes (FTVs), gross tumor volumes (GTVs) and tumor stages before and after PET-imaging. Changes in treatment were categorized as "minor" (adaption of radiation field) or "major" (change of treatment regimen). Immunohistochemistry (IHC) staining for FAP was performed in all patients with available tissue.

RESULTS

Primary tumor was detected in all FAPI-46/dual-tracer scans and in 30/32 (93%) of FDG scans. Compared to the initial staging CT scan, 12/32 patients (38%) were upstaged in nodal status after the combination of FDG and FAPI-46 PET scans. Two lymph node metastases were only visible in FAPI-46/dual-tracer. New distant metastasis was observed in 2/32 (6%) patients following FAPI-4 -PET/CT. Our findings led to larger RT fields ("minor change") in 5/32 patients (16%) and changed treatment regimen ("major change") in 3/32 patients after FAPI-46/dual-tracer PET/CT. GTVs were larger in FAPI-46/dual-tracer scans compared to FDG-PET/CT (mean 99.0 vs. 80.3 ml, respectively (p < 0.001)) with similar results for nuclear medical FTVs. IHC revealed heterogenous FAP-expression in all specimens (mean H-score: 36.3 (SD 24.6)) without correlation between FAP expression in IHC and FAPI tracer uptake in PET/CT.

CONCLUSION

We report first data on the use of PET with FAPI-46 for patients with EC, who are scheduled to receive RT. Tumor uptake was high and not depending on FAP expression in TME. Further, FAPI-46/dual-tracer PET had relevant impact on management in this setting. Our data calls for prospective evaluation of FAPI-46/dual-tracer PET to improve clinical outcomes of EC.

Evaluating the Patterns of FAPI Uptake in the Shoulder Joint: a Preliminary Study Comparing with FDG Uptake in Oncological Studies

BACKGROUND

Fibroblast activation protein inhibitor (FAPI) targeting PET has been introduced as a novel molecular imaging modality for visualizing cancer-associated fibroblasts. There have also been reports suggesting incidental findings of localized accumulation in the shoulder joints. However, further characterization in a larger patient cohort is still lacking.

METHODS

77 consecutive patients (28 females; mean age, 63.1 ± 11.6) who underwent Ga-68 FAPI-04 PET/CT for diagnosis of solid tumors were included. The incidence and localization of tracer uptake in shoulder joints were investigated and compared with available F-18 FDG scans serving as reference.

RESULTS

Ga-68 FAPI-04 uptake was evaluated in 77 patients (154 shoulder joints), of whom 54 subjects (108 shoulder joints) also had available F-18 FDG scans for head-to-head comparison. On FAPI-targeted imaging, 67/154 shoulders (43.5%) demonstrated increased radiotracer accumulation in target lesions, which were distributed as follows: acromioclavicular (AC) joints in 25/67 (37.3%), followed by glenohumeral and subacromial (GH + SA) joints in 23/67 (34.3%), or both (AC and GH + SA joints) in the remaining 19/67 (28.4%). Ga-68 FAPI-04 correlated with quantified F-18 FDG uptake (r = 0.69, p < 0.0001). Relative to the latter radiotracer, however, in-vivo FAP expression in the shoulders was significantly increased (Ga-68 FAPI-04, 4.7 ± 3.2 vs F-18 FDG, 3.6 ± 1.3, p < 0.001).

CONCLUSION

Our study revealed focal accumulation of Ga-68 FAPI-04 in the shoulders, particularly in the AC joints, with higher uptake compared to the inflammatory-directed PET radiotracer F-18 FDG in oncological studies. As a result, further trials are warranted to investigate the potential of FAPI-directed molecular imaging in identifying chronic remodeling in shoulder joints. This could have implications for initiating anti-FAP targeted photodynamic therapy based on PET signal strength.

Application of Nuclear Medicine Techniques in Musculoskeletal Infection: Current Trends and Future Prospects

Nuclear medicine has become an indispensable discipline in the diagnosis and management of musculoskeletal infections. Radionuclide tests serve as a valuable diagnostic tool for patients suspected of having osteomyelitis, spondylodiscitis, or prosthetic joint infections. The choice of the most suitable imaging modality depends on various factors, including the affected area, potential extra osseous involvement, or the impact of previous bone/joint conditions. This review provides an update on the use of conventional radionuclide imaging tests and recent advancements in fusion imaging scans for the differential diagnosis of musculoskeletal infections. Furthermore, it examines the role of radionuclide scans in monitoring treatment responses and explores current trends in their application. We anticipate that this update will be of significant interest to internists, rheumatologists, radiologists, orthopedic surgeons, rehabilitation physicians, and other specialists involved in musculoskeletal pathology.

Synthesis and Preliminary Study of 99mTc-Labeled HYNIC-FAPi for Imaging of Fibroblast Activation Proteins in Tumors

Fibroblast activation protein (FAP) is an emerging target for cancer diagnosis. Different types of FAP inhibitor (FAPI)-based radiotracers have been developed and applied for tumor imaging. However, few FAPI tracers for single photon emission computed tomography (SPECT) imaging have been reported. SPECT imaging is less expensive and more widely distributed than positron emission tomography (PET), and thus, 99mTc-labeled FAPIs would be more available to patients in developing regions. Herein, we developed a FAPI-04-derived radiotracer, HYNIC-FAPi-04 (HFAPi), for SPECT imaging. 99mTc-HFAPi, with a radiochemical purity of >98%, was prepared using a kit formula within 30 min. The specificity of 99mTc-HFAPi for FAP was validated by a cell binding assay in vitro and SPECT/CT imaging in vivo. The binding affinity (Kd value) of 99mTc-HFAPi for human FAP and murine FAP was 4.49 and 2.07 nmol/L, respectively. SPECT/CT imaging in HT1080-hFAP tumor-bearing mice showed the specific FAP targeting ability of 99mTc-HFAPi in vivo. In U87MG tumor-bearing mice, 99mTc-HFAPi had a higher tumor uptake compared with that of HT1080-hFAP and 4T1-mFAP tumor models. Interestingly, 99mTc-HFAPi showed a relatively high uptake in some murine joints. 99mTc-HFAPi accumulated in tumor lesions with a high tumor-to-background ratio. A preliminary clinical study was also performed in breast cancer patients. Additionally, 99mTc-HFAPi exhibited an advantage over 18F-FDG in the detection of lymph node metastatic lesions in breast cancer patients, which is helpful in improving treatment strategies. In short, 99mTc-HFAPi showed excellent affinity and specificity for FAP and is a promising SPECT radiotracer for (re)staging and treatment planning of breast cancers.

Potential utility of [68Ga]DOTA-FAPI-04 as a broad-spectrum benign disease imaging agent-comparison with [18F]FDG and [99mTc]MDP

OBJECTIVES

To compare the detection performance of [68Ga]DOTA-FAPI-04 positron emission tomography/computed tomography (PET/CT) and [18F]FDG PET/CT for patients with various benign diseases and that of [68Ga]DOTA-FAPI-04 PET/CT and [99mTc]MDP single-photon emission computed tomography/computed tomography (SPECT/CT) for patients with benign bone lesions.

METHODS

This prospective study included 89 patients who underwent [18F]FDG and [68Ga]DOTA-FAPI-04 PET/CT and 22 patients who underwent [99mTc]MDP SPECT/CT and [68Ga]DOTA-FAPI-04 PET/CT. Detection performance of [68Ga]DOTA-FAPI-04, [18F]FDG PET/CT, and [99mTc]MDP SPECT/CT for benign lesions was compared using the T/B value, T value, and true positive rate. Paired sample t-tests were conducted for comparisons.

RESULTS

This study comprised 53 men and 58 women (mean age, 53 ± 15.75 years). [68Ga]DOTA-FAPI-04 demonstrated a higher uptake and detection rate for fibrotic disease (SUVmax [FAPI vs FDG: 6.26 ± 1.61 vs 2.38 ± 1.26], p1 < 0.001; T/B value [FAPI vs FDG: 9.38 ± 2.78 vs 1.95 ± 1.33], p2 < 0.001; and true positive rate 100% vs 26.3%), infectious disease (T/B value [FAPI vs FDG: 14.15 ± 12.42 vs 6.70 ± 5.87], p2 < 0.05; and true positive rate 94% vs 82%), and benign tumor (T/B value [FAPI vs FDG: 6.49 ± 5.85 vs 1.96 ± 2.03], p2 < 0.05; and true positive rate 70% vs 52%) than [18F]FDG. [68Ga]DOTA-FAPI-04 demonstrated a lower uptake and a comparable detection rate for benign bone disease (T/B value [FAPI vs FDG: 5.46 ± 2.91 vs 23.58 ± 15.37], p < 0.001; and true positive rate 90% vs 99%) than [99mTc]MDP.

CONCLUSION

[68Ga]DOTA-FAPI-04 could be used as a benign disease imaging agent to complement traditional nuclide imaging agents.

CLINICAL RELEVANCE STATEMENT

Our study showed that [68Ga]DOTA-FAPI-04 PET/CT could provide reliable imaging evidence for clinical detection and diagnosis of various benign lesions, such as inflammatory, infectious, fibrotic diseases, and benign tumors.

KEY POINTS

• [68Ga]DOTA-FAPI-04 demonstrated a higher uptake and detection rate for fibrotic disease, infectious disease, and benign tumor than [18F]FDG. • [68Ga]DOTA-FAPI-04 demonstrated an equivalent detection efficacy to [18F]FDG for inflammatory diseases. • [68Ga]DOTA-FAPI-04 demonstrated a lower uptake and a comparable detection rate for benign bone disease than [99mTc]MDP.

Incidental FAPI Localization in Benign Bone Lesions in a Case of Carcinoma Stomach

ABSTRACT

Recently developed radiolabeled FAPI (fibroblast-activation protein inhibitors) have attracted researcher's attention in diagnosing various tumors because of its high specificity and better tumor-to-background ratio. Increasing use of 68 Ga-FAPI PET/CT has resulted in reporting of incidental benign findings as well. We hereby present a case of suspected carcinoma stomach showing increased uptake in primary tumor in stomach as well as incidental uptake in benign subchondral cysts in bilateral shoulder joints and right hip joint on 68 Ga-FAPI PET CT scan.

Increased 18 F-FAPI-04 Uptake in Vertebral Hemangioma

ABSTRACT

A 54-year-old man with a history of colectomy for colorectal cancer and subsequent liver metastasectomy underwent 18 F-FDG PET/CT and 18 F-FAPI-04 PET/CT scans to evaluate possible hepatic metastasis revealed by contrast-enhanced MRI. Both studies showed similarly increased uptake in liver metastases, and 18 F-FDG detected a metastatic lung nodule. Furthermore, the images showed an incidental finding of increased uptake of 18 F-FAPI-04 in the L3 vertebral, which was not 18 F-FDG-avid. A review of the patient's previous CT and MRI scans suggests vertebral hemangioma.

Increased 18F-FAPI uptake in bones and joints of lung cancer patients: characteristics and distributions

OBJECTIVE

This study investigated the distribution and characteristics of various bone and joint lesions on 18F-FAPI PET/CT in lung cancer patients.

METHODS

Seventy-four lung cancer patients who underwent 18F-FAPI PET/CT were reviewed. Bone and joint lesions with elevated 18F-FAPI uptake were recorded and analyzed. The distribution and maximum uptake value (SUVmax) of different benign lesions or bone metastases were presented. In addition, the SUVmax of bone metastases on 18F-FDG and 18F-FAPI-04 PET/CT were also compared.

RESULTS

In 53 patients, a total of 262 lesions presented 18F-FAPI accumulation. Bone metastases were mainly in vertebrae, pelvis, and ribs, while benign lesions were in vertebral margins, alveolar bone, and shoulder joints. The SUVmax of bone metastases was significantly higher than that of benign lesions ([Formula: see text] vs. [Formula: see text], [Formula: see text]), with NSCLC cases having higher SUVmax values than SCLC cases ([Formula: see text] vs. [Formula: see text], [Formula: see text]). Among benign lesions, arthritis and periodontitis demonstrated higher SUVmax than degenerative lesions (arthritis: [Formula: see text]; periodontitis: [Formula: see text]; degenerative diseases: [Formula: see text]; [Formula: see text] and [Formula: see text], respectively). The SUVmax of bone metastases was comparable between 18F-FDG and 18F-FAPI PET/CT. However, 18F-FAPI PET/CT was found to be superior in identifying cranial metastases compared to 18F-FDG PET/CT (TBRmet/brain: [Formula: see text] vs. [Formula: see text], [Formula: see text]).

CONCLUSION

This study demonstrated that 18F-FAPI PET/CT is a valuable imaging modality for detecting bone and joint lesions in lung cancer patients. The SUVmax of malignant lesions was higher than that of benign lesions, but cannot accurately distinguish benign and malignant lesions. The uptake of FAPI differs among lesions with different pathological types.

FAPI-avid nonmalignant PET/CT findings: An expedited systematic review

Fibroblast activation protein inhibitor (FAPI) is a promising tracer in oncologic positron emission tomography/computed tomography (PET/CT). Numerous studies have demonstrated the superior sensitivity of FAPI PET/CT over fluorodeoxyglucose (FDG) PET/CT in several types of cancer. However, the cancer specificity of FAPI uptake remains understudied, and several cases of false-positive FAPI PET/CT findings have been reported. A systematic search of PubMed, Embase, and Web of Science was conducted for studies published prior to April 2022 reporting nonmalignant FAPI PET/CT findings. We included original peer-reviewed articles of studies in humans using FAPI tracers radiolabeled with 68Ga or 18F that were published in English. Papers without original data and studies with insufficient information were excluded. Nonmalignant findings were presented on a per-lesion basis and grouped according to the type of organ or tissue involved. The search identified a total of 1.178 papers, of which 108 studies were eligible. Eighty studies were case reports (74%), and the remaining 28 were cohort studies (26%). A total of 2.372 FAPI-avid nonmalignant findings were reported, with the most frequent being uptake in the arteries, e.g., related to plaques (n = 1178, 49%). FAPI uptake was also frequently related to degenerative and traumatic bone and joint lesions (n = 147, 6%) or arthritis (n = 92, 4%). For organs, diffuse or focal uptake was often seen in cases of inflammation, infection, fibrosis, and IgG4-related disease (n = 157, 7%). FAPI-avid inflammatory/reactive lymph nodes (n = 121, 5%) and tuberculosis lesions (n = 51, 2%) have been reported and could prove to be potential pitfalls in cancer staging. Periodontitis (n = 76, 3%), hemorrhoids (n = 47, 2%), and scarring/wound healing (n = 35, 2%) also presented as focal uptake on FAPI PET/CT. The present review provides an overview of the reported FAPI-avid nonmalignant PET/CT findings to date. A large number of benign clinical entities may show FAPI uptake and should be kept in mind when interpreting FAPI PET/CT findings in patients with cancer.

Prospective Comparison of the Imaging Value of 99m Tc-MDP Bone Scan and 68 Ga-FAPI-04 PET/CT in Synovitis, Acne, Pustulosis, Hyperostosis, and Osteitis Syndrome

PURPOSE

This study aimed to explore the imaging value of 68 Ga-FAPI-04 PET/CT in synovitis, acne, pustulosis, hyperostosis, and osteitis (SAPHO) syndrome and compare it with that of 99m Tc-MDP bone scan.

METHODS

Nineteen participants with SAPHO syndrome underwent 68 Ga-FAPI-04 PET/CT and 99m Tc-MDP bone scan. Demographic data and clinical features were recorded, SAPHO imaging features were analyzed, and the osteoarticular lesion detection rate in both methods was calculated.

RESULTS

This prospective study recruited 4 men and 15 women aged 52.4 ± 8.6 years. The anterior chest wall was involved in all participants (100%). Palmoplantar pustulosis was the most common (36.8%) skin symptom. 99m Tc-MDP bone scan and 68 Ga-FAPI-04 PET/CT together detected 84 osteoarticular lesions, of which 91.7% (77/84) were detected by the former and 96.4% (81/84) by the latter. Furthermore, 68 Ga-FAPI-04 PET/CT detected 5 cases of knee and hip joint synovitis.

CONCLUSIONS

68 Ga-FAPI-04 PET/CT was more sensitive than 99m Tc-MDP bone scan when evaluating osteoarticular lesions in SAPHO syndrome and could also evaluate synovial lesions. 68 Ga-FAPI-04 PET/CT could be a good imaging method for SAPHO syndrome but requires further verification in a more extensive research cohort.

Q.Clear reconstruction for reducing the scanning time for 68 Ga-DOTA-FAPI-04 PET/MR imaging

PURPOSE

This study aims to determine whether Q.Clear positron emission tomography (PET) reconstruction may reduce tracer injection dose or shorten scanning time in 68Gallium-labelled fibroblast activation protein inhibitor (68 Ga-FAPI) PET/magnetic resonance (MR) imaging.

METHODS

We retrospectively collected cases of 68 Ga-FAPI whole-body imaging performed on integrated PET/MR. PET images were reconstructed using three different methods: ordered subset expectation maximization (OSEM) reconstruction with full scanning time, OSEM reconstruction with half scanning time, and Q.Clear reconstruction with half scanning time. We then measured standardized uptake values (SUVs) within and around lesions, alongside their volumes. We also evaluated image quality using lesion-to-background (L/B) ratio and signal-to-noise ratio (SNR). We then compared these metrics across the three reconstruction techniques using statistical methods.

RESULTS

Q.Clear reconstruction significantly increased SUVmax and SUVmean within lesions (more than 30%) and reduced their volumes in comparison with OSEM reconstruction. Background SUVmax also increased significantly, while background SUVmean showed no difference. Average L/B values for Q.Clear reconstruction were only marginally higher than those from OSME reconstruction with half-time. SNR decreased significantly in Q.Clear reconstruction compared with OSEM reconstruction with full time (but not half time). Differences between Q.Clear and OSEM reconstructions in SUVmax and SUVmean values within lesions were significantly correlated with SUVs within lesions.

CONCLUSIONS

Q.Clear reconstruction was useful for reducing PET injection dose or scanning time while maintaining the image quality. Q.Clear may affect PET quantification, and it is necessary to establish diagnostic recommendations based on Q.Clear results for Q.Clear application.

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.

Hybrid PET/MRI in Infection and Inflammation: An Update About the Latest Available Literature Evidence

PET/MRI has been reported to be promising in the diagnosis and evaluation of infection and inflammation including brain disorders, bone and soft tissue infections and inflammations, cardiovascular, abdominal, and systemic diseases. However, evidence came out manly from anecdotal cases or small cohorts. The present review aimed to update the latest available evidence about the role of PET/MRI in infection and inflammation. The search (January, 1 2018-July, 8 2022) on PubMed produced 504 results. Sixty-five articles were selected and included in the qualitative synthesis. The number of publications on PET/MRI in the 3 years 2018-2020 was comparable, while it increased in 2021 and 2022 (from 11 to 17 and 15, respectively). [¹⁸F]FDG and ⁶⁸Ga-DOTA-FAPI-04 were the most frequently used (42/65) and innovative radiopharmaceuticals, respectively. [¹⁸F]fluoride (9/65), translocator protein (TSPO)-targeted PET agents (6/65), CXCR4 receptor targeting tracer and β-amyloid plaques binding radiopharmaceuticals (2/65 and 2/65, respectively) were also used. Most PET/MRI studies in the period 2018-2022 focused on inflammation (55/65), and cardiovascular diseases represented the most frequent field of interest (30/65), also when considering each year singularly. An increasing trend in bone and joint publications was observed in the considered period (12/65). Other topics included neurology (11/65), inflammatory bowel disease (8/65), and other (4/65). PET/MRI technology demonstrated to be useful in infection and inflammation, being superior to each single modality and/or facilitating diagnosis in a number of conditions (eg, cardiac sarcoidosis, myocarditis, endocarditis), and/or allowing to provide insightful information about disease biology and apply innovative radiopharmaceuticals (eg, neurology, atherosclerosis). Publications focused on PET/MRI in large vessel vasculitis and aortic diseases include both diagnostic and discovery objectives. The current review corroborates the potential of PET/MRI - combining in a single examination the high soft tissue contrast, high resolution, and functional information of MRI, with molecular data provided by PET technology - to positively impact on the management of infectious diseases and inflammatory conditions.

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

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

[18F]FAPI-42 PET/CT versus [18F]FDG PET/CT for imaging of recurrent or metastatic gastrointestinal stromal tumors

PURPOSE

PET has been important for monitoring recurrence and metastasis of Gastrointestinal Stromal Tumors (GISTs) and the selection of therapeutic strategies. A significant portion of GISTs lesions show negative FDG uptake and therefore calls for more tumor-specific imaging biomarkers. This study compared the imaging performance of [18F]FAPI-42 PET/CT and [18F]FDG PET/CT in recurrent or metastatic gastrointestinal stromal tumors (R/M GISTs).

METHODS

This study retrospectively included 35 patients with R/M GISTs who underwent both FAPI PET/CT and FDG PET/CT. The definite diagnosis was confirmed by pathology or follow-up drug treatment effects. The differences in detection rates and tumor-to-background SUVmax ratio (SUVTBR) of different locations between dual-tracer PET/CT were compared. Factors including tumor size, degree of enhancement, type of gene mutation, and targeted treatment potentially influencing the uptake of both tracers were assessed. The excised lesions (n = 3) underwent immunohistochemical staining to verify FAP expression in the tissue.

RESULTS

A total of 106 lesions in 35 patients were identified, out of which 38/106 (35.8%) lesions (FAPI + /FDG -) were additionally detected by FAPI PET/CT as compared to that by FDG, including 26 liver metastases, ten peritoneal metastases, one gastrointestinal recurrence, and one bone metastasis. The positive detection rate of FAPI PET/CT for recurrent or metastatic GISTs was higher than that of FDG (80.2% vs. 53.8%,  P< 0.001), especially in liver metastases (87.5% vs. 33.3%, P< 0.001). Moreover, the SUVTBR of liver metastases of GISTs in FAPI PET/CT was higher than that in FDG [2.4 (0.3 to 11.2) vs. 0.9 (0.3 to 6.5), P< 0.001]. The longest diameter of tumors in the FDG-positive group was higher than that of the FDG-negative group (P= 0.005); still, it did not differ between the FAPI-positive group and the FAPI-negative group. No difference in the degree of enhancement was observed between both tracers' positive and negative groups. Besides, the SUVTBR of FDG but not FAPI differed significantly among various gene mutations (P< 0.001) as well as the targeted therapy and no targeted therapy groups (P= 0.001). FAP was expressed in R/M GISTs, and the uptake of FAPI corresponded to the level of FAP expression.

CONCLUSION

In conclusion, FAPI for imaging of R/M GISTs could be superior to FDG, specifically for liver metastases. The uptake of FAPI could reflect the level of FAP expression, and it was independent of tumor size, degree of enhancement, type of gene mutation, and targeted therapy as compared to FDG.

Fibroblast Activation Protein Inhibitor Imaging in Nonmalignant Diseases: A New Perspective for Molecular Imaging

Fibroblast activation protein-α (FAP-α) is a type II transmembrane glycoprotein that is overexpressed in activated fibroblasts such as those in the stroma of tumors or in the fibrotic processes accompanying various benign diseases. The recent development and clinical implementation of radiolabeled quinolone-based tracers suitable for PET that act as FAP inhibitors (FAPIs) have opened a new perspective in molecular imaging. Although multiple studies have investigated the use of FAPI imaging in cancer, evidence concerning its use in nonmalignant diseases is still scarce. Herein, we provide a comprehensive review of FAPI imaging in nonmalignant diseases to clarify the current and potential role of this class of molecules in nuclear medicine.

Non-tumoral uptake of 68Ga-FAPI-04 PET: A retrospective study

OBJECTIVE

Fibroblast activation protein (FAP)-targeting radiopharmaceutical based on the FAP-specific inhibitor (FAPI) is considered as a potential alternative agent to FDG for tumor-specific imaging. However, FAP is also expressed in normal adult tissues. The aim of this study was to explore the image features of non-tumoral regions with high uptake of ⁶⁸Ga-FAPI-04 in positron emission tomography (PET) imaging and to reveal the physiological mechanisms of these regions.

MATERIAL

A total of 137 patients who underwent whole-body ⁶⁸Ga-FAPI-04 PET/MR (n=46) or PET/CT (n=91) were included in this retrospective study. Three experienced nuclear medicine physicians determined the non-tumoral regions according to other imaging modalities (CT, MRI, ¹⁸F-FDG PET, or ultrasound), clinical information, or pathological results. The regions of interest (ROIs) were drawn manually, and the maximum standardized uptake value (SUV_max) was measured.

RESULTS

A total of 392 non-tumoral uptake regions were included in this study. The included physiological regions were uterus (n=38), submandibular gland (n=118), nipple (n=37), gingiva (n=65), and esophagus (n=31). The incidence of ⁶⁸Ga-FAPI-04 uptake in physiological regions was independent of age, the tracer uptakes in the gingiva and esophagus were more common in male patients (p=0.006, 0.009), while that in the nipple was more common in female patients (p < 0.001). The included benign regions were inflammatory lymph node (n =10), pneumonia (n=13), atherosclerosis (n=10), pancreatitis (n=18), osteosclerosis (n=45), and surgical scar (n=7). No significant difference was observed in SUV_max between physiological and benign regions.

CONCLUSIONS

A number of organs exhibit physiological uptakes of ⁶⁸Ga-FAPI-04. Our study showed that regions with high ⁶⁸Ga-FAPI-04 uptake did not necessarily represent malignancy. Being familiar with physiological and typical benign ⁶⁸Ga-FAPI-04 uptake regions can be helpful for physicians to interpret images and to make an accurate diagnosis.

Physiological tracer distribution and benign lesion incidental uptake of Al18F-NOTA-FAPI-04 on PET/CT imaging

OBJECTIVE

To systematically investigate the physiological distribution and benign lesion incidental uptake of Al18F-NOTA-FAPI-04 (18F-FAPI) in cancer patients to establish the normal uptake range in relevant organs and lesions.

METHODS

Twenty patients who underwent 18F-FAPI PET/CT imaging were retrospectively assessed. Organ and benign lesion tracer uptake was quantified based on standardized uptake values (SUVmax and SUVmean). We compared the variation in tracer uptake in certain organs between men and women, analyzed the possible reasons for diffuse uptake in the thyroid, and assessed tracer uptake variations in the uterus in different menstrual cycle phases. Incidental tracer uptake in benign lesions was also assessed.

RESULTS

Physiological 18F-FAPI uptake was observed in the urinary tract, biliary tract system, submandibular glands, pancreas, thyroid, uterus, intestine, prostate gland, parotid gland, myocardium, kidney cortex, and muscles, but not the brain, lungs, liver, spleen, colon, and breasts. The SUVmean for each organ was similar for women and men (all P > 0.05). Diffuse tracer uptake in the thyroid was caused by normal thyroid or thyroiditis; there were no statistically significant differences between them (SUVmax: t = -1.3, P = 0.25; SUVmean: t = -1.1, P = 0.31). There was a significant difference for uterus uptake among different menstrual cycle phases (SUVmax: F = 5.08, P = 0.04; SUVmean: F = 5.19, P = 0.04). Incidental benign lesion tracer uptake was observed in patients with esophagitis, thyroiditis, arthritis, fractures, and uterine fibroids.

CONCLUSION

This study provides a reference range for 18F-FAPI uptake in relevant organs and benign lesions. Benign lesion 18F-FAPI uptake may reduce 18F-FAPI PET/CT specificity.

18F-FDG and 68 Ga-FAPI PET/CT for the evaluation of periprosthetic joint infection and aseptic loosening in rabbit models

Purpose

We built a joint replacement loosening model based on the original rabbit model of infection and evaluated the performance characteristics of ¹⁸F-FDG and ⁶⁸ Ga-FAPI in evaluating infection and loosening.

Methods

After surgery, the rabbits were divided into four groups, with six individuals in the control group and 10 each in the aseptic loosening, S. aureus and S. epidermidis groups. PET/CT and serological examination were performed three times at two-week intervals. After the rabbits were euthanized, micro-CT, tissue pathology, pullout tests and scanning electron microscopy (SEM) were performed.

Results

The pullout test and SEM showed the feasibility of the aseptic loosening model. ¹⁸F-FDG showed similar performance in the control and loosening groups. The SUVmax of the S. aureus group was consistently higher than that of the S. epidermidis group. As for ⁶⁸ Ga-FAPI, the SUVmax of the control group was lowest in the second week and gradually increased over subsequent weeks. The SUVmax of the loosening group began to exceed that of the control group after the second week. The SUVmax of the S. aureus group in the second week was the lowest among the four groups and increased as the number of weeks increased. The pathology results showed concordance with the performance of PET/CT. Linear regressions between SUVmax and serology showed that ¹⁸F-FDG was positively correlated with CRP and IL-6, while ⁶⁸ Ga-FAPI revealed negative correlations with CRP and IL-6 in the second week and positive correlations in the sixth week. In addition, the SUVmax and MT(target)V of both ¹⁸F-FDG and ⁶⁸ Ga-FAPI were negatively correlated with bone volume/trabecular volume (TV) and bone surface area/TV.

Conclusion

In this longitudinal observation, ⁶⁸ Ga-FAPI showed greater sensitivity than ¹⁸F-FDG in detecting diseases, and ⁶⁸ Ga-FAPI had no intestinal or muscular uptake. The MT(target)V of ⁶⁸ Ga-FAPI was larger than that of ¹⁸F-FDG, which meant that ⁶⁸ Ga-FAPI had the potential to define the scope of lesions more accurately. Finally, the SUVmax of ⁶⁸ Ga-FAPI could not differentiate between loosening and infection; further study of the diagnostic criteria is warranted.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05537-w.

Lipid-Poor Renal Angiomyolipoma Mimicking Renal Cell Carcinoma on 68Ga-FAPI-04 PET/CT

ABSTRACT

FAPI PET/CT findings of renal tumors have been rarely reported. We describe 68Ga-FAPI-04 PET/CT findings in 1 case with lipid-poor renal angiomyolipoma and 1 case with high-grade clear cell renal cell carcinoma. Both tumors showed intense 68Ga-FAPI-04 uptake. These 2 cases indicate that lipid-poor renal angiomyolipoma should be included in the differential diagnosis of FAPI-avid renal tumors.

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.

Validating Cell Surface Proteases as Drug Targets for Cancer Therapy: What Do We Know, and Where Do We Go?

Cell surface proteases (also known as ectoproteases) are transmembrane and membrane-bound enzymes involved in various physiological and pathological processes. Several members, most notably dipeptidyl peptidase 4 (DPP4/CD26) and its related family member fibroblast activation protein (FAP), aminopeptidase N (APN/CD13), a disintegrin and metalloprotease 17 (ADAM17/TACE), and matrix metalloproteinases (MMPs) MMP2 and MMP9, are often overexpressed in cancers and have been associated with tumour dysfunction. With multifaceted actions, these ectoproteases have been validated as therapeutic targets for cancer. Numerous inhibitors have been developed to target these enzymes, attempting to control their enzymatic activity. Even though clinical trials with these compounds did not show the expected results in most cases, the field of ectoprotease inhibitors is growing. This review summarizes the current knowledge on this subject and highlights the recent development of more effective and selective drugs targeting ectoproteases among which small molecular weight inhibitors, peptide conjugates, prodrugs, or monoclonal antibodies (mAbs) and derivatives. These promising avenues have the potential to deliver novel therapeutic strategies in the treatment of cancers.

Head-to-Head Comparison of Fibroblast Activation Protein Inhibitors (FAPI) Radiotracers versus [18F]F-FDG in Oncology: A Systematic Review

Several recent studies comparing radiolabeled fibroblast activation protein inhibitors (FAPI) and fluorine-18 fluorodeoxyglucose ([18F]F-FDG) as positron emission tomography (PET) radiotracers in oncology have been published. The aim of this systematic review is to perform an updated evidence-based summary about the comparison of these PET radiotracers in oncology to better address further research in this setting. Studies or subsets of studies comparing radiolabeled FAPI and [18F]F-FDG as PET radiotracers in oncology were eligible for inclusion in this systematic review. A systematic literature search of PubMed/MEDLINE and Cochrane library databases was performed until August 2021. Literature data about the comparison of [18F]F-FDG and radiolabeled FAPI are rapidly increasing. Overall, taking into account radiotracer uptake and tumor-to-background uptake ratio, compared to [18F]F-FDG PET, an equal or higher detection of primary tumors and/or metastatic lesions was usually demonstrated by using radiolabeled FAPI PET. In particular, the cancer entities with better detection rate of tumor lesions by using radiolabeled FAPI PET, compared to [18F]F-FDG PET, were gastrointestinal tumors, liver tumors, breast cancer and nasopharyngeal carcinoma. Further comparison studies are needed to better evaluate the best field of application of radiolabeled FAPI PET.