Incidental Detection of Elastofibroma Dorsi With 68Ga-FAPI-46 and 18F-FDG PET/CT in a Patient With Esophageal Cancer

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.

Fibroblast activation protein-α expression in fibroblasts is common in the tumor microenvironment of colorectal cancer and may serve as a therapeutic target

Background: Colorectal cancer (CRC) is still one of the leading causes of cancer death worldwide, emphasizing the need for further diagnostic and therapeutic approaches. Cancer invasion and metastasis are affected by the tumor microenvironment (TME), with cancer-associated fibroblasts (CAF) being the predominant cellular component. An important marker for CAF is fibroblast activation protein-α (FAP) which has been evaluated as therapeutic target for, e.g., radioligand therapy. The aim of this study was to examine CRC regarding the FAP expression as a candidate for targeted therapy. Methods: 67 CRC, 24 adenomas, 18 tissue samples of inflammation sites and 28 non-neoplastic, non-inflammatory tissue samples of colonic mucosa were evaluated for immunohistochemical FAP expression of CAF in tissue microarrays. The results were correlated with clinicopathological data, tumor biology and concurrent expression of additional immunohistochemical parameters. Results: 53/67 (79%) CRC and 6/18 (33%) inflammatory tissue specimens showed expression of FAP. However, FAP was only present in 1/24 (4%) adenomas and absent in normal mucosa (0/28). Thus, FAP expression in CRC was significantly higher than in the other investigated groups. Within the CRC cohort, expression of FAP did not correlate with tumor stage, grading or the MSI status. However, it was observed that tumors exhibiting high immunohistochemical expression of Ki-67, CD3, p53, and β-Catenin showed a significantly higher incidence of FAP expression. Conclusion: In the crosstalk between tumor cells and TME, CAF play a key role in carcinogenesis and metastatic spread. Expression of FAP was detectable in the majority of CRC but nearly absent in precursor lesions and non-neoplastic, non-inflammatory tissue. This finding indicates that FAP has the potential to emerge as a target for new diagnostic and therapeutic concepts in CRC. Additionally, the association between FAP expression and other immunohistochemical parameters displays the interaction between different components of the TME and demands further investigation.

One Third of Malignant Pleural Mesothelioma Shows High Immunohistochemical Expression of MSLN or CXCR4 Which Indicates Potent Candidates for Endo-Radiotherapy

Malignant pleural mesothelioma (MPM) is a mainly asbestos-related tumour associated with a very poor prognosis. Therapeutic approaches include multimodal therapy and chemotherapeutics, with cisplatin being the drug of choice, but response rates of only up to 14% indicate very poor outcomes. Effective treatment options are lacking. Besides the diagnostic usage of radioligands in positron emission tomography (PET)/computed tomography (CT), the endo-radioligand therapy with Lu177 has been proven as a powerful tool in cancer therapy. Mesothelin (MSLN) and C-XC chemokine receptor 4 (CXCR4) are membrane-bound proteins, expressed in certain cancers, and thus are promising targets for endo-radiotherapy. A significant portion of high MSLN- or CXCR4-expressing tumors within the MPM may open the field for this sophisticated treatment approach in the near future. Formalin-fixed, paraffin-embedded (FFPE) tumour specimens from 105 patients suffering from MPM and treated at the Lung Cancer Centre of Essen and at the Helios Klinikum Emil von Behring Berlin were screened. The tumour samples were arranged in tissue microarrays. We immunohistochemically stained the tumour samples against MSLN and CXCR4. The protein expressions of the stainings were scored by a pathologist by using a semiquantitative method. The data obtained were correlated with the clinical outcome. Overall, 77.1% of the analysed tumours showed CXCR4 protein expression (25.7% of them at high expression level (Score 3)). 48.6% of all samples showed an overall strong staining (Score ≥ 2), 59% of the investigated tumours showed MSLN protein expression (10.5% of them at high expression (Score 3)), and 36.2% of all samples showed an overall strong staining (Score ≥ 2). Our results show significant tissue expression levels, for both CXCR4 and MSLN protein, in a major portion of clinical MPM samples. One-third of patients showed outstanding immunoexpression of at least one of these markers, making them interesting candidates for radioligand-based PET/CT diagnostics and follow-up and furthermore may profit from endo-radiotherapy.

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.

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.

The potential utility of [68 Ga]Ga-DOTA-FAPI-04 as a novel broad-spectrum oncological and non-oncological imaging agent-comparison with [18F]FDG

PURPOSE

This study aimed to compare the detection performance of [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT in the patients with various oncological and non-oncological lesions.

METHODS

A total of 123 patients underwent contemporaneous [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT were included in this prospective study. The maximum standard uptake value (SUVmax) was measured to compare oncological and non-oncological lesion uptake. The sensitivity, specificity, predictive values, and accuracy of [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT for detecting primary, metastatic, and non-oncological lesions were calculated and compared to evaluate the detection efficacy.

RESULTS

The study subjects consisted of 123 patients (69 men and 54 women; mean age 56.11 ± 11.94 years). Among the 102 patients with either newly diagnosed (82 patients) or previously treated solid tumor (20 patients), a total of 88 solid primary malignant tumors in 84/102 patients were detected. Two patients had two primary tumors each and 1 patient had three primary tumors. Among them, 58/102 and 43/102 patients had nodal (376 lesions) and distant metastases (406 lesions), respectively. Eight patients had hematological neoplasm. No malignant oncological diseases were detected in the remaining 13 patients. A total of 145 non-oncological lesions and benign tumors in 52/123 patients were detected incidentally. [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT demonstrated a significantly higher uptake and detection rate for the primary (SUVmax 10.98 ± 5.83 vs. 8.36 ± 6.43, p < 0.001; sensitivity 97.67 vs. 84.89%; and accuracy 96.59 vs. 82.95%, X² = 0.538, p = 0.021), nodal (SUVmax 10.50 ± 5.98 vs. 8.20 ± 6.29, p = 0.011; sensitivity 97.59 vs. 84.72%; and accuracy 97.34 vs. 84.31%, X² = 2.067, p < 0.001), and distant metastatic lesions (SUVmax 9.64 ± 6.45 vs. 6.74 ± 4.83; p < 0.001; sensitivity 98.01 vs. 65.59%; and accuracy 97.04 vs. 65.51%, X² = 4.897, p < 0.001) of solid tumor than did [¹⁸F]FDG PET/CT. [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT demonstrated a lower activity (SUVmax: 6.84 ± 4.67 vs. 13.09 ± 7.29, p < 0.001) and detection rate (sensitivity 50.65 vs. 96.75%, and accuracy 51.28 vs. 95.51%, X² = 5.166, p < 0.001) for multiple myeloma and lymphoma compared to [¹⁸F]FDG PET/CT. [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT PET/CT demonstrated a comparative activity (SUVmax 6.40 ± 3.95 vs. 5.74 ± 15.78, p = 0.729) and detection efficacy (sensitivity 86.52 vs. 72.34%, and accuracy 84.83 vs. 72.41%, X² = 9.460, p = 0.007) for non-oncological lesion and benign tumor detection.

CONCLUSIONS

Except for myeloma and lymphoma, [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT showed a superior detection efficacy for detecting various primary and metastatic lesions than [¹⁸F]FDG PET/CT. A comparative detection utility for non-oncological lesion was obtained with both tracers. [⁶⁸ Ga]Ga-DOTA-FAPI-04 could be used as a broad-spectrum tumor and inflammatory imaging agent in the clinical especially for various solid tumors and non-oncological lesions.

FAPI Uptake in a Vertebral Body Fracture in a Patient With Lung Cancer: A FAPI Imaging Pitfall

ABSTRACT

A left pulmonary nodule was identified by CT scan in a 53-year-old woman who had a car accident 10 days earlier. 18F-FDG PET/CT showed multiple FDG-avid lesions located at the left lung nodule, mediastinal lymph nodules, and L4 vertebral body. 68Ga-FAPI PET/CT was performed for further evaluation. However, 68Ga-FAPI demonstrated intense FAPI uptake in the accident-related fracture of the L4 vertebral body. This case documents that the fracture of the vertebral body may cause FAPI uptake, and nuclear clinicians evaluating 68Ga-FAPI imaging should be aware of this potential pitfall.

Correlation between microvessel density (MVD) and multi-spiral CT (MSCT) perfusion parameters of esophageal cancer lesions and the diagnostic value of combined CtBP2 and P16INK4A

Background

This article aims to analyze the correlation between microvessel density (MVD) and multi-spiral CT(MSCT) perfusion parameters of esophageal cancer lesions, and the diagnostic value of combining C-terminal binding protein 2 (CtBP2) and P16 inhibitor of cyclin-dependent kinase 4a (P16^INK4A).

Methods

A total of 42 cases of normal esophageal mucosa tissues >5 cm from the cancer tissue were selected as the control group. The expression levels of CtBP2 and P16^INK4A and the values of MSCT perfusion parameters and MVD were compared in the control group and esophageal cancer group. SP immunohistochemical staining was used to detect protein expression levels of CtBP2 and P16^INK4A. The Pearson method was used to analyze the differences and pertinence of MSCT perfusion parameters and MVD in the control group and esophageal cancer group. The receiver operating characteristic (ROC) curve was used to calculate the diagnostic value of CtBP2 and P16^INK4A combined with MVD and MSCT perfusion parameters in esophageal cancer.

Results

The positive expression rate of P16^INK4A in the esophageal cancer group was significantly lower than that in the control group. The positive expression rates of CtBP2, blood volume (BV), mean transit time (MTT), surface permeability (permeability surface, PS), and MVD values were significantly higher than those of the control group (P<0.05). There was no significant difference in blood flow (BF) value between the 2 groups (P>0.05). The BF value of the tumor invading the fibrous membrane was significantly higher than that of the non-invading fibrous membrane (P<0.05), and the PS and MVD values of the patients with lymph node metastasis were higher than those without lymph node metastasis (P<0.05). The MSCT perfusion parameters BF and BV were significantly positively correlated with MVD (P<0.05), while MTT, PS, and MVD were not significantly correlated (P>0.05). ROC results showed that the areas under curve (AUC) of CtBP2, P16^INK4A, and MSCT were 0.625, 0.747, and 0.812, respectively. However, the area under the combined detection curve was larger, at 0.869.

Conclusions

MSCT perfusion imaging of esophageal cancer lesions can indirectly reflect the angiogenesis of esophageal cancer, and the combination of CtBP2 and P16^INK4A can effectively improve the diagnostic efficiency of the disease.

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

BACKGROUND AND PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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