68Ga-FAPI Outperforms 18F-FDG PET/CT in Identifying Bone Metastasis and Peritoneal Carcinomatosis in a Patient With Metastatic Breast Cancer

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.

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.

Applications of FAPI PET/CT in the diagnosis and treatment of breast and the most common gynecologic malignancies: a literature review

The fibroblast activating protein (FAP) is expressed by some fibroblasts found in healthy tissues. However, FAP is overexpressed in more than 90% of epithelial tumors, including breast and gynecological tumors. As a result, the FAP ligand could be used as a target for diagnosis and treatment purposes. Positron emission tomography/computed tomography (PET/CT) is a hybrid imaging technique commonly used to locate and assess the tumor's molecular and metabolic functions. PET imaging involves the injection of a radiotracer that tends to accumulate more in metabolically active lesions such as cancer. Several radiotracers have been developed to target FAP in PET/CT imaging, such as the fibroblast-activation protein inhibitor (FAPI). These tracers bind to FAP with high specificity and affinity, allowing for the non-invasive detection and quantification of FAP expression in tumors. In this review, we discussed the applications of FAPI PET/CT in the diagnosis and treatment of breast and the most common gynecologic malignancies. Radiolabeled FAPI can improve the detection, staging, and assessment of treatment response in breast and the most common gynecologic malignancies, but the problem with normal hormone-responsive organs remains insurmountable. Compared to the diagnostic applications of FAPI, further research is needed for future therapeutic applications.

68 Ga-FAPI and 18 F-FDG PET/CT in Intravenous Leiomyomatosis

ABSTRACT

Intravenous (IV) leiomyomatosis is a rare IV disease. Our case presents 68 Ga-FAPI and 18 F-FDG PET/CT findings of IV leiomyomatosis in a 37-year-old woman. Intravenous leiomyomatosis shows only mild FDG but intense 68 Ga-FAPI activity on PET/CT studies. In this case, 68 Ga-FAPI was superior to 18 F-FDG PET/CT in detecting IV leiomyomatosis.

Other Novel PET Radiotracers for Breast Cancer

Many novel PET radiotracers have demonstrated potential use in breast cancer. Although not currently approved for clinical use in the breast cancer population, these innovative imaging agents may one day play a role in the diagnosis, staging, management, and even treatment of breast cancer.

Malignant Peritoneal Mesothelioma With Butterfly-Shaped Muscle Metastasis: 68Ga-FAPI PET/CT Versus 18F-FDG PET/CT

ABSTRACT

Malignant peritoneal mesothelioma is a rare malignancy that may manifest as extraperitoneal metastasis. We describe the findings of 68 Ga-FAPI and 18 F-FDG PET/CT in a case of malignant peritoneal mesothelioma with widespread muscle metastasis in a butterfly-shaped distribution pattern. We found that FDG and FAPI uptake in the peritoneal tumor and its spread were inhomogeneous. Notably, 68 Ga-FAPI PET/CT more clearly showed relevant lesions compared with 18 F-FDG PET/CT.

The Role of 68 Ga-FAPI PET/CT in Detection of Metastatic Lobular Breast Cancer

PURPOSE

Invasive lobular breast cancer (ILC) may be hard to detect using conventional imaging modalities and usually shows less avidity to 18 F-FDG PET/CT. 68 Ga-fibroblast activation protein inhibitor (FAPI) PET/CT has shown promising results in detecting non- 18 F-FDG-avid cancers. We aimed to assess the feasibility of detecting metastatic disease in patients with non- 18 F-FDG-avid ILC.

METHODS

This prospective study included patients with metastatic ILC, infiltrative to soft tissues, which was not 18 F-FDG avid. The patients underwent 68 Ga-FAPI PET/CT for evaluation, which was correlated with the fully diagnostic CT performed at the same time.

RESULTS

Seven women (aged 57 ± 10 years) were included. Among the 30 organs and structures found to be involved by tumor, the number of findings observed by FAPI PET/CT was significantly higher than that observed by CT alone ( P = 0.022), especially in infiltrative soft tissue and serosal locations.

CONCLUSIONS

This small pilot trial suggests a role for 68 Ga-FAPI PET/CT in ILC, which needs to be confirmed by subsequent trials.

PET/CT with Fibroblast Activation Protein Inhibitors in Breast Cancer: Diagnostic and Theranostic Application-A Literature Review

Growing studies have recently reported on the promising application of radiolabeled-fibroblast activation protein inhibitors (FAPIs) as diagnostic and therapeutic agents in various oncological populations. To exclusively evaluate the current evidence on the diagnostic and therapeutic role of FAPI radiotracers in patients with breast cancer (BC), a narrative review of the available literature was performed. A search algorithm from PubMed/MEDLINE, based on the combination of "PET" OR "positron emission tomography" and "FAPI" and "cancer", with a last update in February 2022, was applied. From 233 identified articles, 33 studies conducted in BC patients and with available data on PET imaging or radiolabeled-FAPI therapy were finally considered, for a total of 191 patients. Despite some clinical and methodological heterogeneity among the reviewed articles, ⁶⁸Ga-FAPI PET/CT emerges as a valuable diagnostic tool in BC patients both at staging and restaging, also demonstrating several technical advantages and an overall better performance than ¹⁸F-FDG, especially in histotypes with well-known low ¹⁸F-FDG avidity. Moreover, although with still limited clinical evidence in BC, radiolabeled FAPIs emerge as promising therapeutic agents in a theranostic perspective, increasing the possibility of more personalized treatments. From these results, future research directions on FAPI radiotracers application in BC patients are suggested.

[68Ga]Ga-FAPI PET/CT Improves the T Staging of Patients with Newly Diagnosed Nasopharyngeal Carcinoma: A Comparison with [18F]F-FDG

PURPOSE

This study aimed to explore the value of [⁶⁸Ga]Ga-labelled fibroblast activation protein inhibitor ([⁶⁸Ga]Ga-FAPI) positron emission tomography/computed tomography (PET/CT) in the initial staging of patients with newly diagnosed nasopharyngeal carcinoma (NPC), compared with 2-deoxy-2[¹⁸F]fluoro-D-glucose ([¹⁸F]F-FDG) PET/CT.

MATERIALS AND METHODS

Forty-seven treatment-naïve patients with newly diagnosed NPC underwent magnetic resonance imaging (MRI), [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT and [¹⁸F]F-FDG PET/CT within 1 week. The diagnostic efficiency of all imaging modalities for evaluating primary tumour extension was compared from the two aspects of soft tissue and bony structure involvement. The accuracy of two PET/CT methods for diagnosing cervical lymph node (CLN) metastases was compared, and MRI served as the standard reference. T and N stages were assessed by MRI, [⁶⁸Ga]Ga-FAPI PET/CT and [¹⁸F]F-FDG PET/CT. Immunohistochemical (IHC) staining for FAP was conducted in 22 of the patients.

RESULTS

[⁶⁸Ga]Ga-FAPI PET/CT outperformed [¹⁸F]F-FDG PET/CT in the assessment of primary tumour invasion in the cavernous sinus (10 vs. 1, p < 0.001) and bony structures (207 vs. 177, p < 0.001). Compared with MRI, [⁶⁸Ga]Ga-FAPI PET/CT upgraded and underestimated T stage in 13 and 2 patients, while [¹⁸F]F-FDG PET/CT upgraded and underestimated T stage in 5 and 13 patients. However, [⁶⁸Ga]Ga-FAPI PET/CT was inferior to [¹⁸F]F-FDG PET/CT in diagnosing positive CLNs based on the analyses of patients, neck sides, neck levels and individual nodes. [⁶⁸Ga]Ga-FAPI PET/CT changed therapeutic schedules in 8 patients because of stage group changes. The presence of FAP with high quantity and intensity in cancer-associated fibroblasts (CAFs) was confirmed in all tumour specimens.

CONCLUSION

[⁶⁸Ga]Ga-FAPI PET/CT outperformed [¹⁸F]F-FDG PET/CT in detecting the cavernous sinus and bony structure involvement of primary NPC tumours, suggesting its value in improving T staging and therapeutic regimen selection. However, the performance of [⁶⁸Ga]Ga-FAPI PET/CT is less promising for N staging because it detected fewer positive CLNs than [¹⁸F]F-FDG PET/CT.

[Oncological theranostics in nuclear medicine]

The theranostic principle describes the diagnostic and therapeutic use of radioactive nuclides linked to biochemically active ligands. The oldest and most prominent field of application of theranostics in oncology is differentiated thyroid cancer treated by radioiodine therapy, which allows imaging of the iodine uptake and thus tumor manifestations by gamma (γ) radiation of radioiodine. Other areas of application include neuroendocrine tumors, castration-resistant prostate cancer and, in the context of individual therapeutic approaches, fibroblastic tumors. Imaging with beta-plus (β⁺) emitters is mainly performed using so-called hybrid imaging: positron emission tomography (PET) in combination with computed tomography or magnetic resonance imaging (PET/CT or PET/MRI). Beta-minus (β^-) emitters are predominantly used in therapy, but the use of alpha (α) emitters is also increasing, thus, enabling targeted cancer treatment with mostly low-grade side effects.

Noninvasive Monitoring of Reparative Fibrosis after Myocardial Infarction in Rats Using 68Ga-FAPI-04 PET/CT

Noninvasively monitoring activated fibroblasts is of great value for understanding the dynamic process of myocardial fibrosis after myocardial infarction (MI). This study aimed to evaluate the feasibility of ⁶⁸Ga-labeled fibroblast activation protein inhibitor 04 (⁶⁸Ga-FAPI-04) for monitoring reparative fibrosis and reactive fibrosis after MI. MI models were prepared by ligation of the left anterior descending (LAD) coronary artery and validated by electrocardiogram and ¹⁸F-FDG PET/CT 1 day after MI and hematoxylin and eosin (HE) staining. ⁶⁸Ga-FAPI-04 PET/CT scans (1, 3, 6, 9, 12, 15, 18, 21, 28, and 35 days after MI) were carried out in MI rats and sham-operated rats without ligation of LAD. Blocking experiments were carried out on MI rats on day 7 after MI with ⁶⁸Ga-FAPI-04 and excessive FAPI-04. Autoradiography, HE staining, Masson's trichrome staining, and immunofluorescence staining were carried out for ex vivo validation. The infarcted area with decreased or defective myocardial metabolic activity in ¹⁸F-FDG PET/CT correspondingly showed high ⁶⁸Ga-FAPI-04 uptake in the MI rats. The myocardial tracer uptake was significantly different between MI and sham-operated rats from day 1 to 28 after MI and reached peak value 6 days after MI (0.806 ± 0.257%ID/cc vs 0.199 ± 0.012%ID/cc, P < 0.05). Tracer uptake at the infarcted myocardium and normal tissues in MI rats decreased significantly after blocking. Obvious tracer uptake was confirmed by autoradiography, and immunofluorescence staining showed FAP+ cells in the infarcted myocardium and border zone. Masson's trichrome staining of the heart sections of MI rats at different times suggested the presence of myocardial fibrosis. ⁶⁸Ga-FAPI-04 uptake was not observed in the distal uninjured myocardium throughout the observation period. In conclusion, ⁶⁸Ga-FAPI-04 PET could noninvasively monitor the activated fibroblasts in the early stage post acute MI and may be helpful for evaluating the degree of reparative fibrosis, while reactive fibrosis monitoring still needs further study.

Uncommon Metastases From Occult Breast Cancer Revealed by 18 F-FDG and 68 Ga-FAPI PET/CT

ABSTRACT

Metastases to gastrointestinal tract from breast cancer are very rare and can be diagnostically challenging. In this case, we reported 18 F-FDG and 68 Ga-FAPI PET/CT findings in a 54-year-old women with such uncommon metastases from breast cancer. Although the primary tumor of breast cancer remained radiologically occult, 68 Ga-FAPI PET/CT showed a far better tumor-to-background ratio than 18 F-FDG PET/CT for detecting the metastatic lesions, especially for detecting the retroperitoneal lymph node and rectal metastases.

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.

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.

Comparison of 68Ga-FAPI and 18F-FDG PET/CT in the Evaluation of Advanced Lung Cancer

Background Gallium 68 (⁶⁸Ga)-labeled fibroblast-activation protein inhibitor (FAPI) has recently been introduced as a promising tumor imaging agent. Purpose To compare ⁶⁸Ga-FAPI PET/CT with fluorine 18 (¹⁸F)-labeled fluorodeoxyglucose (FDG) PET/CT in evaluating lung cancer. Materials and Methods In this prospective study conducted from September 2020 to February 2021, images from participants with lung cancer who underwent both ⁶⁸Ga-FAPI and ¹⁸F-FDG PET/CT examinations were analyzed. The tracer uptakes, quantified by maximum standardized uptake value (SUV_max) and target-to-background ratio (TBR), were compared for paired positive lesions between both modalities using the paired t test or Wilcoxon signed-rank test. Results Thirty-four participants (median age, 64 years [interquartile range: 46-80 years]; 20 men) were evaluated. From visual evaluation, ⁶⁸Ga-FAPI PET/CT and ¹⁸F-FDG PET/CT showed similar performance in the delineation of primary tumors and detection of suspected metastases in the lungs, liver, and adrenal glands. The metabolic tumor volume in primary and recurrent lung tumors showed no difference between modalities (mean: 11.6 vs 10.8, respectively; P = .68). However, compared with ¹⁸F-FDG PET/CT, ⁶⁸Ga-FAPI PET/CT depicted more suspected metastases in lymph nodes (356 vs 320), brain (23 vs 10), bone (109 vs 91), and pleura (66 vs 35). From semiquantitative evaluation, the SUV_max and TBR of primary or recurrent tumors, positive lymph nodes, bone lesions, and pleural lesions at ⁶⁸Ga-FAPI PET/CT were all higher than those at ¹⁸F-FDG PET/CT (all P < .01). Although SUV_max of ⁶⁸Ga-FAPI and ¹⁸F-FDG in brain metastases were not different (mean SUV_max: 9.0 vs 7.4, P = .32), TBR was higher with ⁶⁸Ga-FAPI than with ¹⁸F-FDG (mean: 314.4 vs 1.0, P = .02). Conclusion Gallium 68-labeled fibroblast-activation protein inhibitor PET/CT may outperform fluorine 18-labeled fluorodeoxyglucose PET/CT in staging lung cancer, particularly in the detection of metastasis to the brain, lymph nodes, bone, and pleura. © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Jacobson and Van den Abbeele in this issue.

Benign Bone Cyst Mimicking Bone Metastasis Demonstrated on 68Ga-FAPI

ABSTRACT

A 59-year-old woman with lobular breast cancer was referred to nuclear medicine department for a 68Ga-FAPI PET/CT study after completion of treatment to rule out the possibility of metastatic disease. It revealed a lytic lesion showing increased FAPI accumulation in the right acetabulum, highly suspicious for bone metastasis. In comparison to an initial 18F-FDG PET/CT study done 2 years before, the lytic lesion showing no FDG uptake in the right acetabulum remained unchanged, and the findings were consistent with bone cyst. This case highlighted that benign bone cyst may demonstrate increased 68Ga-FAPI uptake mimicking bone metastasis on 68Ga-FAPI PET/CT.

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.

Hepatic Lesion of Mucosa-Associated Lymphoid Tissue Lymphoma Revealed by Al18F-NOTA-FAPI-04 PET/CT

ABSTRACT

A middle-aged man was newly diagnosed with mucosa-associated lymphoid tissue lymphoma with secondary liver involvement. The hepatic lesion was not shown on FDG PET/CT but FAPI (fibroblast-activated protein inhibitor) PET/CT, which revealed abnormal FAPI accumulation. This case demonstrated that FAPI PET/CT might provide value in hepatic mucosa-associated lymphoid tissue lymphoma.

Al18F-NOTA-FAPI-04 Outperforms 18F-FDG PET/CT in Identifying the Primary Lesion and Rare Metastases From Gastric Cancer

ABSTRACT

A 45-year-old woman with gastric cancer underwent FDG PET/CT for initial staging. However, the primary and the metastatic lesions were observed with low or no FDG uptake. Then, the patient underwent fibroblast-activated protein inhibitor PET/CT 2 days later, which demonstrated more lesions and much higher tumor-to-background contrast than FDG PET/CT did.

A Case with 68Ga-FAPI Positive and 18F-FDG Negative Breast Cancer

A female patient diagnosed of infiltrative breast carcinoma using tru-cut biopsy underwent ¹⁸flourine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) for staging. The tumor was located in the superior external quadrant of the right breast, and did not exhibit pathological uptake in ¹⁸F-FDG PET/CT. Later, gallium-⁶⁸ (⁶⁸Ga) fibroblast activation protein-specific inhibitor (FAPI)-04 PET/CT imaging was performed and the primary tumor showed intense radiotracer accumulation. This presumes that ⁶⁸Ga-FAPI PET/CT imaging is superior to ¹⁸F-FDG imaging in detecting the primary tumor in breast cancer, thereby suggesting the replacement of FAPI by ¹⁸F-FDG in breast-cancer staging in the future.

Simultaneous FAPI PET/MRI Targeting the Fibroblast-Activation Protein for Breast Cancer

Background Integrated PET/MRI is a promising modality for breast assessment. The most frequently used tracer, fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG), is applied for whole-body staging in advanced breast cancer but has limited accuracy in evaluating primary breast lesions. The fibroblast-activation protein (FAP) is abundantly expressed in invasive breast cancer. FAP-directed PET tracers have recently become available, but results in primary breast tumors remain lacking. Purpose To evaluate the use of FAP inhibitor (FAPI) breast PET/MRI in assessing breast lesions and of FAPI whole-body scanning for lymph node (LN) and distant staging using the ligand gallium 68 (⁶⁸Ga)-FAPI-46. Materials and Methods In women with histologically confirmed invasive breast cancer, all primary ⁶⁸Ga-FAPI-46 breast and whole-body PET/MRI and PET/CT examinations conducted at the authors' center between October 2019 and December 2020 were retrospectively analyzed. MRI lesion characteristics and standardized uptake values (SUVs) were quantified with dedicated software. Mann-Whitney U tests were used to compare tumor SUVs across different tumor types. The Pearson correlation coefficient was calculated between SUV and measures of MRI morphologic characteristics. Results Nineteen women (mean age, 49 years ± 9 [standard deviation]) were evaluated-18 to complement initial staging and one for restaging after therapy for distant metastases. Strong tracer accumulation was observed in all 18 untreated primary breast malignancies (mean maximum SUV [SUV_max] = 13.9 [range, 7.9-29.9]; median lesion diameter = 26 mm [range, 9-155 mm]), resulting in clear tumor delineation across different gradings, receptors, and histologic types. All preoperatively verified LN metastases in 13 women showed strong tracer accumulation (mean SUV_max= 12.2 [range, 3.3-22.4]; mean diameter = 21 mm [range, 14-35 mm]). Tracer uptake established or supported extra-axillary LN involvement in seven women and affected therapy decisions in three women. Conclusion This retrospective analysis indicates use of ⁶⁸Ga fibroblast-activation protein inhibitor tracers for breast cancer diagnosis and staging. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Mankoff and Sellmyer in this issue.

Comparison of the Relative Diagnostic Performance of [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT for the Detection of Bone Metastasis in Patients With Different Cancers

Purpose

The present retrospective analysis sought to compare the relative diagnostic efficacy of [⁶⁸Ga]Ga-DOTA-FAPI-04 to that of [¹⁸F]FDG PET/CT as a means of detecting bone metastases in patients with a range of cancer types.

Materials

In total, 30 patients with bone metastases associated with different underlying malignancies were retrospectively enrolled. All patients had undergone [⁶⁸Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT, and the McNemar test was used to compare the relative diagnostic performance of these two imaging modalities. The maximum standard uptake value (SUVmax) was used to quantify radiotracer uptake by metastatic lesions, with the relative uptake associated with these two imaging strategies being compared via the Mann-Whitney U test. The cohort was further respectively divided into two (osteolytic and osteoblastic bone metastases) and three clinical subgroups (lung cancer, thyroid cancer, and liver cancer).

Results

[⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT was found to be significantly more sensitive as a means of diagnosing bone metastases relative to [¹⁸F]FDG PET/CT ([109/109] 100% vs [89/109] 81.7%; P< 0.01), consistent with the significantly increased uptake of [⁶⁸Ga]Ga-DOTA-FAPI-04 by these metastatic lesions relative to that of [¹⁸F]FDG (n=109, median SUVmax, 9.1 vs. 4.5; P< 0.01). [⁶⁸Ga]Ga-DOTA-FAPI-04 accumulation was significantly higher than that of [¹⁸F]FDG in both osteolytic (n=66, median SUVmax, 10.6 vs 6.1; P < 0.01), and osteoblastic metastases (n=43, median SUVmax, 7.7 vs 3.7; P < 0.01). [⁶⁸Ga]Ga-DOTA-FAPI-04 uptakes were significantly higher than that of [¹⁸F]FDG in bone metastases from lung cancer (n = 62, median SUVmax, 10.7 vs 5.2; P < 0.01), thyroid cancer (n = 18, median SUVmax, 5.65 vs 2.1; P < 0.01) and liver cancer (n = 12, median SUVmax, 5.65 vs 3.05; P < 0.01). However, [⁶⁸Ga]Ga-DOTA-FAPI-04 detected 10 false-positive lesions, while only 5 false-positive were visualized by [¹⁸F]FDG PET/CT.

Conclusion

[⁶⁸Ga]Ga-DOTA-FAPI-04 PET/CT exhibits excellent diagnostic performance as a means of detecting bone metastases, and is superior to [¹⁸F]FDG PET/CT in this diagnostic context. Furthermore, [⁶⁸Ga]Ga-DOTA-FAPI-04 tracer uptake levels are higher than those of [¹⁸F]FDG for most bone metastases. However, owing to the potential for false-positive bone lesions, it is critical that physicians interpret all CT findings with caution to ensure diagnostic accuracy.

Novel Tracers and Radionuclides in PET Imaging

The use of PET imaging agents in oncology, cardiovascular disease, and neurodegenerative disease shows the power of this technique in evaluating the molecular and biological characteristics of numerous diseases. These agents provide crucial information for designing therapeutic strategies for individual patients. Novel PET tracers are in continual development and many have potential use in clinical and research settings. This article discusses the potential applications of tracers in diagnostics, the biological characteristics of diseases, the ability to provide prognostic indicators, and using this information to guide treatment strategies including monitoring treatment efficacy in real time to improve outcomes and survival.

68Ga-FAPI Uptake of Thyroiditis in a Patient With Breast Cancer

ABSTRACT

68Ga-fibroblast activation protein-specific inhibitor (FAPI)-04 PET/CT was performed in a 49-year-old woman diagnosed with breast cancer. In PET/CT imaging, intense 68Ga-FAPI uptake was observed in the primary tumor, axillary lymph nodes, and also in the thyroid gland, whereas pathological 18F-FDG uptake was not observed in the thyroid gland. On thyroid ultrasonography, parenchyma was heterogeneous, and an area of focal thyroiditis was observed in the superior part of the right lobe. Biochemical parameters were found to be consistent with thyroiditis. This case shows that FAPI uptake in the thyroid gland may be associated with thyroiditis and should be evaluated clinically.

False-Positive 68Ga-Fibroblast Activation Protein-Specific Inhibitor Uptake of Benign Lymphoid Tissue in a Patient With Breast Cancer

ABSTRACT

68Ga-fibroblast activation protein-specific inhibitor (FAPI)-04 PET/CT was performed in a patient with left lower outer quadrant breast cancer who had 18F-FDG PET/CT imaging. 68Ga-FAPI-04 PET/CT showed higher accumulation of radiotracer in primary tumor and axillary lymph nodes than 18F-FDG PET/CT. In addition, focal increased FAPI uptake was observed in another nodular lesion in the lower inner quadrant in the same breast, which was considered malignant. However, Tru-Cut biopsy of this lesion was reported as benign lymphoid tissue. This case showed that all FAPI accumulation in breast tissue should not be interpreted in favor of malignancy; histopathological confirmation is required.

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

PURPOSE

To describe the uptake of ⁶⁸Gallium-labelled fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) in the bones and joints for better understanding of the role of ⁶⁸Ga-FAPI PET in benign and malignant bone lesions and joint diseases.

METHODS

All 129 ⁶⁸Ga-FAPI PET/MR or PET/CT scans from June 1, 2020, to February 20, 2021, performed at our PET center were retrospectively reviewed. Foci of elevated ⁶⁸Ga-FAPI uptake in the bones and joints were identified. All lesions were divided into malignant and benign diseases. Benign lesions included osteofibrous dysplasia, periodontitis, degenerative bone diseases, arthritis, and other inflammatory or trauma-related abnormalities. The number, locations, and SUVmax of all lesions were recorded and analyzed. The detectability of ⁶⁸Ga-FAPI PET and ¹⁸F-FDG PET in patients who had two scans was also compared.

RESULTS

Elevated uptake of ⁶⁸Ga-FAPI in/around the bones and joints was found in 82 cases (63.57%). A total of 295 lesions were identified, including 94 (31.9%) malignant lesions (all were metastases) and 201 (68.1%) benign lesions. The benign lesions consisted of 13 osteofibrous dysplasia, 48 degenerative bone disease, 33 periodontitis, 56 arthritis, and 51 other inflammatory or trauma-related abnormalities. The spine, shoulder joint, alveolar ridge, and pelvis were the most commonly involved locations. Bone metastases were mainly distributed in the spine, pelvis, and ribs. Among benign diseases, periodontitis and arthritis are site-specific. The mean SUVmax of bone metastases was significantly higher than that of benign diseases (7.14 ± 4.33 vs. 3.57 ± 1.60, p < 0.001), but overlap existed. The differences in SUVmax among subgroups of benign diseases were statistically significant (p < 0.001), with much higher uptake in periodontitis (4.45 ± 1.17). ⁶⁸Ga-FAPI PET identified much more lesions than ¹⁸F-FDG PET (104 vs. 48) with higher uptake value.

CONCLUSION

⁶⁸Ga-FAPI accumulated in both bone metastases and some benign diseases of the bones and joints. Although the uptake of ⁶⁸Ga-FAPI was often higher in bone metastases, this finding cannot be used to distinguish between benign and malignant lesions. ⁶⁸Ga-FAPI PET also has the potential to locate and evaluate the extent of both malignant tumor and benign diseases in bones and joints.

TRIAL REGISTRATION

NCT04554719, NCT04605939. Registered September 8, 2020 and October 25, 2020-retrospectively registered, http://clinicaltrails.gov/show/NCT04554719 ; http://clinicaltrails.gov/show/NCT04605939.

The Role of Fibroblast Activation Protein Ligands in Oncologic PET Imaging

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

68Ga-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.

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.

Intense Diffuse Uptake of 68Ga-FAPI-04 in the Breasts Found by PET/CT in a Patient With Advanced Nasopharyngeal Carcinoma

ABSTRACT

The physiological uptake of 68Ga-FAPI-04 due to the change of the internal environment is little known. We report the case of a 45-year-old woman who was highly suspected to have advanced nasopharyngeal carcinoma. 18F-FDG and 68Ga-FAPI-04 PET/CT was performed for evaluating the disease. Both PET and CT with different tracers detected the primary nasopharyngeal carcinoma and metastases in right neck lymph nodes, liver, and bones. To our surprise, intense diffuse uptake of 68Ga-FAPI-04 was found in both breasts, which might be due to the hormone stimulation because the patient received 68Ga-FAPi-04 PET/CT just at the period of ovulation.

68Ga-FAPI and 18F-FDG PET/CT in Perineum Extramammary Paget Disease

ABSTRACT

Extramammary Paget disease is a rare skin intraepithelial adenocarcinoma. Our case presents 68Ga-fibroblast activation protein inhibitor and 18F-FDG PET/CT imaging in a patient with perineum extramammary Paget disease. Compared with 18F-FDG PET/CT, the primary tumor, enlarged pelvic lymph nodes and right maxillofacial lesion showed higher uptake in 68Ga-fibroblast activation protein inhibitor PET/CT.

Fibroblast Activation Protein Inhibitor PET/CT: A Promising Molecular Imaging Tool

PURPOSE

Fibroblast activation protein (FAP) is a cell membrane-bound serine peptidase, overexpressed in cancer-associated fibroblasts and activated fibroblasts at wound healing/inflammatory sites. Recently, molecular PET/CT imaging with radiolabeled FAP inhibitor (FAPI) has been evaluated in different diseases. We aimed to assess its potential role based on the available literature.

PATIENTS AND METHODS

We conducted a comprehensive review of the available preclinical and clinical data on FAPI PET/CT in an attempt to summarize its current status and potential future role. Based on that, we have discussed the pathophysiology behind FAP-based imaging, followed by a discussion of FAPI radiopharmaceuticals including their synthesis, biodistribution, and dosimetry. Next, we have discussed studies evaluating FAPI PET/CT in different oncological and nononcological pathologies. The potential of FAPI PET/CT in theranostics has also been addressed.

RESULTS

Based on the early scientific evidence available, including preclinical and clinical studies, FAPI PET/CT seems to be a promising molecular imaging tool, especially in oncology. It can be used for imaging different types of cancers and outperforms 18F-FDG PET/CT in some of these. Its potential as a theranostic tool warrants special attention.

CONCLUSIONS

Fibroblast activation protein inhibitor PET/CT has the potential to emerge as a powerful molecular imaging tool in the future. However, as of yet, the available evidence is limited, warranting further research and trials in this field.

A head-to-head comparison of 68Ga-DOTA-FAPI-04 and 18F-FDG PET/MR in patients with nasopharyngeal carcinoma: a prospective study

PURPOSE

To conduct a head-to-head comparison of the diagnostic ability of ⁶⁸Ga-DOTA-FAPI-04 (⁶⁸Ga-FAPI) and ¹⁸F-FDG PET/MR in nasopharyngeal carcinoma (NPC) patients.

METHODS

Patients diagnosed with NPC were prospectively enrolled. All patients underwent head-and-neck ⁶⁸Ga-FAPI PET/MR and ¹⁸F-FDG PET/MR within 1 week. Primary tumor, lymph node numbers, and tracer uptake were compared by SUVmax and visual evaluation. The primary tumor volumes derived from ⁶⁸Ga-FAPI, ¹⁸F-FDG PET, and MRI were also compared.

RESULTS

Fifteen patients were enrolled from June to August 2020. Both ⁶⁸Ga-FAPI and ¹⁸F-FDG PET had 100% detection rate of the primary tumor. The ⁶⁸Ga-FAPI SUVmax of primary tumors (13.87 ± 5.13) was lower than that of ¹⁸F-FDG (17.73 ± 6.84), but the difference was not significant (p = 0.078). Compared with ¹⁸F-FDG, ⁶⁸Ga-FAPI PET improved the delineation of skull-base invasion in eight out of eight patients and intracranial invasion in four out of four patients. When 25%SUVmax of ⁶⁸Ga-FAPI or 20%SUVmax of ¹⁸F-FDG was utilized as a threshold for determining tumor volume, it was highly consistent with MRI. ¹⁸F-FDG PET detected much more positive lymph nodes than ⁶⁸Ga-FAPI (100 vs 48). The SUVmax of 48 paired lymph nodes was significantly lower on ⁶⁸Ga-FAPI than ¹⁸F-FDG (8.67 ± 3.88 vs 11.79 ± 6.17, p < 0.001). Additionally, ⁶⁸Ga-FAPI further detected four highly suspected small, distant metastases in three patients. Compared with ¹⁸F-FDG, ⁶⁸Ga-FAPI changed overall staging in six of fifteen patients, with three patients being up-staged, and three down-staged.

CONCLUSION

⁶⁸Ga-FAPI outperforms ¹⁸F-FDG in delineating the primary tumor and detecting suspected distant metastases, particularly in the evaluation of skull-base and intracranial invasion, suggesting ⁶⁸Ga-FAPI hybrid PET/MR has the potential to serve as a single-step staging modality for patients with NPC. However, its value regarding lymph node and distant metastases evaluation needs further study.

TRIAL REGISTRATION

NCT04554719. Registered September 8, 2020 - retrospectively registered, http://clinicaltrails.gov/show/NCT04554719.

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.