[68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT in the Assessment of Pancreatic Tumors: A Comparison Study

Purpose Pancreatic tumors are characterized by abundant desmoplasia including cancer-associated broblasts (CAFs) that express broblast activation protein (FAP). Gallium-68-labeled broblast-activating protein inhibitor (FAPI) is promising probe for positron emission tomography/computed tomography (PET/CT) imaging of various types of cancers. This work aims to compare the diagnostic performances of [ 68 Ga]Ga-DOTA-FAPI-04 and [ 18 F]FDG PET/CT in detecting primary pancreatic tumors and metastasis prospectively. Methods We collected patients with pancreatic tumors during May 1 to August 1, 2020. All the patients underwent [ 68 Ga]Ga-DOTA-FAPI-04 and [ 18 F]FDG PET/CT within 5 days at diagnosis, recurrence detection or therapeutic evaluation. The clinical information, PET/CT image characteristics, maximum standardized uptake (SUV max ) value of pancreatic tumors and metastases, target-to-background ratio (TBR) of the liver metastases were collected for analysis. The pathological results or follow-up clinical diagnostic results were obtained. Ga]Ga-DOTA-FAPI-04 detected abnormal in 36 patients (LNs: 6.1 ± 2.7 vs. 4.4 ± 1.6, TBR: 5.0 ± 3.1 vs. 2.9 ± 1.4 p < 0.0001), respectively. [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT successfully unregulated the clinical stage in 2 patients, visualized recurrence in 1 patient, and detected residual active tumor tissues in 2 patients with discordant imaging results (FAPI + /FDG - ). In addition, there was nearly no [ 68 Ga]Ga-DOTA-FAPI-04 or [ 18 F]FDG uptake in pancreatic cystic neoplasms. Most of neuroendocrine neoplasms showed negligible [ 68 Ga]Ga-DOTA-FAPI-04 uptake. Conclusion Compared with [ 18 F]FDG PET/CT, [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT detected pancreatic tumors and associated metastases with a higher sensitivity and SUV max value. However, false-positive uptake of [ 68 Ga]Ga-DOTA-FAPI-04 in pancreatitis, cholangitis, some benign liver disease, and inammatory LNs was also prominent.


Introduction
Pancreatic tumors consist of a heterogeneous group of lesions, such as adenocarcinoma, neuroendocrine neoplasms (NEN), and pancreatic cystic neoplasms. Pancreatic ductal adenocarcinoma (PDAC) accounts for above 85% and the incidence continues to increase over the past decade [1].
Unfortunately, the majorities of patients with PDAC are diagnosed with advanced stages. More than 80% of the patients with surgery or adjuvant chemotherapy experienced recurrence in a short time [2]. year relative survival rate for PDAC was 3% when other organs were involved and the corresponding median survival was only 3 months without treatment [1,3]. Consequently, accurate diagnosis, staging, and early detection of the recurrence or metastasis is essential in the precise management of pancreatic tumors.
In the clinical practice, contrast enhanced computered tomography (CT) and magnetic resonance imaging (MRI) are mostly used for the diagnosis of pancreatic tumors [4]. They can detect metastatic lymph nodes (LNs) or liver metastases with high resolution. Compared with CT or MRI, positron emission tomography/computed tomography (PET/CT) imaging with [ 18 F]FDG is not recommended with high priority in the guideline, but it has great advantages in clinical staging, therapeutic evaluation, and detection of recurrence. Some studies have also demonstrated that [ 18 F]FDG uptake of pancreatic tumors is associated with poor prognosis [5][6][7]. However, its speci city is relative low due to that the false positive uptake in in ammatory diseases. And [ 18 F]FDG PET/CT has limited role in detecting LN involvement of PDAC [8]. Moreover, [ 18 F]FDG uptakes is generally low in some types of pancreatic NENs [9].
And the expression of broblast-activating protein (FAP) is relatively high on CAFs [11]. Recent studies have illustrated that 68 Ga-labled FAP inhibitor (FAPI) is promising for noninvasively imaging of various types of tumors [12][13][14]. Furthermore, the high expression of FAP in some tumors is associated with poor prognosis and response to chemotherapy [15,16]

Patients selection and study design
The study was approved by the Ethics Committee of the Fudan University Shanghai Cancer Center (IRB protocol #ZS1810). All the subjects were provided written informed consent to receive [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT scanning. Patients were recruited from May 1 to August 1, 2020 at our hospital. The inclusion criteria were as follows: (1) patients with suspicious pancreatic tumors revealed by non-invasive imaging (CT, MR, or ultrasound); (2) patients with pancreatic cancer who have ful lled chemotherapy within 1 month; (3) patients received surgery before and presented with elevated tumor markers. Exclusion criteria were: (1) the pancreatic tumor turned out to be metastatic lesion; (2)  PET/CT scanning. The nal diagnosis was determined by the pathological assessment of the surgically removed/biopsied tissues or clinical diagnosis based on radiological features, laboratory examinations, and clinical symptoms 6-month later.

Radiopharmaceuticals
Radiolabeling of [ 68 Ga]Ga-DOTA-FAPI-04 was performed according to the published method [12]. In brief, FAPI-04 was dissolved in NaAc solution, to which 68 GaCl 3 was added. After pH adjustment with sodium acetate, the reaction mixture was heated to 100 °C for 10 min. [ 68 Ga]Ga-DOTA-FAPI-04 was processed by solid-phase extraction. The radiochemical purity was assessed by thin-layer chromatography (TLC). . The spiral CT scan was conducted using a standardized protocol (120 kV, 140 mA, 3 mm slice thickness). Then PET scan was conducted with FlowMotion. PET data were reconstructed iteratively using an ordered-subset expectation maximization iterative reconstruction (OSEM) with CT data for attenuation correction.
[ 68 Ga]Ga-DOTA-FAPI-04 and [ 18 F]FDG PET/CT images were analyzed independently by two experienced nuclear medicine physicians by using the software (Taxus, Medivoly technology). Regions of interest (ROIs) were drawn and maximum standard uptake value (SUV max ) were automatically calculated in the ROI of the primary tumors, lymph nodes, and metastatic lesions.

Statistical analysis
All the statistical analyses were conducted using the Graphpad Prism7 software. [ 18 F]FDG and [ 68 Ga]Ga-DOTA-FAPI-04 uptake in the tumors, metastatic LNs, and distant metastases were compared using matched-pairs signed-rank test, respectively. Two-tailed P values < 0.05 was considered statistically signi cant.

Results
Clinical characteristics of the patients Fifty-three patients were enrolled in the current prospective study. Eight patients were excluded for the analysis, ve of which were with metastatic pancreatic tumors and three patients with no abnormal uptake. Finally, forty-ve patients including 18 females and 27 males (median age of 62.4 years; range: 42-84 years) were included. Of the 45 patients, thirty-two patients received PET/CT scans for initial tumor diagnosis and staging, eight patients previously diagnosed with pancreatic cancer for therapy evaluation, and ve patients for detecting the recurrence because of abnormally increased tumor markers. The scheme of the study was shown in Figure 1. Of the 45 patients, thirty-two were PDACs, three were intraductal papillary mucinous neoplasm (IPMN), two were benign lesions, two were NENs (one NET G2 and one NEC), one was autoimmune pancreatitis, and ve patients were clinically diagnosed with pancreatic cancer after 6-month follow-up. Notably, one patient has two primary tumors (PDAC and lymphoma) simultaneously. The characteristics of the patients were shown in Table 1.   Figure 4. 2) and 14.6 ± 5.9 (range 7.3-24.5), respectively. And there was no statistical difference between the uptake. Of the 17 patients fteen patients showed diffuse uptake of FAPI in the body and tail of pancreas while only 2 patients illustrated focal uptake. The representative case was presented in Figure 5. There was no relationship between the

Discussion
Recently, some studies have revealed that [ 68 Ga]Ga-DOTA-FAPI-04 showed advantages over [ 18 F]FDG in tumor management [18][19][20]. Pancreatic cancer was histopathologically characterized by high desmoplastic reactions and therefore FAPI PET/CT seems to be promising to improve diagnostic performance. In our study, we compared the performance of [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT with metastasis with diameter < 1 cm and peritoneum carcinomatosis. In the present study, we found that [ 68 Ga]Ga-DOTA-FAPI-04 had a lower uptake in healthy tissues than that of [ 18 F]FDG. As a result, [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT had a higher detection rate in metastatic LNs, liver, and peritoneum metastases, which was in consistent with the nding of a previous study [17]. The favorable imaging contrast de nitely makes [ 68 Ga]Ga-DOTA-FAPI-04 a tracer with high sensitivity for the diagnosis of pancreatic cancer. However, [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT did not demonstrated more bone and lung metastases with higher SUV max , which was different to the published article [21]. This may due to the different tumor types and the small number of the lesions for statistical analysis.
We also compared the performance of We noticed [ 68 Ga]Ga-DOTA-FAPI-04-avid pancreas in 17 patients (45.9%). And most of them were with diffuse uptake in the body and tail of pancreas. Previous studies have also revealed the similar performance and tumors showed higher average SUV max than pancreatitis [17,23]. However, the SUV max of the tumors and non-tumor pancreas was similar in our study. It is worth noting that the whole pancreas showed diffuse signi cant uptake in some patients, which covered the tumors. As we all know, desmoplasia and in ammation are two major hallmarks of pancreatic cancer [24,25]. In this circumstance, the combination with other examinations or close follow-up is of great importance for the accurate diagnosis. One of the possible reasons for the abnormal uptake in pancreas may be the chronic pancreatitis, as we found that pancreatic body and tail atrophy and pancreatic duct dilatation in the majority of patients. Another potential reason was tumor-associated in ammation [26]. In some patients of our study, the pathology of puncture or surgery showed brin exudation and in ammatory cell in ltration in the pancreatic tissue.  [31]. Some studies have demonstrated that FAPI-PET/CT could be used for targeted radiotherapy in patients with tumors of head and neck and lower gastrointestinal tract [32,33].
As we detected the false-uptake of [ 68 Ga]Ga-DOTA-FAPI-04 in pancreatitis and biliary obstruction that commonly accompanied with pancreatic cancer, we should pay attention to the related side-effect of FAPtargeted radiotherapy. In addition, overexpression of FAP in the stroma is reported to be associated with tumor progression and poor prognosis [34,35]. The prognostic value of [ 68 Ga]Ga-DOTA-FAPI PET/CT uptake in pancreatic cancer remains to be investigated.
There are some limitations to our study. First, the number of the enrolled patients was limited. Future study should be conducted to provide a more comprehensive overview of [ 68 Ga]Ga-DOTA-FAPI PET/CT in managing pancreatic tumors. Second, it is di cult to obtain the pathology of all the metastases. In the present study, we collected and analysis the follow-up of 6 months to con rm the results. Pathologic con rmation would be performed in the future study.