Comparison of 68Ga-FAPI-04 and 18F-FDG for the Detection of Primary Gastric Cancers and Metastasis 


 Introduction Early and precise diagnosis and staging of gastric cancer are important for its treatment and management. However, the low sensitivity of 18F-fluorodeoxyglucose (18F-FDG) for gastric cancer diagnosis limits its application. Currently, the tracer 68Ga-FAPI, which targets fibroblast activation protein (FAP), is widely used to diagnose various cancers. However, the diagnostic value of 68Ga-FAPI in gastric cancer is still unclear. In this study, we aimed to investigate the potential advantage of 68Ga-FAPI-04 over 18F-FDG in the evaluation of gastric cancer.Methods: Thirty-eight patients with gastric cancer (31 with adenocarcinoma and 7 with signet ring cell carcinoma) were recruited for this study. All of the participants underwent 68Ga-FAPI-04 and 18F-FDG imaging by positron emission tomography (PET)/computed tomography (CT) or PET/magnetic resonance (MR). The results were interpreted by two experienced nuclear medicine physicians, and the maximum standardized uptake value (SUVmax) was calculated.Results: For the detection of primary gastric cancer, the sensitivities of 68Ga-FAPI-04 PET and 18F-FDG PET were 100% (38/38) and 81.6% (31/38), respectively. Four cases of adenocarcinoma and three cases of signet ring cell carcinoma were missed by 18F-FDG PET. The SUVmax of 68Ga-FAPI-04 in tumors greater than 4 cm (11.0 ± 4.5) was higher than tumors less than 4 cm (4.5 ± 3.2) (P = 0.0015). The SUVmax of 68Ga-FAPI-04 was higher in T2-4 tumors (9.7 ± 4.4) than in T1 tumors (3.1 ± 1.5) (P = 0.0002). For the detection of metastatic lesions, the sensitivities of 68Ga-FAPI-04 PET and 18F-FDG PET in 10 patients with regional lymph node metastasis and distant metastasis were 6/10 and 5/10, respectively.Conclusion: Compared to 18F-FDG PET, 68Ga-FAPI-04 PET had superior potential in detecting primary gastric cancers and metastatic lymph nodes, 68Ga-FAPI-04 PET also had a better performance on small gastric cancer detection. 68Ga-FAPI-04 PET could provide better performance for gastric cancer diagnosis and staging.


Introduction
Gastric cancer is one of the most common malignant tumors, with a poor prognosis in East Asia and the USA [1,2]. Although the diagnosis and treatment of gastric cancer have improved, the prognostic outcome remains suboptimal. Early diagnosis, accurate staging and quantitative evaluation of gastric cancer are important for its treatment, management, and prognosis. In the clinic, gastroscopy and imaging examinations are the main diagnostic methods for gastric cancer. Computed tomography (CT) and magnetic resonance imaging (MRI) have been used for the primary staging of gastric cancer.
However, CT and MRI are anatomy-based imaging techniques that have been inadequate in staging, such as lymph node involvement and distant metastasis [3].
Positron emission tomography (PET) using 18 F-uorodeoxyglucose ( 18 F-FDG) has been used in various cancer diagnoses and in gastric cancer detection [4,5]. However, up to 53% of primary tumors in gastric cancer are not avid for 18 F-FDG [6,7]. Furthermore, low sensitivity in detecting lymph node metastasis limits the use of 18 F-FDG in gastric cancer diagnosis [7]. Signet ring cell carcinoma and mucinous carcinoma also show lower 18 F-FDG uptake and sensitivity than conventional adenocarcinoma [8][9][10].
Therefore, new tracers such as 3'-deoxy-3'-18 F-uorothymidine ( 18 F-FLT) have been developed for gastric cancer [7,11]. Since the lower uptake of 18 F-FLT in gastric cancer, this tracer is not applied in clinical practice.
Currently, broblast activation protein (FAP), which is highly expressed in the cancer-associated broblasts (CAFs) of many malignant tumors, is used as a new target for tumor tracer development since CAFs are one of the most abundant stromal components in the tumor microenvironment [12]. Several tracers targeting FAP have been developed, among which 68 Ga-FAPI-04 is the most promising, with high a nity towards FAP and suitable kinetics [13][14][15]. 68 Ga-FAPI-04 PET has achieved good outcomes for the diagnosis and staging of several tumor types [12,[16][17][18]. Moreover, previous studies have shown that FAP is overexpressed in the CAFs of gastric cancer and plays an important role in the invasion and migration of gastric carcinomas [19,20]. 68 Ga-FAPI-04 PET was used to image primary gastric carcinoma and metastatic lesions in one patient [21]. However, no research has reported the potential advantage of 68 Ga-FAPI-04 in gastric cancer diagnosis and the difference between 68 Ga-FAPI-04 and 18 F-FDG.
The aim of this study was to assess the potential advantage of 68 Ga-FAPI-04 PET over 18 [12,25]. A low-dose CT scan (120 keV, 100-120 mA) or an MR scan was collected for attenuation correction and image fusion. PET images were acquired in 3D mode and were reconstructed by the ordered subset expectation maximization 3D (OSEM 3D) method. The two PET scans were performed within 48 hours apart (1.6 ± 0.8 days, range: 1-2 days).

Data analysis
For calculation of the maximum SUV (SUV max ), circular regions of interest were drawn around the tumors on transaxial slices and automatically adapted to a three-dimensional VOI with Syngo.via software (Siemens Molecular Imaging, Hoffman Estates, Illinois, USA) at a 60% isocontour. The results of 68 Ga-FAPI-04 PET and 18 F-FDG PET scans were independently evaluated by 2 experienced nuclear medicine physicians (JZ and FH) who were blinded to the clinical data and pathologic ndings. Any difference of opinion between these two physicians was resolved by a consensus. For primary tumors and metastatic lesions, positive uptake was identi ed as areas of focal increase compared to surrounding normal tissue by a visual assessment supported by the ratio of suspicious lesions to surrounding normal tissue [16,26].

Immunohistochemistry of FAP expression
Immunohistochemical staining of FAP was performed on the tumor tissue obtained after surgical resection in one patient with adenocarcinoma. An antibody against FAP (ab207178, Abcam) was used.

Statistical analysis
Statistical analysis was performed using Prism 7 (GraphPad Software, CA, USA) and STATA (version 15.1 StataCorp LLC) software. The comparisons of sensitivity, speci city, and accuracy for detection of gastric cancer between 18 F-FDG and 68 Ga-FAPI-04 were using the McNemar's test. Categorical variables are described as frequencies and percentages. Continuous variables are described as the mean ± standard deviation (SD). The Mann-Whitney U test was used to compare the SUV max between different categorized groups. A P value less than 0.05 was considered statistically signi cant.

Patients
The clinical characteristics of the 38 patients are shown in Table 1 and Supplementary Table 1. Among 38 patients with gastric cancer (29 men and 9 women; age 63.7 ± 15.3 years; age range, 25-86 years) whose diagnosis was con rmed by pathological biopsy under gastroscopy, 31 had adenocarcinoma and 7 had signet ring cell carcinoma. After the PET evaluation, 24 patients (19 with adenocarcinoma and 5 with signet ring cell carcinoma) received surgical treatment, while the other patients received chemotherapy. Among 24 patients treated with surgery, 10 (9 with adenocarcinoma and 1 with signet ring cell carcinoma) were con rmed pathologically with regional lymph node metastasis after surgical resection. According to the depth of tumor invasion based on examination of the surgical specimen after resection, 12 patients were classi ed as T1 (tumor invading the lamina propria, muscularis mucosae, or submucosa), 3 patient was classi ed as T2 (tumor invading the muscularis propria), and 9 patients were The SUV max of 18 F-FDG in tumors greater than 4 cm was higher than tumors less than 4 cm (6.3 ± 1.8 vs. 3.6 ± 1.7, P = 0.0064). The SUV max was higher in T2-4 tumors than in T1 tumors (5.6 ± 1.9 vs. 2.7 ± 0.9, P = 0.0021). The SUV max in poorly differentiated tumors () was signi cantly higher than in well and moderately differentiated tumours (6.2 ± 2.0 vs. 3.8 ± 1.9, P = 0.032).

Immunohistochemistry
One primary tumor sample obtained from one patient with gastric adenocarcinoma was assessed for FAP expression by immunohistochemistry. Stromal cells around the tumor presented prominent FAP expression, while tumor cells showed weak FAP expression (Fig. 2). 68 Ga-FAPI-04 PET showed high uptake (SUV max = 10.2) in the primary gastric tumor of this patient.

Discussion
Gastric cancer is one of the most common cancers and the third leading cause of cancer-related death worldwide [27]. Early and accurate diagnosis is extremely important for its treatment and prognosis. 68 Ga-FAPI-04 PET has achieved good outcomes for diagnosis in various tumors [12,[16][17][18]. These include high expression across a wide range of cancer types including several with typically low FDGavidity, low uptake in almost all normal tissues, where high physiological uptake can obscure primary or metastatic disease [28]. However, the usefulness of 68 Ga-FAPI-04 for the detection of primary gastric cancer remains unclear. In this study, we aimed to evaluate the usefulness of 68 Ga-FAPI-04 PET for the detection of gastric cancer compared with that of 18 F-FDG PET.
Our results demonstrated the potential advantage of 68 Ga-FAPI-04 PET over 18 F-FDG PET for the detection of primary gastric cancer. In all 38 patients with gastric cancer, all primary tumors were detected by 68 Ga-FAPI-04 PET. 68 Ga-FAPI-04 PET (100%, 38/38) provided better sensitivity than 18 F-FDG PET (82%, 31/38), consistent with a previous study reporting that more positive lesions on other type of tumors were discovered with 68 Ga-FAPI-04 than with 18 F-FDG, especially in gastric cancer [29]. In the present study, 68 Ga-FAPI-04 PET detected signet ring cell carcinoma of the stomach in one case that was missed by 18 F-FDG PET (Fig. 3). Furthermore, in another patient with gastric adenocarcinoma, 68 Ga-FAPI-04 displayed focal uptake in the primary tumor, whereas 18 F-FDG showed slightly and diffusely elevated uptake in the gastric wall (Fig. 4). As a result, 68 Ga-FAPI-04 could provide better surgery determination for gastric cancer. Moreover, the background activity of 68 Ga-FAPI-04 was low, resulting in a higher tumor-tobackground contrast than 18 F-FDG.
Previous studies have shown that the sensitivity of 18 F-FDG PET in the detection of gastric cancer ranges from 47% to 96% [7]. One of the reasons for this disparity may be the different histologic types of gastric cancer. 18 F-FDG showed lower sensitivity and uptake in signet ring cell carcinoma and mucinous carcinoma than in conventional adenocarcinoma [8][9][10]. This is due to the relatively low expression level of glucose transporter 1 (GLUT-1) in signet ring cell and mucinous carcinomas [30]. These results are consistent with our nding that signet ring cell carcinoma had lower 18 F-FDG uptake than adenocarcinoma, although the difference was not statistically signi cant. In our study, 68 Ga-FAPI-04 detected 3 cases of signet ring cell carcinoma that were negative in 18 F-FDG, resulting in a sensitivity of 100% (7/7) in detecting signet ring cell carcinoma and outperforming 18 F-FDG (57%, 4/7). Therefore, 68 Ga-FAPI-04 PET has an obvious advantage for the detection of signet ring cell carcinoma, especially when negative with 18 F-FDG. But the sensitivity of 68 Ga-FAPI-04 on signet ring cell carcinoma need larger samples to evaluate.
Tumor size is also an important factor in uencing the detection rate of gastric cancer in PET scans. Nakajo et al. [31] examined the relationship between the SUV max of primary gastric cancers and the size of visible tumors and found that the 18 F-FDG uptake of primary tumors was signi cantly associated with tumor size. In another study, small gastric cancers were reported to be di cult to detect by 18 F-FDG PET [32]. Interestingly, for detection of tumors less than 4 cm, 68 Ga-FAPI-04 PET also provided a better sensitivity than (100% vs. 70%, P = 0.062) 18 F-FDG, although the difference is not signi cant. All 17 tumors ≤ 4 cm in size, including 5 with negative 18 F-FDG uptake, were detected by 68 Ga-FAPI-04 PET in our study. The uptake of 68 Ga-FAPI-04 in small gastric cancers (≤ 4 cm in diameter) was lower than that in large gastric cancers (> 4 cm in diameter), it needs to be further con rmed whether 68 Ga-FAPI-04 PET has advantages in detecting small gastric cancers.
The depth of invasion in primary gastric cancer is essential for prognosis and therapy. The results from previous studies have shown that the SUV max of 18 F-FDG does not correlate with the degree of in ltration [31,33,34]. In addition, other tracers used to detect gastric cancer, such as 18 F-FLT, are not suitable for evaluating the degree of in ltration in gastric cancers [31]. In our study, the SUV max of 68 Ga-FAPI-04 in pathologically T4 tumors was signi cantly higher than that in T1 and T2 tumors (10.9 ± 4.3 vs. 3.8 ± 2.1, P = 0.0002). Thus, 68 Ga-FAPI-04 PET could provide insight into the degree of tumor invasion in gastric cancer.
In our patient-based analysis, the sensitivity of 18 F-FDG PET for detecting regional metastatic lymph nodes was 50% (5/10), which is in line with previous studies of 18 F-FDG PET for detecting lymph node metastasis (mean sensitivity: 45%) [7]. The sensitivity of 68 Ga-FAPI-04 PET for the detection of regional metastatic lymph nodes was 60% (6/10). As shown in Fig. 5, lymph node metastasis at the lesser curvature of the stomach in one patient with moderately differentiated adenocarcinoma presented elevated uptake of 68 Ga-FAPI-04 but negative uptake of 18 F-FDG. Chen et al. [16] reported 12 cases of gastric cancer (4 signet ring cell carcinomas and 8 adenocarcinomas) and found that 68 Ga-FAPI-04 PET/CT showed higher sensitivity than 18 F-FDG PET/CT for the detection of lymph node metastases of gastric cancer from their lesion-based analysis.
In addition, in contrast to 18 F-FDG PET, 68 Ga-FAPI-04 PET has an advantage in detecting distant metastasis in gastric cancer. Although pathological analysis of a biopsy serves as the gold standard for the diagnosis of metastases in gastric cancer, the noninvasive imaging technique has become a standard modality for staging before treatment. In this study, distant lymph nodes in 3 patients showed high 68 Ga-FAPI-04 uptake but negative 18 F-FDG uptake. These distant lymph nodes, which included the posterior peritoneum lymph nodes and supraclavicular lymph nodes, are prevalent metastatic sites of gastric cancer. As shown in Fig. 6, one patient displayed discernible 68 Ga-FAPI-04 uptake in the peritoneum that was not detected by 18 F-FDG PET. It is known that distant metastasis has an important impact on treatment and prognosis. In gastric cancer, peritoneal metastasis is associated with a poor prognosis [35]. According to previous studies, the sensitivity of 18 F-FDG PET for the detection of metastatic peritoneal disease is low [36,37]. Overall, 68 Ga-FAPI-04 PET may play an important role in detecting metastases of gastric cancer with or without con rmation by pathological biopsy.
There were several limitations to our study. The small sample size limited the power of the analysis, and not all histological types of gastric cancer were analyzed. Moreover, as there was no histological veri cation available for some cases of highly suspicious distant metastases, latent bias may be present. We also intended to investigate the advantage of PET/MR in the detection of gastric cancer metastases, but it seems infeasible due to the lack of contrast-enhanced MRI data in this study. Nevertheless, the usefulness of 68 Ga-FAPI-04 PET for the detection of distant metastases of gastric cancer needs to be further investigated on a larger cohort.

Conclusion
In summary, 68 Ga-FAPI-04 PET presents superior potential in detecting primary gastric cancers and metastatic lymph nodes than 18         A 65-year-old male patient with gastric adenocarcinoma and regional lymph node metastasis con rmed by pathology postoperatively. a, b, c 68Ga-FAPI-04 PET/MR displayed increased uptake in the primary lesion (SUVmax = 9.7) and lymph nodes (SUVmax = 3.3) at the lesser curvature of the stomach (arrow).
d, e, f 18F-FDG PET/MR showed high uptake in the primary tumor (SUVmax = 6.9) but no uptake in the regional lymph nodes.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. SupplementaryInformation.docx