68Ga-DOTA-FAPI-04 PET/MR in the Evaluation of Gastric Carcinomas: Comparison with 18F-FDG PET/CT

Abstract

We sought to evaluate the performance of 68Ga-DOTA-FAPI-04 ( 68 Ga-FAPI) PET/MR for the diagnosis of primary tumor and metastatic lesions in patients with gastric carcinomas and to compare the results with those of 18F-FDG PET/CT. Methods: Twenty patients with histologically proven gastric carcinomas were recruited, and each patient underwent both 18F-FDG PET/CT and 68 Ga-FAPI PET/MR. A visual scoring system was established to compare the detectability of primary tumors and metastases in different organs or regions (the peritoneum, abdominal lymph nodes, supradiaphragmatic lymph nodes, liver, ovary, bone, and other tissues). The original SUVmax and normalized SUVmax (calculated by dividing a lesion's original SUVmax with the SUVmean of the descending aorta) of selected lesions on both 18F-FDG PET/CT and 68Ga-FAPI PET/MR were measured. Original/normalized SUVmax-FAPI and SUVmax-FDG were compared for patient-based (including a single lesion with the highest activity uptake in each organ/region) and lesion-based (including all lesions [≤5] or the 5 lesions with highest activity [>5]) analyses, respectively. Results: The 20 recruited patients (median age: 56.0 y; range: 29-70 y) included 9 men and 11 women, 14 patients for initial staging and 6 for recurrence detection. 68Ga-FAPI PET was superior to 18F-FDG PET for primary tumor detection (100.00% [14/14] vs. 71.43% [10/14]; P = 0.034), and the former had higher tracer uptake levels (P < 0.05). 68Ga-FAPI PET was superior to 18F-FDG PET in both patient-based and lesion-based evaluation except for the metastatic lesions in supradiaphragmatic lymph nodes and ovaries. Additionally, multiple sequences of MR images were beneficial for the interpretation of hepatic metastases in 3 patients, uterine and rectal metastases in 1 patient, ovarian lesions in 7 patients, and osseous metastases in 2 patients. Conclusion:68Ga-FAPI PET/MR outperformed 18F-FDG PET/CT in visualizing the primary and most metastatic lesions of gastric cancer and might be a promising method, with the potential of replacing 18F-FDG PET/CT.

Keywords: 18F-FDG; 68Ga-FAPI; PET/MR; fibroblast activation protein; gastric cancer.

© 2022 by the Society of Nuclear Medicine and Molecular Imaging.

Figures

Graphical abstract

FIGURE 1.

Representative images of 68Ga-FAPI PET (A) and 18F-FDG PET (B) in patients with gastric cancer for initial staging (patients 1, 2, 3, and 10) and recurrence detection (patients 18 and 19) (from left to right). 68Ga-FAPI images are superior to 18F-FDG images in visualization of primary tumors and metastases. 68Ga-FAPI total scores were 8, 7, 6, 5, 3, and 2, respectively, and all of the 18F-FDG total scores were 0.

FIGURE 2.

Comparison of visual assessment between 68Ga-FAPI PET and 18F-FDG PET. n(n) in each bar refers to patient number (scores); M = metastases.

FIGURE 3.

A 61-y-old man (patient 7) with moderately differentiated gastric adenocarcinoma. In addition to the primary tumor (A, white arrow, SUVmax = 11.0), 2 foci of elevated activity in the liver were noted on the 18F-FDG PET/CT images (A, yellow arrows, SUVmax = 5.8). On the 68Ga-FAPI PET/MR images, the primary tumor had more intense uptake (B and C, white arrows, SUVmax = 14.2), and the 2 hepatic lesions had more prominent 68Ga-FAPI accumulation (B, yellow arrows, SUVmax = 7.6). Additionally, multiple foci of increased 68Ga-FAPI activity were also revealed in the liver (C, red outline, yellow arrows), which corresponded to multiple high signals on DWI (yellow arrows), suggesting multiple hepatic metastases.

FIGURE 4.

A 45-y-old woman (patient 20) with a surgical history of gastric cancer 4 y previously. 18F-FDG PET/CT (A) showed a mass in the uterus with heterogeneous density and intense 18F-FDG activity (yellow arrows, SUVmax = 9.1), which involved the adjacent rectum (white arrows, SUVmax = 5.7). On 68Ga-FAPI PET/MR (B), there was intense 68Ga-FAPI uptake throughout the uterus (SUVmax = 12.7), which may be physiologic uptake. When signal changes on multiple sequences of MR are interpreted, uterine masses (yellow arrows), disappearance of the fat space, and rectal involvement (white arrows) can be observed and diagnosed as metastases. ADC = apparent diffusion coefficient.

FIGURE 5.

Comparison between 18F-FDG PET/CT (A) and 68Ga-FAPI PET/MR (B) for ovarian metastases in a 38-y-old woman (patient 1). Ovaries had slightly increased 18F-FDG accumulation (SUVmax = 2.6) (A, arrows); it was difficult to determine whether this accumulation was physiologic uptake or metastases. On 68Ga-FAPI PET/MRI, increased uptake (SUVmax = 4.1) was observed in enlarged ovaries with significant MR signal changes (heterogeneous signal on T2-weighted images [T2WI], high signal on DWI, and low signal on apparent diffusion coefficient [ADC]), which enhanced the confidence in making a diagnosis of metastases.

FIGURE 6.

Corpus luteum as false-positive on 18F-FDG PET/CT and 68Ga-FAPI PET/MR in a 35-y-old woman (patient 12). Two foci of increased 18F-FDG activity (A, arrows, SUVmax = 5.0 [right], 5.7 [left]) and 68Ga-FAPI activity (B, arrows, SUVmax = 4.3 [right], 3.3 [left]) were observed in ovaries, but without obvious abnormal morphology and signal changes on MR images (B). Operative exploration confirmed these “lesions” as corpus luteum. ADC = apparent diffusion coefficient.