European Radiology: Comparison of uptake of [68Ga]Ga-FAPI and [18F]FDG in patients with gastric ring-ring cell carcinoma

Gastric ring cell carcinoma (GSRCC) is a relatively rare pathological subtype of gastric cancer, accounting for less than 10% of all gastric cancers, with a very high degree of malignancy and poor
prognosis.
Studies have shown that patients with early-stage GSRCC have a higher 5-year survival rate than non-GSRCC patients
.
Due to the lack of specific clinical manifestations, more than 90% of patients with GSRCC are diagnosed with advanced disease; Therefore, accurate and early disease staging is critical
to treatment planning and prognosis for patients with GSRCC.

At present, 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) positron emission tomography ([18F]FDG PET) plays an important role
in the staging, re-staging, treatment planning, and treatment response of various cancers.
However, the application of [18F]FDG PET imaging in gastric cancer is challenging because of the disturbance
of uneven distribution of gastric absorption caused by different inflammatory and physiological states.
[18F] The suboptimal sensitivity of FDG PET/CT for detecting affected lymph nodes, liver metastases, and peritoneal metastases further limits the use of [18F]FDG PET/CT for tumor staging
in GSRCC.

Fibroblast activating protein (FAP) is a matrix antigen that is highly expressed by activated fibroblasts, including cancer-associated fibroblasts (CAFs), while FAP is limited
in normal healthy tissues.
FAP-specific inhibitors (hereinafter referred to as FAPI) have recently been developed as PET tracers ([68Ga]Ga-FAPI) that can be used to target FAP and visualize
tumor stroma.
In previous studies, [68Ga]Ga-FAPI PET/CT has shown greater tumor-to-background ratio (TBR) and lesion detection rates than [18F]FDG imaging in various types of cancer, particularly gastric cancer
.

Recently, a study published in the journal European Radiology explored the tumor detection and accuracy of [68Ga]Ga-FAPI PET imaging in primary and metastatic GSRCC and compared
the results with [18F]FDG PET.

This retrospective multicentre analysis included 34 patients
with histologically confirmed GSRCCs from four medical centres.
The maximum standard uptake value (SUVmax), tumor-to-background ratio (TBR), and diagnostic accuracy were compared
between the two methods.
The uptake of [18F]FDG and [68Ga]Ga-FAPI was compared using the Wilcoxon signature rank test
.
The McNemar test is used to compare the diagnostic accuracy
of the two techniques.

Data were analyzed from 27 pairs of PET/CT and 7 pairs of PET/MRI scans from 34 patients with GSRCC (16 men and 18 women), with a median age of 51 years (range: 25-85 years).

[68Ga] Ga-FAPI PET showed higher SUVmax and TBR values (SUVmax: 5.
2 vs 2.
2, p = 0.
001; TBR: 7.
6vs 1.
3, p < 0.
001) and lymph nodes in primary tumors than [18F]FDG PET Involvement (SUVmax: 6.
8 vs.
2.
5, p < 0.
001; TBR: 5.
8 vs.
1.
3, p < 0.
001) and bone and visceral metastases (SUVmax: 6.
5 vs.
2.
4, p < 0.
001; TBR: 6.
3 vs.
1.
3, p < 0.
001).

。 In terms of diagnostic performance, [68 Ga]Ga-FAPI PET was instrumental in detecting primary tumors (73% [16/22] vs.
18% [4/22], p < 0.
001), local recurrence (100% [7/7] vs.
29% [2/7], p = 0.
071), lymph node metastasis (77% [59/77] vs.
23% [18/77], p < 0.
001).
Distant metastases (93% [207/222] vs.
39% [86/222], p < 0.
001) were higher than [18F]FDG PET.

Figure A representative case of extensive gastric signet ring cell carcinoma (GSRCC) detected by [68Ga]Ga-FAPI PET/CT.

a A 49-year-old male, known to have GSRCC, underwent a preliminary stage of [18F]FDG PET/CT, showing low to moderate uptake of the primary tumor (white arrow), affected abdominal lymph nodes (green arrow), and bone metastases (yellow arrow).

This study showed that [68Ga]Ga-FAPI PET exhibited significantly greater radionuclide uptake and TBRs than [18F]FDG in primary and metastatic GSRCC, thereby improving the sensitivity and accuracy
of GSRCC diagnosis.

Original source:

Haojun Chen,Yizhen Pang,Junpeng Li,et al.
Comparison of [ 68 Ga]Ga-FAPI and [ 18 F]FDG uptake in patients with gastric signet-ring-cell carcinoma: a multicenter retrospective study.
DOI:10.
1007/s00330-022-09084-9