18F-AlF-NOTA-FAPI-04 PET/CT as a complementary tool for detecting occult signet-ring cell gastric adenocarcinoma when FDG imaging is inconclusive: a case report.
Cheng Yuan, Mingzhe Lai, Yanzhi Fu, Peng Wang, Meini Pan, Rui Bai, Wenli Dai, Eyi Ben, Lu Xu, Xiang Dong, Tingzhuang Yi, Lihua Ni
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Abstract
Background: Accurate identification of the primary site in metastatic cancer remains a significant challenge when conventional imaging, such as fluorine-18 fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT), yields inconclusive results. This issue is particularly evident in stroma-rich tumors, such as signet-ring cell carcinoma, which exhibit low glycolytic activity and are characterized by an abundance of cancer-associated fibroblasts (CAFs). 18F-AlF-NOTA-FAPI-04 PET/CT, which targets fibroblast activation protein (FAP), offers a promising alternative by providing detailed imaging of the tumor microenvironment (TME).
Case description: A 34-year-old male presented with altered bowel habits and right inguinal lymphadenopathy. Histology of the lymph node revealed metastatic poorly differentiated carcinoma, suggesting gastrointestinal origin. However, abdominal CT and 18F-FDG PET/CT failed to identify the primary lesion, showing only mild gastric wall thickening with weak FDG uptake. One week later, fluorine-18 fibroblast activator protein inhibitor (18F-FAPI) PET/CT revealed significantly increased uptake in the gastric fundus and body, and a hypermetabolic lesion in the right iliopsoas muscle. Guided by these findings, targeted biopsies confirmed poorly differentiated gastric adenocarcinoma with signet-ring cell features. After eight cycles of capecitabine plus oxaliplatin (XELOX) chemotherapy over a 6-month period, follow-up 18F-FAPI PET/CT showed marked tumor regression.
Conclusions: 18F-FAPI PET/CT can provide complementary biological and diagnostic information when 18F-FDG imaging is inconclusive, particularly in stroma‑rich gastric cancers with signet‑ring cell features. In this patient, FAPI PET/CT enabled targeted biopsy and demonstrated treatment response, suggesting its potential value in clinical management. Larger prospective studies are warranted to further define the role of 18F-FAPI PET/CT in the diagnostic algorithm of occult primary tumors.
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