[68Ga]Ga-FAPI-04 PET/CT Delineates Peritoneal Carcinomatosis in Invasive Lobular Breast Cancer with Limited FDG Avidity

Case Report Invasive lobular breast carcinoma (ILC) has a recognized tendency for infiltrative metastatic involvement of the peritoneum, retroperitoneum, gastrointestinal tract, and gynecologic organs [ 1 ] [ 2 ]. Because ILC may show low or heterogeneous [ 18 F]FDG avidity, particularly in hormone receptor-positive disease and in infiltrative growth patterns, peritoneal disease can be subtle on FDG PET/CT [ 3 ]. Fibroblast activation protein inhibitor (FAPI) PET/CT may provide additive lesion conspicuity in this setting because of fibroblast activation protein expression in cancer-associated fibroblasts and generally low physiologic abdominal background activity [ 4 ] [ 5 ]. A 48-year-old woman with a history of left breast ILC had initially undergone breast-conserving surgery and sentinel lymph node biopsy approximately four years earlier. Pathology showed grade 2 ILC measuring approximately 1.3 cm, estrogen receptor-positive, progesterone receptor-positive, HER2-negative disease with loss of E-cadherin expression and one metastatic sentinel lymph node. Adjuvant breast and axillary radiotherapy was subsequently administered. Approximately three years after the initial diagnosis, the patient developed ascites and suspected peritoneal disease. Cytologic examination of ascitic fluid demonstrated malignant cells compatible with metastatic breast carcinoma. Peritoneal and omental biopsies confirmed malignant epithelial tumor infiltration consistent with metastatic ILC. Immunohistochemistry showed estrogen receptor positivity, progesterone receptor positivity, GATA3 positivity, PAX8 negativity, HER2 negativity by fluorescence in situ hybridization, and loss of E-cadherin expression. These findings supported metastatic lobular breast carcinoma rather than a primary gynecologic malignancy. Systemic therapy for metastatic hormone receptor-positive breast cancer was initiated. During follow-up, [ 18 F]FDG PET/CT performed for treatment response assessment demonstrated ascites with only minimal peritoneal metabolic abnormality. No clearly delineated or measurable FDG-avid peritoneal implant was identified for SUVmax assessment. A small left axillary lymph node showed minimal stable FDG uptake, and previously treated osseous lesions were metabolically inactive. One month later, [ 68 Ga]Ga-FAPI-04 PET/CT was performed because of persistent clinical concern for peritoneal disease. The scan was acquired after administration of approximately 100 MBq (2.70 mCi) [ 68 Ga]Ga-FAPI-04 using a Siemens Biograph Trinion EB2 PET/CT scanner. In contrast to the preceding FDG PET/CT, FAPI-04 PET/CT demonstrated moderate-to-intense tracer uptake along the gastric and intestinal serosal surfaces and within mesenteric/peritoneal regions in the presence of ascites; the highest peritoneal lesion SUVmax was 17.1. In the upper abdomen, perihepatic/peritoneal-serosal uptake and a small FAPI-avid hepatic focus were clearly visualized on FAPI-04 PET/CT, whereas the corresponding FDG findings were subtle and poorly delineated ([ Fig. 1 ]). In the lower abdomen and pelvis, FAPI-04 PET/CT similarly demonstrated conspicuous uptake along mesenteric and peritoneal/serosal surfaces, compatible with peritoneal carcinomatosis ([ Fig. 2 ]). Whole-body maximum-intensity projection images further highlighted the greater conspicuity of multifocal abdominal peritoneal and mesenteric uptake on FAPI-04 PET compared with FDG PET ([ Fig. 3 ]). The FAPI-04 findings were concordant with the previously confirmed peritoneal metastasis from ILC. Intense uterine corpus uptake was also observed and was interpreted primarily as physiologic uptake. Fig. 1 Comparative upper abdominal [ 18 F]FDG and [ 68 Ga]Ga-FAPI-04 PET/CT findings in metastatic invasive lobular breast carcinoma. Coronal fused PET/CT images ( a, b ) and axial fused PET/CT images ( c, d ) are shown. On [ 18 F]FDG PET/CT ( a, c ), peritoneal disease is subtle and poorly delineated. On [ 68 Ga]Ga-FAPI-04 PET/CT ( b, d ), tracer uptake is clearly seen along the perihepatic/peritoneal-serosal surfaces (thick arrows), and a small FAPI-avid hepatic focus is also noted (thin arrows), suspicious for metastatic involvement. Fig. 2 Comparative lower abdominal and pelvic [ 18 F]FDG and [ 68 Ga]Ga-FAPI-04 PET/CT findings. Coronal fused PET/CT images ( a, b ) and axial fused PET/CT images ( c, d ) are shown. [ 18 F]FDG PET/CT ( a, c ) demonstrates only limited or equivocal abnormality in the pelvis, whereas [ 68 Ga]Ga-FAPI-04 PET/CT ( b, d ) reveals conspicuous tracer uptake along mesenteric and peritoneal/serosal surfaces in the lower abdomen and pelvis (arrows), compatible with peritoneal carcinomatosis. Fig. 3 Maximum-intensity projection images of [ 18 F]FDG PET and [ 68 Ga]Ga-FAPI-04 PET. Compared with [ 18 F]FDG PET ( a ), [ 68 Ga]Ga-FAPI-04 PET ( b ) demonstrates clearly increased tracer uptake in multifocal abdominal peritoneal and mesenteric regions (arrows), highlighting the extent of peritoneal carcinomatosis. Recent studies have reported the potential value of FAPI PET/CT in ILC. Sahin et al. found higher tumor uptake and tumor-to-background ratios with [ 68 Ga]Ga-FAPI PET/CT than with [ 18 F]FDG PET/CT in patients with ILC [ 6 ]. In a pilot study of metastatic non-FDG-avid ILC, Eshet et al. reported that [ 68 Ga]Ga-FAPI PET/CT detected additional disease sites, particularly in infiltrative soft-tissue and serosal locations [ 7 ]. Our case extends prior reports by demonstrating marked [ 68 Ga]Ga-FAPI-04 uptake in serosal and mesenteric peritoneal disease in a patient with previously cytologically and histopathologically confirmed hormone receptor-positive, HER2-negative ILC, despite only subtle peritoneal findings on the preceding [ 18 F]FDG PET/CT. The potential role of [ 68 Ga]Ga-FAPI-04 PET/CT in peritoneal carcinomatosis has also been reported beyond breast cancer [ 8 ], and breast cancer case reports have described more conspicuous visualization of peritoneal carcinomatosis and other metastatic sites on FAPI PET/CT than on FDG PET/CT [ 9 ] [ 10 ]. Because the two PET/CT examinations were performed sequentially rather than as a same-day head-to-head comparison, the observed difference should be interpreted as improved lesion conspicuity on FAPI-04 PET/CT in this clinical context rather than definitive evidence of diagnostic superiority. Nevertheless, FAPI uptake is not tumor-specific. Benign or physiologic uptake may be seen in inflammatory, fibrotic, post-treatment, muscular, mammary, degenerative, and uterine conditions [ 11 ]. Therefore, FAPI-positive findings should be interpreted together with clinical history, morphologic imaging, and histopathology whenever possible. In the present case, the peritoneal and omental biopsy results strongly supported the interpretation of the FAPI-avid serosal and mesenteric lesions as metastatic peritoneal carcinomatosis, whereas the uterine corpus uptake was interpreted primarily as physiologic. In conclusion, [ 68 Ga]Ga-FAPI-04 PET/CT may provide complementary information for evaluating peritoneal carcinomatosis in metastatic ILC, particularly when [ 18 F]FDG PET/CT findings are subtle or equivocal. Publication History Received: 05 May 2026 Accepted: 19 May 2026 Article published online: 11 June 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany