Added value of regional ¹⁸F-FDG PET/MRI-assisted whole-body ¹⁸F-FDG PET/CT in malignant ascites with unknown primary origin

Background Comparing to PET/CT, integrative PET/MRI imaging provides superior soft tissue resolution. This study aims to evaluate the added value of regional delayed F-18-FDG PET/MRI-assisted whole-body F-18-FDG PET/CT in diagnosing malignant ascites patients.Results The final diagnosis included 22 patients with ovarian cancer (n = 11), peritoneal cancer (n = 3), colon cancer (n = 2), liver cancer (n = 2), pancreatic cancer (n = 2), gastric cancer (n = 1), and fallopian tube cancer (n = 1). The diagnosis of the primary tumor using whole-body PET/CT was correct in 11 cases. Regional PET/MRI-assisted whole-body PET/CT diagnosis was correct in 18 cases, including 6 more cases of ovarian cancer and 1 more case of fallopian tube cancer. Among 4 cases that were not diagnosed correctly, 1 case had the primary tumor outside of the PET/MRI scan area, 2 cases were peritoneal cancer, and 1 case was colon cancer. The diagnostic accuracy of regional PET/MRI-assisted whole-body PET/CT was higher than PET/CT alone (81.8% vs. 50.0%, kappa (2) = 5.14, p = 0.023). The primary tumor conspicuity score of PET/MRI was higher than PET/CT (3.67 +/- 0.66 vs. 2.76 +/- 0.94, P < 0.01). In the same scan area, more metastases were detected in PET/MRI than in PET/CT (156 vs. 86 in total, and 7.43 +/- 5.17 vs. 4.10 +/- 1.92 per patient, t = 3.89, P < 0.01). Lesion-to-background ratio in PET/MRI was higher than that in PET/CT (10.76 +/- 5.16 vs. 6.56 +/- 3.45, t = 13.02, P < 0.01).Conclusion Comparing to whole-body PET/CT alone, additional delayed regional PET/MRI with high soft tissue resolution is helpful in diagnosing the location of the primary tumor and identifying more metastases in patients with malignant ascites. Yet larger sample size in multicenter and prospective clinical researches is still needed.