A lesson in humility: the added values of PET-MRI over PET-CT in detecting malignant hepatic lesions

Abstract

Purpose: The recent introduction of integrated PET-MRI systems into practice seems promising in oncologic imaging. Efforts are made to specify the added-values of this modality. The current study evaluates the added-values of PET-MRI over PET-CT in detecting active malignant hepatic lesions.Methods: As part of an ongoing prospective study in our institution that assesses the added-values of PET-MRI over PET-CT, subjects undergo whole-body PET-CT and subsequent dedicated PET-MRI after single radiotracer injection. The current study included 97 consecutive paired [18F]FDG PET-CT and liver PET-MRI scans (of 61 patients) interpreted as showing active malignant hepatic involvement. Primary malignancies were of colorectal/biliary/pancreatic/breast/other origins in 19/9/9/7/17 patients. Eightysix paired scans were performed for monitoring response to therapy. When PET-MRI detected additional malignant lesions over PET-CT, lesions size, the main advantage PET-MRI offered, and the in uence on the nal report were recorded.Results: In 37/97 (38.1%) cases, a total of 78 malignant lesions were reported based on PET-MRI data only. The improved detectability on PET-MRI was evident in three groups of lesions: (1) [18F]FDG-avid lesions seen on PET of PET-MRI but not on PET of PET-CT (11 cases, 19 lesions); (2) small lesions (≤0.8cm) identi ed by MRI only (14 cases, 37 lesions); (3) lesions >0.8cm with low/no [18F]FDG-uptake categorized as viable based on MRI (12 cases, 22 lesions). These lesions caused major effect on nal reports in 11/97 (11.3%) cases, changing reported response assessment category (10/86 cases) or de ning malignant hepatic disease on staging/restaging scans (1/11 cases).Conclusion: PET-MRI offers several advantages over PET-CT in assessing the extent and response to therapy of malignant hepatic involvement. Additional malignant lesions are detected by PET-MRI due to better PET performance, greater spatial resolution provided by MRI, and improved multi-parametric viability assessment of lesions. In around one-tenth of cases, such ndings signi cantly change the nal report's conclusion.

The use of PET/MRI in radiotherapy

“…Due to its high soft tissue contrast, PET/MRI may be extremely accurate in T-staging of cancer, such as head and neck, abdominal, and pelvic tumors. Recent studies have begun to show that it is also valuable in treating lymphoma, liver, and breast cancer [ 7 – 9 ]. When it comes to finding lymph node metastases and faraway metastases, PET/MRI is also superior to CT, MRI, and PET/CT, as PET/MRI can provide dynamic enhanced imaging, diffusion-weighted imaging (DWI), and signal strength (SI) assessment that can display non-fluorodeoxyglucose (non-FDG) uptake lesions and avoid misdiagnosis of sites with physiologic FDG uptake, such as the adrenal glands and mucous membranes [ 10 – 13 ].…”