PET/CT vs. PET/MRI for Whole-Body Cancer Imaging

PET/CT (positron emission tomography / computed tomography) is the current gold standard imaging technique for whole-body assessment of the majority of cancers. Recently, PET/MRI (positron emission tomography / magnetic resonance imaging) systems have become commercially available and are gradually being introduced into routine clinical practice. At present, however, it is unclear in which situations i.e., for which cancer types PET/MRI will replace PET/CT, due to a higher lesion detection rate, or due to a comparable performance, but a significantly lower radiation dose. After all, unlike CT, MRI does not rely on ionizing radiation, which is especially relevant for younger patients. Since MRI is generally superior to CT in terms of soft-tissue contrast, it seems plausible that PET/MRI may outperform PET/CT for the whole-body assessment of several cancers, in particular those that involve the liver, the brains, or the bones. Thus, the goal of the present study is to establish, in a large scale prospective study, for which subgroups of oncologic patients PET/MRI may be the future imaging technique of choice.

It was the goal of the FWF project to compare the two hybrid imaging techniques PET/CT (positron emission tomography / computed tomography) and PET/MRI (positron emission tomography / magnetic resonance imaging) for evaluation of cancer patients. PET/MRI generally provided a moderately better accuracy, which also led to changes in patient management in some patients. PET/CT and PET/MRI were performed consecutively on the same day to achieve optimal comparability of the techniques. On the one hand, patient subgroups with distinct cancer subtypes such as neuroendocrine tumors and lymphomas (a type of blood cancer typically affecting lymph nodes) were examined; on the other hand, the comparison between PET/CT and PET/MRI was compared in the entire population of cancer patients that comprised 263 individuals. Our results show that, in lymphoma, PET/MRI to PET/CT, mainly because the MRI component of PET/MRI can visualize slowly growing types of lymphoma, such as the so-called MALT lymphoma. PET may, however, be able to visualize these MALT lymphomas if images are obtained at a later time point after injection of the radiolabeled glucose analogue ([18F]FDG). With regard to neuroendocrine tumors, the main advantage of PET/MRI lies in the detection of secondary cancer spread to the liver (metastases). In the entire study population of 263 cancer patients (in which also lung cancer and black skin cancer were larger groups) that underwent a total of 330 PET/MRI and PET/CT examinations, a 13% higher accuracy (per examination) for PET/MRI with regard to detection of cancer was observed. In particular, PET/MRI was superior to PET/CT for the assessment of brain metastases (which are frequently seen in patients with lung cancer and black skin cancer) and liver metastases; on the other hand, it was slightly inferior to PET/CT for detection of lung metastases. In 8% of the patients, the cancer manifestations detected only by PET/MRI, but not by PET/CT, had an impact on patient management (e.g., type of treatment, follow-up interval, need for additional tests). Contrary to that, the additional lung metastases that were only detected by PET/CT, but not PET/MRI, did not affect patient management in a single case. Thus, overall, PET/MRI appears to be moderately superior to PET/CT for imaging of cancer patients.