PET/MRI and PET/CT: is there room for both at the top of the food chain?

Abstract

In this issue of the journal, Vrachimis and colleagues demonstrate that positron emission tomography combined with Xray computed tomography (PET/CT) outperforms PET combined with magnetic resonance imaging (PET/MRI) in restaging radioiodine-negative patients with biochemical relapse of differentiated thyroid carcinoma [1]. This comes as a surprise, at least to the uninitiated: In a world predominantly driven by material considerations, common sense would presume that diagnostic accuracy correlates with costs, and PET/ MRI indeed Boutperforms^PET considerably with regard to its demands on resources spent on acquisition and operation.A closer look at the data proffered by Vrachimis et al. reveals that the poorer performance of PET/MRI compared to PET/CT for this indication resides in its shortcomings in the ability to diagnose pulmonary metastases, a frequent occurrence in progressive thyroid carcinoma. MRI aficionados will argue that this problem may be overcome by using MRI sequences dedicated to lung imaging. This contingent may ultimately be correct, but such arguments are purely speculative at the moment, as empirical proof for the case of PET/MRI is lacking.A comprehensive review on the evidence reporting the use of PET/MRI in a clinical setting is beyond the scope of this editorial. However, a short appraisal of the hitherto published literature comparing the utility of PET/MRI and PET/CT can be attempted: At the time of this writing, 166 papers featuring the keyword combination BPET/MR PET/CT^currently reside in the Web of Science (ISI) Internet library. Of these, roughly 22 are original articles comparing the diagnostic performance of these two hybrid imaging modalities in oncology. Six report data from mixed groups of patients [2][3][4][5][6][7], with five finding an equivalence of diagnostic performance of the two hybrid imaging technologies and one a slight superiority of PET/MRI over PET/CT with regard to influence on further clinical management. None deals with thyroid cancer directly, allowing us to consider the new publication under discussion by Vrachimis et al. the first to study this neoplastic entity specifically. However, the utility of PET/MRI and PET/CT to detect and characterize small pulmonary nodules more generally has been previously investigated in three papers, with the data tipping the scales in favour of PET/CT [8][9][10]. The remaining 13 publications report some evidence that PET/ MRI is superior to PET/CT for characterizing primary tumours, e.g. primary bone tumours [11][12][13] or oesophageal carcinoma [14], but do not demonstrate significant advantages of whole-body MRI over whole-body PET/CT for N or M staging [15][16][17][18][19][20][21][22][23][24].The introduction of PET/MRI was heralded as a technological breakthrough, and many expected a subsequent clinical revolution, with the newer modality vying to supersede PET/CT atop the diagnostic food chain just as the Burmese python is supplanting the American alligator in the Florida Everglades. However, none of the above-mention...

Additional Clinical Value for PET/MRI in Oncology: Moving Beyond Simple Diagnosis

“…Si nce the introduction of clinical PET/MRI in 2010, much of the research into this modality has focused on diagnostic performance. However, comparative studies to date have largely shown equivalent diagnostic capabilities for these modalities (1,2). Where benefits for PET/MRI have been demonstrated, they are typically small or apply to relatively small populations of patients.…”