Indeterminate Findings on Oncologic PET/CT: What Difference Does PET/MRI Make?

Positron emission tomography/computed tomography (PET/CT) with 2-deoxy-2-[¹⁸F]fluoro-D-glucose (FDG) has become the standard of care for the initial staging and subsequent treatment response assessment of many different malignancies. Despite this success, PET/CT is often supplemented by MRI to improve assessment of local tumor invasion and to facilitate detection of lesions in organs with high background FDG uptake. Consequently, PET/MRI has the potential to expand the clinical value of PET examinations by increasing reader certainty and reducing the need for subsequent imaging. This study evaluates the ability of FDG-PET/MRI to clarify findings initially deemed indeterminate on clinical FDG-PET/CT studies.

A total of 190 oncology patients underwent whole-body PET/CT, immediately followed by PET/MRI utilizing the same FDG administration. Each PET/CT was interpreted by our institution's nuclear medicine service as a standard-of-care clinical examination. Review of these PET/CT reports identified 31 patients (16 %) with indeterminate findings. Two readers evaluated all 31 PET/CT studies, followed by the corresponding PET/MRI studies. A consensus was reached for each case, and changes in interpretation directly resulting from PET/MRI review were recorded. Interpretations were then correlated with follow-up imaging, pathology results, and other diagnostic studies.

In 18 of 31 cases with indeterminate findings on PET/CT, PET/MRI resulted in a more definitive interpretation by facilitating the differentiation of infection/inflammation from malignancy (15/18), the accurate localization of FDG-avid lesions (2/18), and the characterization of incidental non-FDG-avid solid organ lesions (1/18). Explanations for improved reader certainty with PET/MRI included the superior soft tissue contrast of MRI and the ability to assess cellular density with diffusion-weighted imaging. The majority (12/18) of such cases had an appropriate standard of reference; in all 12 cases, the definitive PET/MRI interpretation proved correct. These 12 patients underwent six additional diagnostic studies to clarify the initial indeterminate PET/CT findings. In the remaining 13 of 31 cases with indeterminate findings on both PET/CT and PET/MRI, common reasons for uncertainty included the inability to distinguish reactive from malignant lymphadenopathy (4/13) and local recurrence from treatment effect (2/13).

Indeterminate PET/CT findings can result in equivocal reads and additional diagnostic studies. PET/MRI may reduce the rate of indeterminate findings by facilitating better tumor staging, FDG activity localization, and lesion characterization. In our study, PET/MRI resulted in more definitive imaging interpretations with high accuracy. PET/MRI also showed potential in reducing the number of additional diagnostic studies prompted by PET/CT findings. Our results suggest that whole-body PET/MRI provides certain diagnostic advantages over PET/CT, promotes more definitive imaging interpretations, and may improve the overall clinical utility of PET.