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

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Abstract

Background

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.

Methods

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.

Results

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).

Conclusions

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.

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Abbreviations

ADC:

apparent diffusion coefficient

CT:

computed tomography

DWI:

diffusion-weighted imaging

ECO:

external cervical os

FA:

flip angle

FDG:

2-deoxy-2-[18F]fluoro-D-glucose

FOV:

field of view

FS:

fat saturation

HASTE:

half-Fourier acquisition single-shot turbo spin echo

PET:

positron emission tomography

Q-fat sat:

quick fat saturation mode

SPACE:

sampling perfection with application of optimized contrasts using different flip angle evolution

ST:

slice thickness

T1W:

T1-weighted

T2W:

T2-weighted

TE:

time to echo

TR:

time to repetition

TSE:

turbo spin echo

VIBE:

volumetric interpolated breath-hold examination

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Authors and Affiliations

  1. Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S. Kingshighway Blvd., Campus Box 8131, Saint Louis, MO, 63110, USA

    Tyler J. Fraum, Kathryn J. Fowler, Jonathan McConathy & Farrokh Dehdashti

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  1. Tyler J. Fraum
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  2. Kathryn J. Fowler
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  3. Jonathan McConathy
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  4. Farrokh Dehdashti
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Correspondence to Tyler J. Fraum.

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Conflict of Interest

TJF – None; KJF – Research Support, Bracco Group; JM – Research Support, Eli Lilly & Co.; Research Consultant, General Electric Healthcare; Research Consultant, Blue Earth Diagnostics Ltd.; Research Consultant, Siemens AG; FD – None

Ethical Statement

The study was approved by an institutional review board or equivalent and has been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. All subjects in the study gave written informed consent or the institutional review board waived the need to obtain informed consent.

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Fraum, T.J., Fowler, K.J., McConathy, J. et al. Indeterminate Findings on Oncologic PET/CT: What Difference Does PET/MRI Make?. Nucl Med Mol Imaging 50, 292–299 (2016). https://doi.org/10.1007/s13139-016-0405-1

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  • DOI: https://doi.org/10.1007/s13139-016-0405-1

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