Limitations of dual time point PET in the assessment of lung nodules with low FDG avidity
Introduction
18F-FDG PET has repeatedly demonstrated its utility for differentiating benign and malignant lung lesions. Typically, malignant pulmonary nodules will have increased FDG uptake due to overexpression of glucose transporter I [1]. In addition, the intracellular concentration of hexokinase is elevated which assists in the trapping of FDG within these malignant cells. Unfortunately, inflammatory processes have the same mechanism of action and cannot be easily distinguished from malignancy [2]. In theory, benign pulmonary nodules should not overexpress either glucose transporter I or hexokinase and thereby should not have increased FDG avidity. Unfortunately, some well-differentiated malignancies (classically, bronchoalveolar carcinoma, well-differentiated adenocarinoma, and carcinoid tumors) may not demonstrate increased FDG avidity [3]. Further confounding the issue is that some benign inflammatory and infectious processes can demonstrate increased FDG avidity as well [2]. In order to distinguish whether a low FDG avid pulmonary nodule (maximum SUV (max SUV) < 2.5 on initial imaging) is benign or malignant, dual time point imaging has been used [4], [5]. Theoretically, over a short time frame, malignant nodules would continue to accumulate and trap FDG as opposed to benign processes which would either maintain stable or even decreased FDG activity [2].
Some authors found dual-time-point imaging may improve the sensitivity and/or specificity of FDG PET and thus have the potential to improve accuracy in the evaluation of lung nodules [2], [6], [7]. Others recently have shown less promising results when looking specifically at lesions with low FDG avidity [8]. The purpose of our study was to further investigate the accuracy of dual time point FDG PET imaging for determining the nature of pulmonary lesions when the initial 1 h maximum SUV is less than 2.5.
Section snippets
Materials and methods
All reports from PET/CT examination using dual time point (DTP) technique performed at our institution from the period of June 2006 to September 2008 were retrospectively reviewed. Institutional review board (IRB) approval was obtained for this research and patient consent was waived due to the retrospective nature of the study.
The goal of the activity was to quantify the diagnostic benefit of DTP. During this 15-month period, 113 patients underwent a total of 130 DTP PET/CT with 152 lesions
Results
A total of 67 lesions were assessed with subsequent definitive diagnosis based upon tissue sampling or imaging follow-up. Lesion sizes measured 0.7–11.0 cm. Of these, 42 met criteria for low FDG avidity lesions with maximum SUV < 2.5 (size 0.7–3.6 cm, mean 1.6 cm). 20 of these lesions (48%) demonstrated <10% change in their avidity. 22 of these lesions (52%) demonstrated >10% change in their avidity (Table 2, Table 3). Utilizing the threshold of 10%, the resulting sensitivity is 61%, specificity is
Discussion
In general, utilization of SUVs in PET has become an accepted practice and useful adjunct in the clinical management of pulmonary nodules, where metabolic activity correlated with likelihood of malignancy [4]. An accepted practice in the evaluation of pulmonary nodules is that those which demonstrate a maximum SUV of greater than 2.5 are suspicious for malignancy in the appropriate clinical setting [5]. However, there are numerous reports of malignant nodules which are non-FDG avid such as
Conflict of interest statement
None of the authors have any actual or potential conflicts of interest to declare.
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