ReviewFDG-PET Determination of Metabolically Active Tumor Volume and Comparison with CT☆
Introduction
A recurring problem in anatomic imaging methods such as CT or MRI is in differentiating between metabolically active tumor mass and non-malignant or scar tissue, especially in treated patients. Functional imaging methods such as positron emission tomography (PET) using the glucose analog 2[18-F] fluoro-2-deoxy-D-glucose (FDG) have been shown to be useful in distinguishing between malignant and benign processes,1, 2, 3, 4 and successful FDG-PET scanning of a wide variety of cancers has been performed.5, 6, 7, 8, 9, 10, 11, 12, 13 In vitro and in vivo uptake of FDG has been shown to be strongly related to the number of viable cancer cells.14 The regional distribution of FDG within a tumor reflects regions of high and low metabolic rate and varying fractions of active tumor.15 On the other hand, while conventional CT has been shown to reliably determine untreated tumor or normal organ volumes,16, 17 CT alone cannot consistently distinguish between active and inactive tumor masses. Therefore, the purpose of this study is to determine if tumor volume, in addition to tumor metabolic activity, can be assessed with attenuation-corrected FDG-PET using a semiautomated method.
Section snippets
Materials and Methods
Fourteen patients were studied with CT and FDG-PET, with a mean time between CT and PET of 9 days. For patients to be included, the digital image data sets for both CT and PET had to be available. These patients provided written informed consent as part of clinical studies evaluating the accuracy of PET for staging lymph node involvement with cancer, or assessing tumor response to therapy. Eight of the fourteen patients studied had newly diagnosed cancers with no previous treatment (7 patients
Results
Figure 1a shows a CT section of a patient at the level of a primary lung cancer and Figure 1b shows the corresponding FDG-PET slice reconstructed to match the same anatomic location as the CT slice using an image co-registration technique.9, 12, 18 In this case there is excellent visual agreement between the anatomical image of the tumor mass and the metabolic FDG-PET image. By contrast, Figure 1c shows a CT image of a bulky abdominal tumor site in a patient with non-Hodgkin’s lymphoma (NHL)
Discussion
For the fourteen tumor sites studied here, volumes determined by FDG-PET and CT were significantly correlated. However, for newly diagnosed untreated tumors within this group , correlation between the two imaging methods was strongest. Therefore, it appears possible to reliably determine tumor volume by the FDG-PET thresholding method outlined for untreated tumors of the range of sizes studied here (26–237 cm3). Since initial tumor volume as been shown to be of prognostic significance in
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This work was supported in part by CA53172 and CA52880.