Focal F-18 Fluoro-Deoxy-Glucose Accumulation in the Lung Parenchyma in the Absence of CT Abnormality in PET/CT

Division of Nuclear Medicine, Department of Radiology, Mayo Clinic, Rochester, MN, USA.
Journal of Computer Assisted Tomography (Impact Factor: 1.41). 09/2007; 31(5):800-5. DOI: 10.1097/RCT.0b013e3180340376
Source: PubMed


To demonstrate 3 cases of artifactual focal F-18 fluoro-deoxy-glucose accumulation in the lung parenchyma in the absence of any computed tomographic (CT) abnormality.
Three patients were examined: a 30-year-old man who had a positron emission tomography (PET)/computed tomography for restaging a biopsy-proven recurrence of head and neck cancer, a 68-year-old woman who was referred for initial staging of esophageal carcinoma, and a 57-year-old man who had a PET/computed tomography for initial staging of melanoma. In each case, there was intense focal activity in the lung parenchyma with no corresponding CT abnormality. Each patient was further evaluated with a repeat PET scan in days 1 and 3 in the first 2 cases and with a delayed repeat acquisition in the third case. Patients were followed for 24, 10, and 1 month, respectively.
In the first 2 cases, the abnormal focal activity in the lungs had resolved in the repeat study. In the third case, the focus of increased activity in the lung had moved more peripherally in the delayed acquisition. Clinical follow-up was negative for disease in the corresponding pulmonary parenchymal sites.
The finding of significant focal accumulation of fluoro-deoxy-glucose in the lung parenchyma in the absence of corresponding CT abnormality was artifactual. This was likely due to injection technique and the creation of particulate embolus. Positron emission tomography/Computed tomographic readers should be aware of this type of artifact to avoid misinterpretation.

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    • "The technical aspects that can determine an underestimation of the true 18 F-FDG activity, especially in nodules smaller than 1 cm, are: the respiratory motion because of the displacement caused by shallow breathing, particularly in nodules located in the periphery and in the base of the lungs; the partial volume effect because nodules smaller than the resolution of the PET scanners (ranging from 6 to 10 mm in clinical applications) are not, or only faintly visualized [8,16,17]. Likewise, 18 F-FDG PET specificity for characterizing pulmonary nodules as probably malignant can be compromised by ‘metabolic’ causes and ‘technical’ aspects, which lead to false-positive results. "
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    ABSTRACT: Background 18F-fluoro-deoxy-glucose (18 F-FDG) positron emission tomography integrated/combined with computed tomography (PET-CT) provides the best diagnostic results in the metabolic characterization of undetermined solid pulmonary nodules. The diagnostic performance of 18 F-FDG is similar for nodules measuring at least 1 cm and for larger masses, but few data exist for nodules smaller than 1 cm. Case presentation We report five cases of oncologic patients showing focal lung 18 F-FDG uptake on PET-CT in nodules smaller than 1 cm. We also discuss the most common causes of 18 F-FDG false-positive and false-negative results in the pulmonary parenchyma. In patient 1, contrast-enhanced CT performed 10 days before PET-CT did not show any abnormality in the site of uptake; in patient 2, high-resolution CT performed 1 month after PET showed a bronchiole filled with dense material interpreted as a mucoid impaction; in patient 3, contrast-enhanced CT performed 15 days before PET-CT did not identify any nodules; in patients 4 and 5, contrast-enhanced CT revealed a nodule smaller than 1 cm which could not be characterized. The 18 F-FDG uptake at follow-up confirmed the malignant nature of pulmonary nodules smaller than 1 cm which were undetectable, misinterpreted, not recognized or undetermined at contrast-enhanced CT. Conclusion In all five oncologic patients, 18 F-FDG was able to metabolically characterize as malignant those nodules smaller than 1 cm, underlining that: 18 F-FDG uptake is not only a function of tumor size but it is strongly related to the tumor biology; functional alterations may precede morphologic abnormalities. In the oncologic population, especially in higher-risk patients, PET can be performed even when the nodules are smaller than 1 cm, because it might give an earlier characterization and, sometimes, could guide in the identification of alterations missed on CT.
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    ABSTRACT: A routine feature of positron emission tomography-computed tomography imaging is a whole-body acquisition that results in many unexpected findings identified outside of the primary region of abnormality. Furthermore, 18 F-fluorodeoxyglucose (FDG) is a marker of glycolysis and does not specifically accumulate in malignancy. Understanding the physi-ology and pathophysiology of a normal FDG distribution and common incidental findings is therefore essential to the physician interpreting whole-body FDG positron emission to-mography-computed tomography studies. Whereas many incidental findings are benign and of limited clinical significance, others represent uncommon manifestations of the primary malignancy, second malignancies, or various clinically significant pathologic pro-cesses. Patients with a single malignancy are at greater risk of developing synchronous or metachronous second malignancies, possibly related to exposure to shared carcinogenic agents or presence of prooncogenic mutations. The decision of how to pursue an inter-vention on the basis of an incidental finding is generally left to clinical judgment. Semin Nucl Med xx:xxx © 2010 Published by Elsevier Inc. P ositron emission tomography (PET) has experienced ex-plosive growth as a clinical modality, especially in the realm of oncology. A component of this success has been the introduction of scanners which integrate PET and computed tomography (CT), allowing both studies to be performed sequentially, without need to reposition the patient. 1 The most-commonly used PET radiotracer, 18 F-fluorodeoxyglu-cose (FDG), is a glucose analogue that accumulates in malig-nant tumors exhibiting increased glucose transport and glycolysis. 2 Therefore, FDG-PET is well-established as an im-aging modality in oncology for tumor staging, restaging, sur-veillance of recurrence, and the monitoring of treatment re-sponse. 3-5 A routine feature of PET/CT imaging is whole-body acqui-sition ("eyes to thighs"), which results in many unexpected findings identified outside of the primary region of abnor-mality. Furthermore, FDG is a marker of glycolysis and does not specifically accumulate in malignancy. Various degrees of FDG uptake are therefore present in normal tissues and are noted in benign processes, such as inflammation and infec-tion. Generation of attenuation-corrected PET images is also an intricate multistep procedure; various deviations from ideal will result in artifactual areas of apparent uptake. The accurate interpretation of unexpected findings remains a challenge. Understanding the physiology and pathophysiol-ogy of incidental findings on FDG-PET/CT is essential to the interpreting physician. The purpose of this article is to pro-vide a brief review of common incidental findings, providing insight into their physiology and pathophysiology wherever possible.
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