Focal fluorine-18 fluorodeoxyglucose-avid lesions without computed tomography correlate at whole-body positron emission tomography-computed tomography in oncology patients: how often are they malignant?

OBJECTIVE: To retrospectively evaluate the rate of malignancy of focal fluorine-18 fluorodeoxyglucose (18F-FDG)-avid lesions without computed tomography (CT) correlate at whole-body positron emission tomography (PET)-CT in oncology patients, because better defining these abnormalities could potentially lead to improved patient management algorithms that rely on PET-CT for detection, staging, and treatment monitoring of malignancies.

METHODS: We performed a computer search of all PET-CT studies performed at our institution from 2006 to 2009, and identified 87 studies with findings of focal 18F-FDG-avid lesions without correlate at CT. The rate of malignancy of such lesions was determined by reviewing findings at follow-up imaging or by clinical or histopathological follow-up. Rates of malignancy were categorized and compared by lesion location and by the type of primary malignancy.

RESULTS: The most common locations for focal 18F-FDG-avid lesions without CT correlate were: lymph node location (without visible lymph nodes; 27/87), bone (21/87), soft tissue (17/87), liver (9/87), and gastrointestinal tract (8/87). Forty-one percent (36/87) of the focal FDG-avid lesions without CT correlate were malignant (either metastatic disease or a second malignancy) at follow-up (mean follow-up: 5 months, range: 1-25 months). Focal FDG-avid lesions in lymph node location and in bone without CT correlate had higher rates of malignancy (56%, 15/27 and 52%, 11/21, respectively) than lesions in all other locations (26%, 10/39, P=0.028). In 15 of 87 cases, the only significant finding at PET-CT was an FDG-avid lesion without CT correlate. Of those, 53% (8/15) was positive for malignancy. There were no significant differences in the rates of malignancy for the focal FDG-avid lesions without CT correlate when stratified by the type of primary malignancy in this series.

CONCLUSION: Focal FDG avid lesions without CT correlate were malignant in 41% of cases in our series of oncology patients. Lesions in lymph node location and in bones had the highest rates of malignancy. Knowledge of the patterns and risk of malignancy of focal FDG-avid lesions without CT correlate in oncology patients may facilitate the management of oncology patients with such lesions on PET-CT, and could lead to an improved interpretation of PET-CT scans by imaging specialists.