Objective: This study was performed to show the malignant potential and clinical value of incidental focal 18F-fluorodeoxyglucose (18F-FDG) uptake on positron emission tomography/computed tomography (PET/CT) with the confirmation of histopathologic findings or with a series of consequent correlative imaging methods.
Methods: A total of 2370 18F-FDG-PET/CT studies performed over a 16-month period in patients with various malignant diseases were retrospectively reviewed. Unexpected PET foci that were in an unusual site for metastatic spread of known malignancy and not considered to be physiologic uptake were evaluated by histopathologic findings or follow-up with correlative imaging methods for a period of 1 year or longer. There were 121 such incidental PET lesions in 116 patients.
Results: Seventy-four incidental PET lesions in 70 PET/CT patients were investigated further. Forty-seven lesions in 46 patients were lost to follow-up. Fifty-nine lesions were confirmed histopathologically. Fifteen lesions were evaluated with radiologic methods or instrumental examination. Thirty-six incidental PET foci were unexpected malignant or premalignant lesions (1.5% of 2370 patients and 49% of 74 PET foci). Nineteen of these 36 malignant or premalignant lesions were synchronous carcinomas, 14 lesions were unusual or unexpected malignant spread of known malignancy and three lesions were premalignant colonic adenomas. Thirty-three of 74 incidental PET lesions were of benign origin (1.4% of 2370 patients and 44% of 74 PET foci). Five PET foci were false-positive findings (7% of 74 lesions).
Conclusion: The detection of incidental 18F-FDG-PET foci on PET/CT may reflect unexpected malignant lesions related to unusual malignant spread of primary malignancy or synchronous tumor. Follow-up of these PET foci may result in significant change in therapy management and early diagnosis of various abnormalities that require a medical approach or surgical intervention.