PET/CT assessment of clinically unsuspected, incidental FDG-avid lesions in oncological patients

Clinically unsuspected foci of abnormal FDG uptake are often detected on PET/CT; without careful assessment, these may be interpreted as metastatic disease given the background of pre-existing malignancy in these patients. Knowledge of normal physiological biodistribution patterns of FDG is essential to avoid confusing normal uptake with that caused by pathologies. As an index of increased tissue metabolism, FDG uptake accompanies a wide variety of benign reactive or inflammatory conditions that can mimic malignancy; interpretation of findings in the light of the clinical picture helps to minimize this possibility. Close correlation with the anatomical information provided by the CT component of the study, and comparison of the intensity of uptake of primary and suspected secondary lesions, will help to maximize diagnostic accuracy. Increased uptake of 18F-FDG may reflect normal organ-specific physiology or a wide variety of benign or malignant pathologies. Knowledge of the normal variations, artefacts and processes that may mimic pathology on PET/CT studies is essential to their accurate interpretation, as is an appreciation of the common patterns of metastatic spread of different primary tumours. Furthermore, it is important to be familiar with both the normal physiological distribution and uptake of 18F-FDG on PET/CT and the common causes and clinical significance of incidentally detected abnormal 18F-FDG uptake that may be encountered on PET/CT imaging in oncology patients.