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To evaluate the additional value of contrast-enhanced multiphase CT in comparison with low-dose non-contrast CT in combined positron emission tomography (PET)/CT protocols for oncological imaging, we retrospectively analysed 100 patients with different malignant tumours. All patients underwent a PET/CT consisting of a multiphase CT protocol including a low-dose non-enhanced attenuation scan and an arterial and portal-venous contrast-enhanced scan followed by a whole-body PET. PET/CT studies were analysed by different categories to determine the added value of contrast-enhanced CT. The additional value was defined as new information provided by diagnostic CT and not available from the low-dose CT, resulting in change of PET/CT interpretation. The results were validated either by histopathology or by clinical-radiological follow up at > or =6 months. The clinical impact was evaluated with respect to changes in patient management. Diagnostic multiphase CT was of additional value in 52 out of 100 patients with 85 suspected lesions. In 40 out of 100 patients, no additional value could be detected. Eight patients were excluded due to inconclusive diagnosis in both methods including fusion. The analysis showed the greatest benefit of diagnostic CT in the categories localization of pathological fluorodeoxyglucose (FDG) uptake and precise tumour delineation, changing PET/CT interpretation in 42% and 31% of patients, respectively. The benefit of diagnostic CT was influenced by the tumour type demonstrating the highest impact in gastrointestinal, lung and neuroendocrine tumours. Diagnostic CT changed clinical management in 21 patients (21%). Diagnostic multiphase CT as part of the combined PET/CT protocol has the potential to provide considerable additional value in specific clinical conditions with resultant change of management in a substantial proportion of patients. [ABSTRACT FROM AUTHOR]