Clinically significant incidental findings on the unenhanced CT portion of PET/CT studies: frequency in 250 patients

PET/CT technology is in rapid evolution. It remains unclear if the unenhanced CT portion, performed for attenuation correction and lesion localization, provides additional independent diagnostic information not apparent on PET alone. The objective of the current study was to evaluate the incremental added value and frequency of potentially clinically significant incidental findings from the independent reading of the unenhanced CT portion of PET/CT studies by an expert CT radiologist.

PET/CT was performed on 250 patients (123 men and 127 women; mean age, 56.5 y) referred for clinical evaluation of known or suspected cancer. Unenhanced CT studies were read without knowledge of findings from PET and PET/CT fused images. Findings from unenhanced CT were considered clinically significant if they were not detected or explained by PET findings and were considered, after examination of all available clinical data, to clearly require additional work-up. Small pulmonary nodules < 7 mm were not considered to require immediate work-up.

Unenhanced CT revealed potentially clinically significant incidental findings in 7 patients. Three patients had indeterminate renal lesions, 1 patient had a solid renal mass, 1 patient had sclerotic bone metastases (albeit inactive on PET), 1 patient had liver cirrhosis with portal hypertension, and 1 patient had a 5 cm abdominal aortic aneurysm. These findings were generally not detected on PET.

Clinically significant findings from the unenhanced CT portion of PET/CT are relatively infrequent (3%) but could be serious enough to warrant major alterations in clinical management. Thus, we believe it is most appropriate for the CT portion to be interpreted by a physician skilled in CT interpretation with special attention to the lesions that PET alone can fail to detect.