[Malignant tumor with false negative 18F-FDG PET image].

Objective To investigate the FDG uptake characteristics, the factors affecting ^18F-FDG uptake and the extra CT diagnostic value of ^18F-FDG PET/CT scan in the malignant tumor with false negative ^18F-FDG PET image. Methods The data of PET/CT image in 17 patients with various kinds of cancers were reviewed and analyzed by visual observation and semi-quantity analysis （SUV）. The results were compared with the CT and histopathological diagnosis, respectively. Results Of 6 well-differentiated HCC patients confirmed by histopathological diagnosis, one had two lesions in the right lobe of the liver. One of these two lesions showed low FDG uptake on ^18F-FDG PET scan and low density on CT scan. The other one was not shown on either ^18F-FDG PET or plain CT scan. But on enhanced CT scan, these two lesions were found to be inhomogeneous with high density at arterial phase. The false negative ^18F-FDG PET images of one gastric signet ring cell carcinoma in the gastric fundus with right adnexa metastasis, 3 renal cell carcinoma, one greater omentum and peritoneal metastatic adenocarcinoma and one well-differentiated prostate cancer were caused by normal physical uptake in the digestive tract or FDG retention in the urinary system due to normal excretion. The size of three metastases was smaller than or equal to 1 cm in diameter, however, two primary lesions of these metastases showed high FDG uptake and only one was negative on either ^18F-FDG PET or CT scan. In this series, 68.8% of the primary tumors and 66.7% of metastases were found to show abnormal density on CT scan, and 31.2% of the primary tumors and 33.3% of metastases were not detectable on either PET or CT images. Conclusion False negative ^18F-FDG PET in malignant tumor may be correlated with the pathologic type, differentiation degree and the lesion size. Combining CT information with PET or paying attention to the scan methods during ^18F-FDG PET examination may reduce the rate of false negative ^18F-FDG PET diagnosis in various kinds of malignant tumors. Key words: Malignant tumor ;  Positron emission tomography ;  ^18F-FDG