Evaluation of F-18 fluorodeoxyglucose (FDG) PET scanning for pulmonary nodules less than 3 cm in diameter, with special reference to the CT images
Introduction
In recent years, low-dose helical computed tomography (CT) have enabled the detection of small peripheral pulmonary nodules [1], [2], [3]. Despite developments in CT technology, there are still a significant number of cases in which surgical resection is needed to differentiate lung cancer from benign lesions [4]. On the other hand, recent advances in positron emission tomography (PET) with 2-deoxy-2-fluoro-[F-18]-d-glucose (FDG) has made a significant contribution to differentiate between benign and malignant nodules, and several reports have suggested that PET examinations reduce the number of patients with indeterminate nodules undergoing unnecessary surgical biopsy [5], [6], [7], [8], [9]. However, FDG-PET is neither usually specific nor sensitive, particularly for small pulmonary nodules, because low-grade malignant tumor such as bronchiolo-alveolar carcinoma and carcinoid are frequently negative for FDG-PET due to their low glucose metabolism, while active inflammation sometimes showed positive due to their high glucose metabolism [5], [6], [10], [11], [12], [13], [14], [15]. To clarify the effectiveness of FDG-PET, it is necessary to examine the CT images of small pulmonary nodules that are false positive or false negative for FDG-PET.
Both malignant and benign pulmonary nodules can be substantially classified into ground-glass opacity (GGO) and solid ones on CT. For the GGO nodules, focal pneumonia, atypical adenomatous hyperplasia (AAH) and bronchiolo-alveolar carcinoma could be differential diagnoses [16], [17]. For the solid nodules, granuloma, benign lung tumor, and malignant tumors with solid growth could be differential diagnoses. Because those nodules have different biological natures, FDG-PET could show different findings. Recently, we have found that a CT density histogram can clearly divide lung adenocarcinomas into GGO and solid ones on CT images and can also predict lymph node metastasis and tumor invasiveness [18]. In the present study, to clarify the effectiveness of FDG-PET for pulmonary nodules less than 3 cm in size, we classified them into GGO and solid nodules with using a CT density histogram and compared the results with the data of FDG-PET.
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Materials and methods
Between December 2001 and August 2003, prospective FDG-PET and CT scans were undertaken for 151 non-calcified pulmonary nodules less than 3 cm in diameter, which were consulted for surgical treatment or surgical biopsy on the Department of Thoracic Surgery of Saiseikai Central Hospital. After taking an informed consent, FDG-PET scan was performed at the Nishidai Clinic within 2 weeks after CT scan was done. Of the 151 nodules, 136 could be diagnosed as malignant or benign, while the remaining 15
Results
Mean sizes were 1.9±0.7 cm in the 81 malignant nodules and 1.4±0.7 cm in the 55 benign nodules, of which difference was not significant. Table 1 showed the histological type and sizes (<1 cm versus 1–3 cm) of malignant nodules. Of the 81 malignant nodules, 78 were diagnosed by surgical resection and the remaining 3 were by bronchoscopic or needle biopsy. Histologically, 68 nodules were diagnosed as primary lung cancer and 13 as metastatic lung cancer.
Table 2 showed the kind of diseases and sizes (<1
Discussion
There are few data about the lower limit size of pulmonary nodules that can be correctly identified by PET imaging. A meta-analysis for 1474 nodules by Gould et al. showed that in the eight nodules less than 1 cm in diameter, three were true positive, two were true negative, and three were false negative [20]. The present study showed that all of the eight malignant nodules less than 1 cm were false negative. The spatial resolution of current generation of PET scanners is 7–8 mm, which can hardly
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