Diagnostic Efficacy of PET-FDG Imaging in Solitary Pulmonary Nodules: Discussion

Our study has demonstrated that the use of PET-FDG imaging, a new noninvasive test, has a high degree of accuracy in differentiating benign from malignant solitary pulmonary nodules less than 3 cm in size. Analysis of the two false-positive cases revealed the following data. In one patient, review of the chest radiographs showed a 2-cm nodule that was not present on chest radiographs done 12 weeks earlier, suggesting rapid growth. In the second patient, the nodule was not well defined, and a repeat CT scan of the chest done 6 weeks later, prior to thoracotomy, showed a slight change in the character of the nodule. Both patients underwent thoracotomy, and histologic study demonstrated caseating granulomas with active inflammation and Histoplasma.
Although some of the previous reports have demonstrated increased FDG uptake on PET imaging studies in patients with aspergilloma, lung abscess, and granulomas. However, not all granulomas in our study showed increased FDG uptake. This suggests that FDG uptake may be increased to some extent in some granulomas that demonstrate acute inflammatory process with epithelial cells and histiocytes.
There is some debate about the lower limit of the size of the solitary pulmonary nodule that can be correctly identified by PET imaging. The detectability of a lesion on PET scan may depend on the size of the lesion, instrument resolution, and radio-tracer concentration. A small lesion with very high FDG uptake could potentially be detected even though its size is close to the resolution limit. Additional factors that may hamper the detection of very small lesions include partial volume effect and respiratory motion. In our study, a lesion as small as 0.6 cm demonstrated increased uptake of FDG. However, one false-negative result was seen in a patient with a 1-cm solitary pulmonary nodule. Review of the histologic features indicated that most of the nodule was occupied by fibrotic scar tissue, with a few islets of malignant cells at the periphery. This suggests that a critical mass of metabolically active malignant cells may be required for PET imaging to demonstrate increased uptake of FDG.
The primary interpretation of PET scans was done by qualitative visual analysis of the areas of increased FDG uptake in the lung nodule. Semiquantitative analysis was also performed by computing DUR for all lung nodules. While the mean DUR index in malignant nodules was significantly greater than benign nodules, there was some overlap. The DUR indices were greater than 2 for all true-positive cases and were less than 1 for all true-negative cases. Two false-positive cases had DURs between 1 and 4 and one false-negative case had a DUR of 1.67. The use of DUR indices as compared with visual analysis for differentiating benign from malignant nodules needs further investigation.
Current options in the evaluation and management of indeterminate solitary pulmonary nodules include bronchoscopy, transthoracic fine needle aspiration biopsy, thoracotomy, and observation. Observation is usually favored in individuals with low probability for malignancy or in those individuals who are high-risk candidates for thoracotomy, either due to poor lung function or associated cardiovascular disease.

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