Clinical Significance of Small Pulmonary Nodules with Little or No ¹⁸F-FDG Uptake on PET/CT Images of Patients with Nonthoracic Malignancies

Abstract

Small pulmonary nodules with little or no perceptible ¹⁸F-FDG uptake are relatively common findings on combined PET/CT images of patients with nonthoracic malignancies. Interpreting such nodules is often a diagnostic challenge, and this study aimed to evaluate the clinical significance of the nodules. Methods: Patients with pulmonary nodules ≤1 cm in diameter showing no ¹⁸F-FDG uptake or uptake less than the mediastinal background were included. Nodules with clearly benign or metastatic findings on CT were excluded. One hundred twenty-one patients had either tissue confirmation or clinical follow-up with additional chest images. The subjects were studied by 3 variables: (i) solitary versus multiple nodules, (ii) presence of accompanying benign lung lesion versus absence, and (iii) imperceptible ¹⁸F-FDG uptake versus faint ¹⁸F-FDG uptake. The malignancy rates were calculated for each variable. Results: Of the 121 patients, 24 had malignancy, with a strong possibility of pulmonary metastasis (19.8%). Six of the 44 patients with solitary nodules (13.6%) and 18 of the 77 patients with multiple nodules (23.4%) had malignancies, though there was no statistically significant difference in the incidences of malignancy between the solitary and multiple groups. On the other hand, there was a statistically significant difference (P = 0.040) between the accompanying lung lesion present (8.3%) and absent (24.7%) groups. No statistically significant difference was noted between the ¹⁸F-FDG uptake imperceptible group and faint ¹⁸F-FDG uptake group (20.7% vs. 17.2%). Conclusion: For patients with incidental lung nodules of indeterminate nature with no ¹⁸F-FDG uptake or uptake less than that of the mediastinum on PET/CT images, >19% of the cases turned out to be malignant. The nodule was more likely to be malignant when no other benign pulmonary lesions could be identified elsewhere in the lung field. Thus, regardless of the number of nodules and ¹⁸F-FDG uptake, tissue confirmation or close imaging follow-up is necessary when small nodules with imperceptible or faint ¹⁸F-FDG activity are present on the PET/CT images, especially in the absence of accompanying benign lung lesions.