Significance of small pulmonary nodules with little or no 18F-FDG uptake on PET-CT images of patients with non-thoracic malignancies

Abstract

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Objectives: Small pulmonary nodules with little or no fluorodeoxgyglucose (FDG) uptake are relatively common findings in PET-CT images of patients with non-thoracic malignancies. Interpreting the presence of such nodules is often a diagnostic challenge, and this study aims to examine their significance.

Methods: Cases with pulmonary nodules less than 1 cm in diameter showing no FDG uptake or uptake less than the mediastinal background were reviewed. Nodules with CT findings indicating benign nature were excluded. Total of 75 cases had sufficiently long clinical follow-up period (average 175.2 days) with additional chest images or biopsy results. The cases were subdivided by three categories: first by the presence or absence of other accompanying lung abnormalities such as tuberculosis scars or other inflammatory lesions; secondly by the presence or absence of perceptible FDG uptake; and thirdly into a group of solitary nodule and a group of two or more accompanying nodules. The incidences of metastases were compared between the paired groups for each of the three categories.

Results: Of the 75 cases, 11 proved to be metastastic nodules (14.6%). Nine of the 58 imperceptible FDG cases (15.5%), and 2 of the 17 cases with faint FDG uptake (11.8%) were metastases, though there was no statistically meaningful difference in the incidences of metastases. No statistically significant difference was noted between the groups with or without accompanying lung lesions (5.3% vs. 17.9%), or between the groups of solitary or multiple nodules (13.8% vs. 15.2%).

Conclusions: Over 14% of cases with incidental, small lung nodules with no FDG uptake or uptake less than the mediastinal background seen on PET-CT images turned out to be hematogenous lung metastases when no findings exclusive to benign nature could be found. Thus, when such nodules are present, whether the nodule shows FDG uptake faintly or imperceptibly, has accompanying benign lung lesion or none, or is seen alone or in group, close follow-up is mandatory.