Solitary Pulmonary Nodule: Morphological Effects on Metabolic Activity Assessment

We aimed to evaluate the effects of morphological characteristics of the solitary pulmonary nodules (SPN) on metabolic activity assessment. To the best of our knowledge, this is the first study to compare the volumetric metabolic activity parameters according to the morphologic parameters of the nodules.

In this retrospective study, ¹⁸F-FDG positron emission tomography and computed tomography scans performed between 2011 and 2018 were evaluated by a nuclear and diagnostic radiologist. One hundred thirteen patients with SPNs with biopsy-proven diagnosis were included. SPNs were classified as solid, partially solid (PS), and ground glass opacity (GGO).

SPN diameter, SUV_max, metabolic tumor volume (MTV), total lesion glycolysis (TLG), and density were significantly higher in the malignant group. SUV_max, MTV, TLG increased in direct proportion to the diameter. There was no a significant difference between GGO, PS, and solid nodules in terms of SUV_max values. MTV and TLG values increased in parallel with the density of the nodules, but this increase was only significant in the malignant group. There was a significant difference between SPNs <2 cm and SPNs ≥2 cm in terms of MTV, while there was no difference in terms of SUV_max. The cut-off value determined by the ROC curve was found to be 4.39 for SUV_max, 7.33 mL for MTV and 31.88 g for TLG. The cut-off values for SUV_max of solid and subsolid nodules were close to each other, but cut-off values for MTV and TLG were higher in solid nodules.

SUV_max, MTV, and TLG are affected by diameter and attenuation. We suggest using different MTV and TLG cut-off values for solid and subsolid nodules, but we suggest using same SUV_max values. MTV can be a more reliable parameter than SUV_max in prediction of malignancy in smaller nodules.