## 1 Introduction  
Lung nodules represent a common finding on chest computed tomography (CT), and their incidence is steadily increasing [1; 2; 3]. With improvements in temporal and spatial resolution, multidetector CT (MDCT) scanners are now able to detect a larger number of nodules, especially small nodules and subsolid nodules (SSNs) [1; 3; 4; 5].  
SSNs, also called ground-glass nodules, are classified as nonsolid or part-solid nodules (PSNs) according to the absence or presence of a solid component within the ground-glass opacity [6; 7].  
SSNs are a major diagnostic challenge because they may represent a wide variety of malignant and benign lesions [8]. However, when an SSN persists, although the size and density remain unchanged, the likelihood of malignancy is very high, particularly in PSNs [9; 10].  
Persistent PSNs have a high likelihood of being minimally invasive or invasive adenocarcinomas [7; 8; 9]. However, the behavior of PSNs is quite heterogeneous and is related to the size of the nodule and the size of the intralesional solid component [6; 7; 11]. As a result, in lung cancer screening programs,the American College of Radiology Lung CT Screening Reporting and Data System (Lung-RADS) guidelines recommend a different follow-up protocol based on the overall size of the nodule and the size of the intralesional solid component [12; 13].  
In clinical practice, the new Fleischner Society guidelines recommend a first follow-up CT scan at 3-6 months for any PSN \(\geq\)6 mm in diameter [6]. Conversely, no routine follow-up CT scan is required for PSN \(<\)6 mm in diameter [6]. However, these new guidelines state that only PSNs \(\geq\)6 mm in diameter can actually be defined as part-solid [6].  
According to the size of the solid component, PSNs can be further categorized as PSNs with a solid component \(<\)6 mm and PSNs with a solid component \(\geq\)6 mm [6].  
PSNs with a solid component \(<\)6 mm usually represent minimally invasive adenocarcinomas and are significantly less aggressive than PSNs with a solid component \(\geq\)6 mm [6; 14]. Therefore, PSNs with a solid component \(<\)6 mm could be safely managed with a conservative approach (i.e., annual or biennial follow-up CT scans) [6; 14]. Moreover, the \"wait and see\" approach does not seem to affect the prognosis of patients with PSNs with small solid components [15].  
The indications for surgery in PSNs vary according to different guidelines. According to the American College of Chest Physicians, surgical resection is considered for persistent PSNs that range from 8 to 15 mm or for any PSNs \(>\)15 mm regardless of their persistence [16]. In contrast, in the new Fleischner Society guidelines, surgical resection is considered only for persistent PSNs with a solid component \(\geq\)6 mm in diameter [6]. In addition, some authors recommend surgery only for PSNs with a solid component \(\geq\)5 mm or with an overall size \(\geq\)15 mm [8].  
However, not all PSNs with a small solid component behave in the same way: some nodules exhibit an indolent course, whereas others exhibit more aggressive behavior. Thus, predicting the future behavior of this subtype of PSNs remains a complex and fascinating diagnostic challenge.  
Although computerized analyses and various quantitative features have been tested to predict the future behavior of SSNs [9; 14; 17; 18; 19; 20; 21; 22], to our knowledge, there have been no published studies investigating quantitative CT analyses to predict the behavior of PSNs with a solid component \(<\)6 mm.  
In addition to commercial and in-house image analysis software, several open-source image processing programs have become easily accessible and reliable for performing quantitative analyses on CT images [23].  
Therefore, the main purpose of this study was to apply open-source software to investigate which quantitative CT features may be useful for predicting the behavior of a select group of PSNs with a solid component \(<\)6 mm and a total diameter \(<\)15 mm.