In recent years, the widespread utilization of chest CT has prompted detection of pulmonary GGNs in otherwise asymptomatic individuals. For GGNs, we need to further judge whether they are benign or malignant. However, there is no uniform standard for judging GGNs, which is full of controversy. Clinicians often make judgments based on experience, which often leads to misdiagnosis of GGNs. Therefore, there is a growing need for a benign and malignant prediction model of pulmonary nodules.
There have been several studies that proposed pulmonary nodules predicting models by identifying some relative risk factors based on clinical and imaging features. Swensen et al established Mayo Clinic model in 1997, which found that 3 clinical features (age, smoking history, history of previous malignancy) and 3 imaging features (nodule size, speculation, located in the upper lobe) were independent risk factors for malignant pulmonary nodules [16]. Gould et al. established a Veterans Affairs (VA) model using part of data from a multicenter VA study, including risk factors such as smoking history, older age, large nodule diameter, and time since quitting smoking [17]. Although these prediction models have a good prediction effect, the calculation process required is relatively complex, and a simple and easy to implement risk model is urgently needed in clinical practice for the differentiation of benign and malignant GGNs.
In the present study, our institution retrospectively collected and analyzed 574 GGNs from patients who were pathologically diagnosed. To ensure that the included pulmonary GGNs in the analysis have corresponding pathological diagnoses, for our analysis, we examined patients who underwent preoperative CT-assisted HOOK-WIRE puncture to locate nodules over the years. Our study demonstrated that vacuole sign, air bronchogram, and intra-nodular vessel sign in HRCT were associated with malignancy. According to the obtained three significant variables and their coefficients, we derived a simple scoring model (the VBV Score) for assessing pulmonary GGNs as follows:
VBV Score = vacuole sign + air bronchogram + intra-nodular vessel sign (it added one point if the nodule had one of these characteristics; otherwise, none).
If a nodule got at least one point, it was considered a malignant nodule, otherwise it was a benign nodule. This predictive model showed good sensitivity (95.6%), specificity (80.6%), accuracy (93.2%), and diagnostic concordance (the Kappa Coefficient = 0.753) in 574 GGNs and was validated via three independent cohorts of patients.
In our study, vacuole sign was more frequent in malignant GGNs than in benign GGNs (67.6% vs 12.9%), which was probably caused by the lepidic tumor growth along the alveolar structure in adenocarcinomas. Several other reports have also revealed that the presence of vacuole sign is suggestive of adenocarcinoma [18, 19]. Besides, the frequency of air bronchogram was significantly higher in malignant GGNs than in benign GGNs (21.6% vs 1.1%). F Wu. found air bronchogram could be helpful for distinguishing invasive pulmonary adenocarcinomas from pre-invasive lesions [20]. Intra-nodular vessel sign was essential for the malignancy of GGNs. As we all know, vascular invasion may promote tumor growth and metastasis, leading to recurrence or shortened survival in lung cancer patients [21]. The rate of malignant GGNs showing intra-nodular vessel sign was high (78.6%, 378/481); however, this characteristic was also observed in 11.8% (11/93) of benign nodules. Our findings are consistent with C Lin’s study that the infiltration of pulmonary arteries into lung lesions could indicate a higher blood supply, which may be conducive to the growth of malignant tumors [22].
However, lobulation, spiculated margin, and pleural indentation were not significantly associated with malignant GGNs in this study; which was inconsistent with previous studies [23]. Most of the GGNs we collected were 1–2 cm in size, which mean lobulation and spiculation were relatively inconspicuous in these small nodules. Y Silva reported that with the increase in nodule diameter, the probability of the nodules being malignant also increases [24]. Although the results of our study indicated that malignant GGNs showed a markedly larger diameter than benign nodules, the nodule diameter was not significant with regard to the malignancy status of the nodules, as revealed by the logical regression analysis. On the other hand, for the nodules with pathological diagnosis of inflammation, lymph nodes or benign tumor, they may present as homogeneous patches, cloudy sign rather than ground-glass appearance (Fig. 3A、B). Usually, the pathology of homogenized patch nodules is specific to benign tissues, such as lymph nodes or hamartomas [25]. Cloudiness is another indication that the nodule is benign and presents as a blurred boundary. Nodules presenting as homogenized patch shadows and cloudy sign lacked the significant features mentioned above and probably absorbed after several months.
The purpose of the present study is to establish a score model that allows inexperienced clinician to make a preliminary judgment regarding GGNs on the basis of CT imaging features. Based on the VBV Score, if a pulmonary GGN got at least one point, in other words, had at least one morphological characteristic we describe, we considered that the GGN be malignant. On the contrary, if the score is zero, the GGN is likely to be benign. In order to facilitate the popularization and application of this scoring model, we named it the VBV Score. Features included in this study such as vacuole sign, air bronchogram, and intra-nodular vessel sign are easier to observe than well-defined margin, lobulation and speculation sign for patients with early-stage lung cancer, which may reduce the error caused by subjective judgment. Besides, the specificity performs good (80.6%), while our model has high sensitivity (95.6%), which is better than previous GGN predictive models with a sensitivity of 93.4% and a specificity of 66.7% [26]. For GGNs, improving specificity and reducing false-positive patients is particularly important, which can avoid overtreatment. For patients who have not been diagnosed with malignant nodules, as long as they are regularly followed up, treatment will not be delayed.
However, there are still several limitations. First, this is a multi-center retrospective study. All the cases we collected were considered to be malignant nodules and required pulmonary surgery after a period of follow-up, making the majority of benign cases already screened out. Moreover, it may increase the proportion of benign nodules that all the 574 patients we analyzed underwent preoperative CT-assisted HOOK-WIRE puncture to locate nodules. Both of these can lead to selection bias. Another limitation is that while the scoring model established in this study is accurate, we should recognize that, for now, the model can only be used as an auxiliary tool to help assessing GGNs. In the future, we hope to conduct a prospective multicenter study to further verify the value of VBV Score in the differential diagnosis of benign and malignant GGNs.