This was the first large-sample study on the visualization of the PH–bronchus relationship on MSCT. We found that 88.9% (8/9) of central endobronchial PHs obstructed the bronchus, whereas only 3.8% (8/209) of intraparenchymal PHs obstructed the bronchus. No air bronchogram sign was observed.
Previous studies found that the sign wherein the bronchus is cut off by a lesion (type I) is significantly more common in lung cancers than in benign lesions [5, 6, 11]. Qiang et al. found that the type 1 pattern is shown by 58.5% of malignant nodules and by only 16.0% of benign nodules [6]. Choi et al. reported that type I is most common pattern in squamous cell carcinoma; it is presented by 45.8% of squamous cell carcinomas and is especially common in central squamous cell carcinomas (55%) [11]. Bronchus amputation in lung cancer can be ascribed to two reasons. One is the destructive power of the tumor. Tumor cells proliferate and invade the surrounding lung parenchyma, destroying the lesion area and adjacent bronchi [6]. The other reason is that lung cancer arises mainly from the bronchial mucosal epithelium; therefore, the bronchus is easily cut off in the early stage [14]. Amputated bronchi also occur in inflammatory nodules, such as granulomatous inflammation or common inflammatory lesions, due to the filling of bronchi by mucinous or granulomatous tissue, the inflammation of bronchi, or the destruction of bronchi by the lesion [5, 6]. In this study, we found that the type I pattern was common in central endobronchial PHs (8, 88.9%) but was rare in intraparenchymal PHs (8, 3.8%). Central endobronchial PHs originate in the bronchi, leading to bronchial stenosis, peripheral inflammation, and the filling of bronchi by mucus. These phenomena lead to the type I sign. Only 1 central endobronchial PH in this study did not cut off bronchi because it was located in the trachea, which is wide, and the lesion itself was small. Moreover, no sign of infection was observed. We hypothesized that the bronchus might have been cut off by the intraparenchymal PH because of its origin in the bronchus. Specifically, as the tumor grew expansively, it amputated the small bronchus. Central endobronchial PHs should be distinguished from malignant tumors, especially squamous cell carcinomas. Other relatively rare diseases include infectious lesions and foreign bodies. Among the 9 central endobronchial PHs in this study, only 5 contained fat (2 of which were accompanied by calcification). This characteristic is helpful for the diagnosis of PHs on MSCT. Given that the type I sign was rarely observed for intraparenchymal PHs, we concluded that the presence of this sign in a solitary intraparenchymal lesion should preclude the first diagnosis of PHs. However, the presence of fat and/or popcorn-like calcification on CT is the suggestive finding of PHs. In this study, for type 1, fat was found in 2 intraparenchymal PHs and popcorn-like calcification was found in 3 intraparenchymal PHs. These characteristics are helpful for the diagnosis of PHs.
The air bronchogram sign is an important radiologic sign. Initial studies indicated that the presence of this sign is suggestive of pneumonia or benign nodules, but malignant tumors were also later found to be common [5–13]. The incidence of the air bronchogram sign in malignant solitary pulmonary nodules (28.7–81.5%) is usually higher than that in benign nodules (5–33.3%) [7–10]. In lung cancer, the air bronchogram sign is more common in adenocarcinoma, and different types of air bronchogram signs (normal, tortuous, and ectatic) are associated with the cell types and malignancy of adenocarcinoma [7, 8, 10–13]. In the early stage of adenocarcinomas, the growth pattern of tumors is mainly lepidic, tumor cells spread along the alveolar wall and the interalveolar septum, and the bronchi are surrounded but not destroyed, leading to an air bronchogram sign [6, 15]. Furthermore, with the retraction of tumoral fibrosis, the bronchi within the tumor are not only unobstructed and uncompressed but remain patent and even irregularly dilated [16, 17]. As their malignant degree increases, tumors may encroach on the walls of bronchi, causing the bronchi to become irregular, narrow, or truncate. In addition, the air bronchogram sign can be seen in inflammatory nodules, such as nodules associated with ordinary inflammation, cryptococcus infection, and tuberculosis [6–9, 10, 18, 19]. Takanashi et al. reported that the air bronchogram sign is more frequently found in pneumonia (40%) than other lesions [8]. An air bronchogram sign is present in 63% of inflammatory pseudotumors in the findings of Qiang et al. [6]. Given that the early formation of inflammatory nodules is related to the exudation and consolidation of the alveolar space, bronchi can exist without being destroyed or filled, and some inflammatory nodules can produce contractile force due to the proliferation of fibrous tissue in the chronic stage; this effect causes bronchiectasis. Bronchi may be natural or dilated, narrowed, or amputated when filled with granulation tissue, mucus, or bleeding [20]. The present study included 209 intraparenchymal PHs, and none presented the air bronchogram sign likely because of the expansive, rather than invasive growth of PHs. Thus, changes to the bronchi were pushing rather than enveloping. Therefore, we concluded that PH should not be diagnosed if the air bronchogram sign is present in a solitary lesion.
Benign and malignant nodules can present the sign of bronchi run at the tumor periphery or are compressed by the tumor; compressed bronchi with walls that remain soft and intact are more common in benign tumors, whereas bronchi with rigid and thickened walls are more common in malignant tumors [6]. Given that PHs cannot invade the parenchyma, the bronchi around the nodule are usually patent or compressed, whereas in malignant nodules, tumor cells always infiltrate around the bronchial wall, resulting in wall rigidity and thickening [6]. In this study, 54 (25.8%) intraparenchymal PHs presented bronchi that ran at the tumor periphery or were compressed by the tumor. None of these PHs had rigid and/or thickened bronchi. In addition, the majority (147, 70.3%) of intraparenchymal PHs in this study lacked a tumor–bronchus relationship likely because most of the intraparenchymal PHs were located in the periphery of the lung, and the peripheral bronchi were small and difficult to observe on CT. In this study, 114 (54.5%) of the intraparenchymal PHs were located at distances of less than 5 mm from the pleura and 76 (36.4%) of the intraparenchymal PHs were attached to the pleura.