Bronchial anthracofibrosis (BAF) is defined as bronchial narrowing with black pigmentation of the bronchial mucosa1. It is associated with chronic biomass exposure and active or old tuberculosis (TB). Although the actual prevalence of BAF remains unknown, several studies have reported that it is less than 5% among patients who have undergone bronchoscopy2,3. Although BAF is a rare disease, it is worthy of notice because it is often misdiagnosed as endobronchial lung cancer (EBLC)4,5 due to the fact that these two diseases share several similarities. First, their radiologic similarities make diagnosis difficult. Common findings of BAF and TB include bronchial narrowing and obstruction that can be shown in endobronchial tumors. Second, most BAF patients are the elderly who usually complain of cough, sputum, and dyspnea. Patients with cancer also present these clinical features. Thus, it can be difficult to differentiate BAF from EBLC.

Bronchoscopy is one of the most detailed tests to diagnose BAF and EBLC. However, performing bronchoscopy in all suspected patients can be risky because most patients with BAF are the elderly. Therefore, more convenient and simpler diagnostic methods for characterizing BAF are needed. Today, computed tomography (CT) scans can be performed at a lower cost with reduced hazards. They can be used to detect characteristic radiologic features of BAF and EBLC. Characteristic CT features and clinical findings of both diseases can help us make better decisions and avoid unnecessary tests for BAF patients. Despite the necessity of a detailed study about these two diseases, studies that directly compare BAF and EBLC have not been reported yet. Therefore, the objective of this comparative study was to identify clinical and radiological features that could differentiate between BAF and EBLC, both of which were major causes of bronchial stenosis.

Among 3,214 patients who underwent bronchoscopy, finally 167 patients with BAF were enrolled for the BAF group and 117 patients with EBLC were enrolled for the EBLC group (Figure 1). Demographic and clinical characteristics of these patients are summarized in Table 1. Compared to the EBLC group, the BAF group had older patients (76.7 years vs. 68.2 years, p<0.05), higher proportion of females (67.7% vs. 15.4%, p<0.05), and lower smoking rate (11.4% vs. 82.1%, p<0.05). Patients with BAF were more likely to have active TB (38.3% vs. 6.0%, p<0.05). BAF patients showed better values of forced expiratory volume in 1 second (FEV₁) and forced vital capacity (FVC).

Table 2 shows main symptoms of BAF and EBLC. Common symptoms reported by patients were cough, sputum, and dyspnea in both groups. Meanwhile, the BAF group had lower percentages of blood-tinged sputum (2.4% vs. 15.4%, p<0.05), weight loss (5.4% vs. 11.1%, p=0.078), and chest pain (3.0% vs. 12.0%, p<0.005) than the EBLC group.

Anatomical distributions of both diseases in the bronchial tree are shown in Table 3. The BAF group had the following distributions: right upper lobe (RUL), 47.9%; right middle lobe (RML), 53.9%; right lower lobe (RLL), 21.6%; left upper lobe (LUL), 56.3%; and left lower lobe (LLL), 24.6%. The EBLC group had the following distributions: RUL, 37.6%; RML, 12.0%; RLL, 32.5%; LUL, 51.4%; and LLL, 17.1%. Upper lobe predominance was observed in both groups. However, the rate of upper lobe involvement was much higher in the BAF group than in the EBLC group (146 [87.4%] vs. 76 [65.0%], p<0.005). In addition, lesions in the BAF group tended to be more common as multiple lobar bronchi (85.0%) or bilateral bronchi (73.7%) than those in the EBLC group.

Radiological characteristics of bronchial trees (such as bronchial stenosis, bronchial wall thickening, atelectasis, peribronchial calcification, and pleural calcification) and lymph nodes (such as lymph node enlargement and location, number, and calcification of lymph nodes) are summarized in Table 4.

Bronchial wall thickening (25.1% vs. 6.8%, p<0.05) and peribronchial calcification (10.2% vs. 2.6%, p=0.014) were more commonly detected in the BAF group. Although bronchial stenosis was commonly founded in both groups, the detection rate of bronchial narrowing was higher in the BAF group than in the EBLC group (78.6% vs. 49.1%, p<0.05). Lymph node enlargements were common in both groups (59.9% vs. 63.2%, p=0.566). Calcified lymph nodes were observed in 61 patients (36.5%) with BAF, much higher than that in the EBLC group (19 patients, 15.0%). Calcified lymph nodes involving more than two sites were more commonly found in the BAF group. After reviewing detailed characteristics, calcified lymph nodes larger than 1.0 cm, focally calcified lymph nodes, and lymph nodes with the extent of calcification less than 50% were found to be more common in the BAF group than in the EBLC group. Figure 2 shows representative cases of BAF and EBLC.

Biopsy was performed for 17 cases (10.2%) in the BAF group and 117 cases (100%) in the EBLC group. Pathologic findings were as follows: TB (n=7, 41.2%), chronic inflammation (n=5, 29.4%), and non-neoplastic bronchial epithelium (n=5, 29.4%) in the BAF group; squamous cell carcinoma (n=65, 55.6%), adenocarcinoma (n=14, 12.0%), small cell lung cancer (n=22, 18.8%), large cell carcinomas (n=1, 0.9%), both squamous and adenocarcinoma (n=1, 0.9%), both squamous and small cell carcinoma (n=1, 0.9%), and others (n=13, 11.0%) in the EBLC group. Lung cancer stages at diagnosis were as follows: for non-small cell lung cancer, stage I in 17 cases (17.9%), stage II in 13 cases (13.7%), stage III in 28 cases (29.5%), and stage IV in 37 cases (38.9%); for small cell lung cancer, limited stage in 12 cases (54.5%), and extended stage in 10 cases (45.5%).

BAF can occasionally mimic other respiratory diseases such as lung cancer and TB. Therefore, it is important for clinicians to know specific characteristics of BAF to make a proper differential diagnosis and rule out possible conditions. In this study, we evaluated differences in clinical and radiological findings between BAF and EBLC. It was found that BAF occurred predominantly in older non-smoking female individuals, with a higher chance of a history of TB than in patients with EBLC. In comparison, the EBLC group was younger with a higher rate of male-smokers than the BAF group. These findings are consistent with previous studies6,7. In one study7, among BAF patients with a long-term biomass exposure, 80% of them were females with a mean age of 72 years and most patients (78%) were non-smokers. In the present study, common symptoms were cough, sputum, and dyspnea in both groups. Although clinical symptoms were not specific, blood-tinged sputum, weight loss, and chest pain were noted more frequently in the EBLC group.

Previous studies have also shown an association between BAF and TB1,8-10, consistent with our research. In the present study, the rate of patients with coexistent TB was 38.3%. In addition, 15.6% of patients had a history of TB. In pathologic findings, TB and chronic inflammation occurred in nearly 70% of patients in the BAF group. Meanwhile, in the EBLC group, the rate of coexistent TB was only 6.0% and the rate of EBLC patients with a history of TB was 14.5%.

Because the coexistence of pulmonary TB and lung cancer has been estimated to occur in only about 2% of cases11, BAF can be a highly probable diagnosis compared to EBLC in cases of coexistent active TB.

In pulmonary function test, many reports have stated that BAF is significantly associated with obstructive pattern of airway dysfunction6,7,12,13. In our study, the BAF group did not show a substantial decrease of FEV₁. This finding might be related to distal small airway dysfunction rather than large airway dysfunction11. On the other hand, EBLC patients showed lower values of FEV₁ and FVC. Direct airway involvement or external airway compression of lung cancer might have led to a significant airflow obstruction. In addition, space-occupying mass lesion might have reduced compliance of the lung.

In bronchoscopic examination, the BAF group usually involved multiple (85.0%) or bilateral (73.7%) lobar or segmental bronchi, showing upper and middle lobe predominance as reported previously7. Upper lobe predominance of BAF tends to be related to TB because the upper lobe is the most commonly affected site of TB14. Meanwhile, involvement of lung cancer is generally unilateral without showing a specific predominance.

In the present study, significant radiologic findings of BAF were bronchial stenosis, atelectasis, bronchial wall thickening, peribronchial calcification, and lymph node calcifications. Although the frequency of different radiologic findings of BAF varied according to studies, stenosis (48%-100%), atelectasis (20%-76%), bronchial wall thickening (20%-30%), and peribronchial calcification (62%) were common findings7,13,15-18. In our study, bronchial stenosis was the most frequently found lesion in both groups (49.1% vs. 78.6%). Rates of peribronchial calcification and bronchial wall thickening were high in the BAF group. Additionally, the rate of patients with lymph node calcification was higher in the BAF group than in the EBLC group. To review detailed characteristics of lymph nodes, calcified lymph nodes involving more than two sites, larger than 1.0 cm, focally calcified lymph nodes, and lymph nodes with the extent of calcification less than 50% were more common in the BAF group than in the EBLC group. Previous studies have reported that inflammatory necrosis of the lymph node can cause calcium deposit and that lymph node calcification often results from exposure to mineral dust or prior granulomatous infections, especially TB19,20. A higher rate of lymph node calcification in the BAF group might be associated with a higher proportion of TB coexistence. We also investigated the pattern and extent of lymph node calcification because such characteristics of lymph node calcification could made disease more specific. Considering the fact that BAF can arise from exposure of biomass fuels and occupational dust, we anticipated that calcification pattern of BAF could be similar to lymph node pattern of TB or occupational lung disease such as silicosis7,21,22. However, eggshell calcification, a classic pattern of occupational lung disease, was not detected in this study. Moreover, focal calcification pattern was more common in this study. These findings were inconsistent with results of a previous study showing that complete nodal calcification was the most common finding in TB23. We hypothesized that granular or partially calcified lymph nodes might correspond to the focal pattern category and that these partial dense foci might result from shrinking of inflammatory lesions of relatively large volume lymph nodes in our study group24. Metastatic lymph node calcifications can have a granular, amorphous, stippled, or punctate pattern. However, it is difficult to distinguish between benign and malignant diseases depending on the presence or pattern of lymph node calcification25. In summary, there were no radiological findings to differentiate between BAF and EBLC precisely. Calcified lymph nodes adjacent to the involved bronchi or peribronchial calcification might be helpful in differentiating this condition from lung cancer. If there are no clinical predisposing factors of BAF, bronchoscopy and biopsy seem to be accurate diagnostic methods. In case of mediastinal lymph node enlargement, which often mimics a metastatic lymphadenopathy in positron emission tomography, endobronchial ultrasound-guided transbronchial needle aspiration can be a more reliable tool for diagnosis26.

This study has some limitations. First, it was a small-sized single-center study with a retrospective design. Therefore, the statistical power might be small for findings of the present study. Second, the history of biomass exposure was unavailable. Finally, we used different CT scanners. Despite these limitations, this study described characteristic findings of EBLC and BAF groups. A thorough understanding of clinic-radiologic findings of both diseases can help clinicians make a more accurate diagnosis.

In summary, characteristics of BAF include the following: (1) more likely to occur in non-smoking older women, (2) multiple, bilateral involvement with upper and middle lobe predominance, (3) evidence of active TB or a history of TB, and (4) bronchial stenosis, atelectasis, bronchial wall thickening, calcified lymph node, and peribronchial calcification on CT findings. For EBLC, its characteristics are as follows: (1) more likely to occur in male patients with a history of smoking, (2) unilateral involvement without specific predominance, (3) no definite association with TB, and (4) lack of lymph node calcification or peribronchial calcification. Our study demonstrated characteristics of clinical radiologic findings of BAF and EBLC. Increasing awareness of both diseases may help clinicians differentiate them, thus avoiding unnecessary invasive diagnostic and therapeutic procedures.