Low-dose CT is generally used to screen for lung cancer in high-risk individuals, using minimal ionizing radiation compared with a conventional chest CT scan [14, 15]. Thus far, no study has been performed regarding the feasibility of low-dose CT for peripheral bronchoscopy. This is the first report in which the optimal effective dose CT scan has been evaluated for peripheral bronchoscopy without assistance of novel navigation modalities. In addition, this investigation used ultra-low dose CT protocols with an estimated effective dose of < 1 mSv.
In the present study, the ultra-low dose CT protocols were designed to reduce the effective doses from group 1 to group 4. The results showed that the median estimated effective dose of ultra-low dose CT gradually decreased, from 0.88 mSv in group 1 to 0.12 mSv in group 4. Accordingly, the differences in image noise between standard dose and ultra-low dose CT, as measured by two radiologists, gradually increased from group 1 to group 4 (P < 0.001 for radiologists 1 and 2).
In general, reduction of the radiation dose inevitably increases image noise, which reduces image quality and spatial resolution. Therefore, bronchial walls are more likely to be found spread and their margins are more likely to appear unclear when a lower effective dose is used. Our study showed that the visibility of the bronchial wall leading to the peripheral lung lesion decreased in group 4, compared with groups 1–3. Median differences in bronchial wall thickness between ultra-low dose and standard dose CT were 0.4–0.5 mm in groups 1–3. In addition, median differences in the wall area ratio of the bronchus leading to the peripheral lung lesions were only 3–6% in groups 1–3.
The presence of a bronchus sign is reportedly a reliable predictor of a successful peripheral bronchoscopy procedure [7, 16]. In the current study, the bronchus sign on ultra-low dose CT in groups 1 and 2 was well correlated with the bronchus sign of a standard dose CT scan. Our results suggested that both radiologists and pulmonary physicians could accurately identify the bronchus sign using reconstructed ultra-low dose CT scans.
This study had several limitations. First, it was designed to focus solely on the interpretation of ultra-low dose CT scans by radiologists and pulmonary physicians. Novel navigation modalities for bronchoscopy (e.g., virtual bronchoscopy navigation and electromagnetic navigation bronchoscopy) have been widely used to diagnose peripheral lung lesions under the guidance of artificial intelligence [17–19]. Therefore, bronchoscopy for peripheral lung lesions that depends solely on the interpretation of a CT scan by a doctor may be regarded as an outdated method. However, newer navigation modalities are not always 100% accurate [10, 20]. In addition, navigation systems (e.g., electromagnetic navigation bronchoscopy) are quite expensive; thus, they are not available at all hospitals. Second, although all study patients were randomly assigned to one of the four groups and the database was updated prospectively, the current study was performed retrospectively with a relatively small study population at a single center. We acknowledge that potential selection bias might have influenced the results of our study. A randomized prospective study with a large number of patients is therefore needed to confirm our findings.