Positive bronchus sign | Radiology Signs

Answer:

The positive bronchus sign is the demonstration on CT of a bronchus leading directly to, or contained within, a pulmonary nodule or mass. Its importance is procedural rather than diagnostic: a nodule with a bronchus sign is far more likely to be reached and sampled successfully by bronchoscopic techniques, and reported diagnostic yields with the sign present are of the order of twice those without it, which is why the sign is used to choose between transbronchial sampling and CT-guided percutaneous biopsy. It underpins the planning of radial endobronchial ultrasound, electromagnetic navigation and cone-beam CT-guided bronchoscopy, all of which depend on an airway route to the lesion, and the calibre and course of that airway on thin-section reconstructions determine which tool will reach it. The sign also carries some diagnostic weight, since an airway that is abruptly cut off, irregularly narrowed or encased favours malignancy, whereas a patent airway coursing through a lesion is more typical of a benign or inflammatory process such as organising pneumonia or lepidic-growing adenocarcinoma. Where the sign is absent and the lesion is peripheral, percutaneous CT-guided biopsy is generally preferred, accepting its higher pneumothorax rate, and the choice should be made in a multidisciplinary setting alongside PET-CT staging.

Why is it called so?

The sign is called positive when a bronchus can be traced into the lesion and negative when no airway reaches it.

Pathophysiology

A bronchus that reaches a nodule provides a lumen along which a biopsy instrument, brush or needle can be advanced under fluoroscopic, radial ultrasound or navigational guidance, so the anatomical relationship translates directly into sampling success. Lesions that engulf rather than destroy an airway, such as adenocarcinoma growing along alveolar walls or organising pneumonia filling airspaces, keep the airway patent and visible. Invasive tumours that arise from or invade the bronchial wall obliterate the lumen and produce the cut-off appearance, which is itself an indicator of malignancy and often of central location. Because the sign depends on demonstrating airways of 1 to 2 mm, it requires thin-section acquisition with multiplanar or curved reformats along the airway axis.

Alternative names: Bronchus sign; CT bronchus sign

Other associated named signs: Air bronchogram within a nodule and the bronchial cut-off sign of endobronchial obstruction

References

1. Bilaçeroğlu S, Kumcuoğlu Z, Alper H, Osma E, Cağirici U, Günel O, et al. CT bronchus sign-guided bronchoscopic multiple diagnostic procedures in carcinomatous solitary pulmonary nodules and masses. Respiration. 1998;65(1):49-55. PMID: 9523368.