Axillary Subsegment of the Right Upper Lobe

In about 15% percent of people, an independent limb of the posterior right upper lobe bronchus supplies an axillary subsegment. Disease localized to the axillary subsegment has a characteristic appearance on chest radiographs and CTs.

On PA images of the chest, disease of the axillary subsegment can resemble that of the anterior segment: a triangular opacity with its base abutting the right lateral chest wall, and the apex pointing toward the right hilum.

Differentiation can be made on the lateral view: the opacity is 4-sided and has sharp inferior (minor fissure) and posterior (major fissure) borders whose junction forms a wing-shaped lower border.

Reference

MacGregor JH, et al. Imaging of the axillary subsegment of the right upper lobe. Chest 1986;90;763-765