Lower border of aortic arch

In the IASLC lymph node map, the lower border of the aortic arch serves as a critical anatomic landmark that demarcates the boundary between the upper zone (stations 2-4) and the aortopulmonary zone (stations 5 and 6) of mediastinal lymph nodes used for lung cancer staging.

Anatomic definition and role in the IASLC map

The lower border of the aortic arch corresponds to the inferior margin of the aortic arch as seen on axial CT imaging, at the level where the arch transitions into the descending thoracic aorta. In the IASLC classification, this landmark is used in the following ways:

• Station 4L (left lower paratracheal): Its upper boundary is defined at the level of the upper margin (top) of the aortic arch, and it extends inferiorly to the upper border of the left main pulmonary artery, medial to the ligamentum arteriosum. Thus, the top of the aortic arch marks where station 4L begins superiorly.

• Station 5 (subaortic / aortopulmonary window): Defined as nodes lateral to the ligamentum arteriosum or the aorta or left pulmonary artery, lying between the lower border of the aortic arch and the upper border of the left main pulmonary artery. The lower border of the aortic arch therefore constitutes the upper boundary of station 5.

• Station 6 (para-aortic / ascending aorta or phrenic): Nodes lying anterior and lateral to the ascending aorta and the aortic arch, beneath a line tangential to the upper border of the aortic arch.

In practical terms, on axial CT the lower border of the aortic arch is identified at the level where the aortic arch is no longer visible as a complete arch and begins to appear as the descending aorta posteriorly and the ascending aorta anteriorly essentially the most inferior axial slice on which the arch is still continuous.