Upper rim of left main pulmonary artery

In the IASLC lymph node map, the upper rim of the left main pulmonary artery is defined as the cranial-most aspect of the left pulmonary artery as it courses from the main pulmonary trunk toward the left hilum. On axial CT imaging, this corresponds to the level at which the left pulmonary artery first becomes visible as it passes beneath the aortic arch, typically at or just below the aortopulmonary window.

This landmark serves a dual role in the IASLC map:

• Lower boundary of Station 4L :The left lower paratracheal nodes (4L) are located medial to the ligamentum arteriosum, between the upper margin of the aortic arch (superiorly) and the upper border of the left main pulmonary artery (inferiorly).

• Upper boundary of Station 5: The subaortic (aortopulmonary window) nodes (5) lie lateral to the ligamentum arteriosum, bounded superiorly by the lower border of the aortic arch and inferiorly by the upper rim of the left pulmonary artery.

The ligamentum arteriosum itself serves as the medial-lateral dividing line between stations 4L (medial) and 5 (lateral) at this level.