Left lateral wall of trachea

The left lateral wall of the trachea as the anatomic boundary that separates rights-ided from left-sided paratracheal lymph node stations in the IASLC lymph node map notably, this is not the midline of the trachea.

This landmark is critical for distinguishing station 4R (right lower paratracheal) from station 4L (left lower paratracheal) nodes, and similarly for stations 2R versus 2L. Specifically:

• Station 4R extends from the intersection of the caudal margin of the left innominate vein with the trachea superiorly to the lower border of the azygous vein inferiorly, and its left boundary is the left lateral border of the trachea.

• Station 4L lies to the left of the left lateral wall of the trachea, extending from the upper margin of the aortic arch superiorly to the upper border of the left pulmonary artery medial to the ligamentum arteriosum inferiorly.

Because the dividing line is the left lateral tracheal wall rather than the tracheal midline, station 4R encompasses a larger anatomic region it includes nodes both anterior and to the right of the trachea. This means that nodes located directly anterior to the trachea (pretracheal) are classified as 4R, not 4L.

This distinction has important staging implications: a rightsided lung cancer with nodes in the pretracheal space is classified as ipsilateral N2, whereas a leftsided lung cancer with pretracheal nodes would be classified as contralateral N3.