Analysis of the anatomical relationship between the anterior segment (S3) and adjacent segments in the left upper lobe

S 3 of the left upper lobe (LUL), situated between S 1 + 2 and S 4 + 5 , serves as a vital anatomical bridge. This study examines the anatomical relationships of S 3 with surrounding segments to enhance surgical accuracy and minimize complications. A total of 120 patients with LUL lesions scheduled for surgery from October 2024 to March 2025 were included. Three-dimensional CT bronchography and angiography (3D-CTBA) allowed us to analyze S3 anatomy, its spatial relationships with S 1 + 2 and S 4 + 5 , and lesion locations. Trans-segmental and trans-subsegmental arteries were defined. S 3 -related lesions accounted for 46% (61/133) of all LUL lesions. B 3 branching was classified into three types, with B 3 a; B 3 b + c being the most common (86%, 103/120). An independent origin of B 1 + 2 c limited S 3 a to the central region of the LUL, while the two-branch LUL bronchus and semi-central vein were associated with S 3 a extension to the oblique fissure ( p = 0.029, 0.001, 0.002). The predominant pattern was independent A 3 (73%, 88/120), with trans-subsegmental arteries most frequently seen (31%, 22/72). Only 24% (19/79) matched the typical B 3 bronchial pattern. Arterial common trunks between S 3 –S 1 + 2 and S 3 –S 4 + 5 occurred in 55% (66/120) and 30% (36/120), respectively; venous common trunks occurred in 70% (84/120) and 31% (37/120). Frequent trans-subsegmental arteries, variable S 3 a boundaries, and shared vascular trunks complicate surgery, making preoperative 3D reconstruction essential for precise segmentectomy planning.