Optimizing cervical target volume in patients with nasopharyngeal cancer based on nodal drainage distance.

Abstract Objectives: To determine the potential nodal drainage distances of nasopharyngeal carcinoma (NPC) by investigating spatial distribution of metastatic lymph nodes (LNs). Methods: NPC patients with at least two ipsilateral metastatic LNs were enrolled. LN spreading distances were analyzed in non-restricted direction, cranial-to-caudal direction, and between the two most caudal LNs. Euclidean distance (ED) and vertical distance (VD) between any two LNs were computed. The nearest-neighbor ED and VD covering 95% of LNs or patients (p95-ED and p95-VD) were considered drainage distances, and were further validated by independent internal and external cohorts with recurrent LNs. Results: In all, 5836 metastatic LNs in 948 patients were contoured. Corresponding to the three scenarios, Per-LN level, the p95-EDs were 2.83, 3.28, and 3.55 cm; and p95-VDs were 2.17, 2.32, and 2.63 cm, respectively. Per-patient level, the p95-EDs were 3.25, 3.95, and 3.81 cm; and p95-VDs were 2.67, 2.81, and 2.73 cm, respectively. In internal validation, over 95% of recurred-LNs occurred within ED of 2.91 cm and VD of 0.82 cm to the neighbor LN, and the corresponding distances in external validation were 2.77 and 0.67 cm, respectively. Conclusions: In NPC, the maximum LN drainage distance was 3.95 cm without considering the direction. Specifically, in cranial-to-caudal direction, the sufficient vertical drainage distance was 2.81 cm, indicating that a 3-cm extension from the most inferior node may be rational as caudal border of the prophylactic clinical target volume (CTV). These findings promote in-depth understanding of nodal spreading patterns, uncovering paramount evidence for individualized CTV.