Dosimetric Comparison of Nodal Clinical Target Volume for Locally Advanced Non-Small Cell Lung Cancer: Options for Geometric Expansion vs. Lymph Node Stations

Abstract

We investigated whether dosimetric differences existed in nodal clinical target volume (CTV) using options for geometric expansion and lymph node stations based on the European Society for Radiotherapy and Oncology (ESTRO) guideline for locally advanced non-small cell lung cancer (NSCLC). With treatment planning computed tomographic images in 17 patients who underwent radiotherapy for cT4N2M0 NSCLC from 2017 to 2022 at our institution, we retrospectively contoured nodal CTVs based on the guideline's options of: (1) geometric expansion, CTV including the nodal gross tumor volume plus 5 mm margin: (2) lymph node stations, CTV including the affected lymph node stations. The 5 mm margins for planning target volume (PTV) were added to the nodal and primary tumors' CTVs. Treatment planning of 60 Gy in 30 fractions to the PTV D50% was performed using volumetric modulated arc therapy; Dn% was irradiated dose to n% of volume of the structure; VnGy was percentage of volume of the structure at least irradiated n Gy. We compared dose-volume parameters between the two options using Wilcoxon rank sum test. Primary tumors were located in the right and left lobes in 12 and five patients, and in the upper and lower lobes in 13 and four patients, respectively. Median PTV size/D95% of the options for geometric expansion and lymph node stations were 569 cc (range, 149-2005 cc)/58.2 Gy (range, 57.5-58.7 Gy) and 635 cc (range, 184-2109 cc)/58.1 Gy (range, 57.5-58.7 Gy), respectively. Median mean dose of the esophagus/heart in the two options were 12.2 Gy (range, 6.6-24.9 Gy)/3.3 Gy (range, 0.6-19.1 Gy) and 16.1 Gy (range, 13.6-30.5 Gy)/4.6 Gy (range, 0.7-18.9 Gy), respectively. Median V20 Gy/mean dose of the lungs in the options for geometric expansion and lymph node stations were 20.5% (range, 14.8-33.9%)/12.2 Gy (range, 8.7-18.4 Gy) and 24.0% (range, 15.1-36.7%)/13.5 Gy (range, 9.5-19.4 Gy), respectively. In the eight patients (47%) with lymph node metastases in stations 2 or 3, a significant dosimetric difference between the two options existed on V20 Gy of the lungs; median values of the difference were 2.8% and 0.5% with and without lymph node metastases in stations 2 or 3, respectively (p = 0.027). Of the ESTRO guideline for the nodal CTV, using the option for geometric expansion might be able to reduce V20 Gy of the lungs in patients with lymph node metastases in stations 2 or 3.