Respiratory Motion Management to Improve Cardiac Substructure Sparing for Lung Cancer Radiotherapy

Summary: Radiotherapy has been shown to increase the risk of cardiotoxicity in lung cancer patients. The objective of this study was to investigate cardiac substructure sparing comparing radiation treatment planning based on free breathing (FB) 4DCTs vs breath hold CTs obtained with active breathing control (ABC). Ten patients treated with concomitant radiochemotherapy for stage IIB to IIIB non-small cell lung cancer who underwent CT imaging using FB 4D CT and 3 repeat ABC CTs on an IRB-approved protocol were included. Heart, 18 cardiac substructures (chambers, valves, coronary arteries (CA), sinoatrial (SAN) and atrioventricular nodes (AVN), pulmonary veins (PV)), lungs, esophagus, spinal cord, primary tumor (PT), and lymph nodes (LN) were manually contoured on the average 4DCT and average ABC scans for each subject using established contouring guidelines. Dose planning was performed with VMAT to 60 Gy in 30 fractions using standard clinical dose constraints. Significance was assessed using Wilcoxon signed rank test. On average, ABC led to smaller volumes for most target and organ at risk (OAR) volumes except for lung. Volume reductions were significant for the following target structures iGTV_PT, GTV_LN, iGTV_LN, ITV_LN, PTV_LN with average reductions between 9.6% and 28.2% (p=0.006 – 0.03). ABC also led to significant volume reductions for small cardiac substructures including the four valves and left main CA (p=0.002 – 0.04). Volume reductions for heart and chambers ranged between -1% (increased volume L atrium) and 4% (n.s.). Lung volumes increased with ABC by on average 25% (n.s.). Average mean and max dose parameters for OARs including heart substructures were lower for most structures on ABC. Dose reductions were significant for pulmonic valve, right inferior PV, mean lung and spinal cord PRV doses (p=0.0019 – 0.04). For heart and chambers, dose reductions > 5 Gy were observed in more patients with ABC in 12/19 structures and with 4DCT in 4/19 structures (AVN, L circumflex CA, mitral and tricuspid valves). Left superior PV Dmax was > 5 Gy higher in more patients with 4DCT and Dmean in more patients with ABC. The same number of patients had dose reductions > 5 Gy with either planning method for left ventricle and left atrium. No dose dependence on tumor motion (range < 2mm to 9mm) was observed. In our patient cohort, ABC resulted in smaller target and OAR volumes and reduced doses compared to FB 4DCT-based planning for the majority of OARs and target structures. If tolerated by patients, ABC may therefore reduce cardiotoxicity risks by minimizing exposure to critical cardiac substructures. Further investigation with larger cohorts studying the effect of tumor location and clinical outcomes is suggested to validate these results. Vaani Shah, n/a (Presenting Author) - Virginia Commonwealth University School of Medicine; Shahad Al-Ward, n/a (Co-Author) - Massey Comprehensive Cancer Center at VCU; Mihaela Rosu-Bubulac, PhD, DABR, FAAPM (Co-Author) - Massey Comprehensive Cancer Center at VCU; Elisabeth Weiss, MD (Co-Author) - Massey Comprehensive Cancer Center at VCU