Title: PD5-1-8: 4D CT imaging impacts lung volume definitions and dose-volume relationships in radiation treatment of non-small cell lung cancer
Abstract: To characterize lung volume definitions at various CT imaging conditions for advanced non-small-cell lung cancer (NSCLC) patients. Due to respiratory motion, lung volumes measured by CT imaging can exhibit significant volumetric variations, which can affect the assessment of dose-volume relationships for radiotherapy. The goal of this study was to quantify the range of uncertainties in the dose-volume relationships and suggest a unified correction procedure. Forty stage III/IV NSCLC patients were randomly selected for this study. For all patients, a fast CT scan and a 4D CT scan were acquired under natural breathing conditions. The fast CT scan (free breathing, or FB) represents the typical clinical practice for CT-based radiotherapy treatment planning. The 4D CT scan represents the state-of-art volumetric imaging technique to explicitly include respiratory motion. We define the lung volumes measured in 4D CT at the end of inspiration and at the end of expiration as end inspiratory volume (EIV) and end expiratory volume (EEV), respectively. The percent of volumes at 5Gy (V5), 20Gy (V20), and 50Gy (V50) were calculated and compared for each volume definition at FB, EEV, EIV, and a specially computed average density volume from 4D CT. The total lung EIV was on average 13.9% larger than the EEV (1SD 5.1%; range: 5.7% to 28%). While the average density of the lung at the end of inspiration was 8% lower than the density of the lung at the end of expiration, the total mass enclosed in the lung volume was 4.6% higher at the end of inspiration than at the end of expiration, possibly due to the blood inflow caused by the negative pressure. There was an excellent linear relationship between the density ratio and the volume ratio at the two breathing extremes among all patients (R=0.86; p≤0.0001). We found a strong volume impact to the dose-volume relationship. The V5, V20, and V50 between EIV and EEV varied from −15% to +12%, −21% to +17%, and −6% to +3%, respectively. The V5, V20, and V50 defined on FB CT scans were systematically smaller than those defined in EEV (average=−1.9% for V5; −1.3% for V20; and −0.8% for V50) and varied significantly (range from −12% to +1%), representing the uncertainties in the dose-volume analysis. These variations were primary due to volume differences at different breathing phases. The impact of dose distribution by anatomy changes was minimal (V5, V20, and V50 variations were less than 1% for the same volume definition). We found that the use of a simple scaling factor can effectively correct or convert the dose-volume parameters from one phase to another using the population data obtained in this study. Significant variations in dose-volume parameters were found in current practice when using different volume definitions. We recommend reporting dose-response data on standardized volume definitions (such as the end of expiration, which is more nature for individual patients). Parameters found in this study can be used to convert these dose-volume parameters from one volume definition to another based on population average.