Inter-Breath Hold Displacements Of Daily Gtv Positions With Abc For Lung Sbrt

The active breathing control (ABC) device aims to achieve temporary and reproducible immobilization of respiratory motion by monitoring the patient’s breathing cycle & implementing a breath hold at a predefined relative lung volume level and air flow direction. The purpose of this study was to quantitatively evaluate the ability of the ABC device to immobilize peripheral lung tumors at a reproducible position, increase total lung volume, and thereby reduce lung mass within the planning target volume (PTV). 15 patients with peripheral non–small-cell lung cancer tumors undergoing SBRT had CT scans of their thorax with and without ABC inspiration breath hold during the first 5 days of treatment. Total lung volumes were determined from the CT data sets. Each peripheral lung tumor was contoured by one physician on all CT scans to generate gross tumor volumes (GTVs). The lung density and mass contained within a 0.5-cm PTV margin around each peripheral tumor was calculated using CT numbers. Using the center of the GTV from the Day 1 ABC scan as the reference, the displacement of subsequent GTV centers on Days 2 to 5 for each patient with ABC applied was calculated in three dimensions. With the use of ABC inspiration breath hold, total lung volumes increased by an average of 39%. This resulted in an average decrease in lung mass of 17% within a standard 0.5-cm PTV margin around the GTV. The average (± standard deviation) displacement of GTV centers with ABC breath hold applied was 0.3 mm (± 1.4 mm), 1.4 mm (± 1.9 mm), and 1.3 mm (± 2.3 mm) in the lateral direction, anterior-posterior direction, and superior-inferior direction, respectively. Results from this study indicate that there remains some inter– breath hold variability in peripheral lung tumor position with the use of ABC inspiration breath hold, which prevents significant PTV margin reduction. However, lung volumes can significantly increase, thereby decreasing the mass of lung within a standard PTV.