Volumetric and Motion Differences of Organs-at-Risk Between Seated and Supine Patient Positioning for the Delivery of Thoracic Radiation Therapy

Abstract

The toxicity profile of conventional thoracic radiotherapy delivered in the supine patient position can be impacted by lung and heart anatomy. Vertical computed tomography (CT) permits both planning and delivery of thoracic radiotherapy in the upright seated position. This is hypothesized to impact volume and motion of the lung and heart relative to supine positioning during thoracic radiotherapy. Eleven patients with thoracic tumors were prospectively simulated in both the supine position with conventional CT and in the upright seated position with vertical CT, both with four-dimensional (4D) phase binning. For each patient, the bilateral lungs and heart were contoured in 10 phases of the respiratory cycle. Heart movement was assessed by tracking the center of boundary of the heart contours during each phase of the respiratory cycle in the medial-lateral (ML), anterior-posterior (AP), and superior-inferior (SI) directions. Linear mixed effects regression was performed to analyze the impact patient position on lung volume and heart movement, while accounting for by-patient and by-respiratory phase variations with random intercepts and slopes. Median age at simulation was 70 years. Tumor locations comprised of central defined as within 2cm of the proximal bronchial tree (42.4%), peripheral (28.8%), and anterior mediastinal (28.8%). Overall, the upright position was associated with increased combined lung volume (3,431.3 cm3 vs. 3,119.2 cm3, p=0.002). Linear mixed effects regression with random intercepts and slopes, considering patient and respiratory phase as random effects, found an association with upright positioning and increased right lung volume (β=120.3 cm3 [7.5% of total volume], p=0.04), left lung volume (β=307.4 cm3 [15.3% of total volume], p=0.002), and combined lung volume (β=120.3 cm3 [10.9% of total volume], p=0.005). Upright position was also associated with trend toward decreased heart movement in the SI direction (-0.96 mm, p=0.06). There were no significant differences in heart volume, or heart movement in the ML or AP directions between seated and upright positioning. Upright treatment positioning made possible by vertical 4DCT simulation is associated with significantly increased lung volumes even after accounting for by-patient and by-respiratory phase variations, with a trend toward less heart motion. Relative increases in normal tissue lung volume and decreased heart motion may lead to improved dosimetry and sparing of these normal tissues during definitive-intent radiotherapy of thoracic malignancies.