Simplified strategies to determine the mean respiratory position for liver radiation therapy planning

Establishing the time-weighted mean respiratory position in the liver is challenging due to poor tumor contrast on 4-dimensional (4D) imaging. The purpose of this study is to validate simplified strategies in determining the mean position of liver tumors for radiation therapy planning, and quantify the potential for planning target volume (PTV) reduction.

Full, 10-phase 4D computed tomography (CT) data sets from 10 liver radiation therapy patients were analyzed to compare 2 techniques. First, a mid-ventilation CT was chosen from the initial reconstruction of the 4DCT. This was based on the minimum displacement of the diaphragm at each phase relative to its mean respiratory position, calculated using rigid registration over all 4DCT phases. Second, the exhale 4DCT was deformed to the inhale 4DCT using biomechanical-based deformable registration. The diaphragm's mean cranio-caudal position in the respiratory cycle (normalized as a percentage relative to exhale) was applied to the exhale-to-inhale deformation map assuming a linear trajectory to reconstruct a mid-position CT. These strategies were compared with the time-weighted mean respiratory position, calculated with deformable registration over all 10 4DCT phases. PTVs incorporating respiratory motion were then compared for 2 planning strategies: exhale 4DCT using the internal target volume (ITV), or mid-position CT using dose-probability margins.

Compared with the mean respiratory tumor position, the mid-ventilation CT and mid-position CT had mean (maximum) tumor vector errors of 1.0 ± 0.5 (2.1) mm and 0.6 ± 0.3 (1.4) mm, respectively, within the image resolution. Compared with ITV-based PTV, dose-probability PTV reduced the irradiated volume by 34% ± 7% on average, up to 43%.

Simplified strategies to select a mid-ventilation CT or reconstruct a mid-position CT for the liver were validated with respect to the mean respiratory position. These data sets require significantly smaller PTVs, potentially allowing for dose-escalated liver stereotactic body radiation therapy to improve local control.