Development of a geometry-based respiratory motion-simulating patient model for radiation treatment dosimetry

Temporal and spatial anatomical changes caused by respiration during radiation treatment delivery can lead to discrepancies between the prescribed and actually received radiation doses. This paper presents a study to construct a respiratory-motion-simulating, four-dimensional (4D) patient anatomical and dosimetry model for the study of dosimetric effects of organ motion on various radiation treatment plans and delivery strategies. A 3D VIP-Man (VIsible Photographic Man) model has been reconstructed using the Non-Uniform Rational B-Splines (NURBS) method to reflect the deformation of organs during respiration by manipulating surface control points as time-dependent equations. The 4D model is applied to dose simulation using the Monte Carlo code EGS4 (Electron Gamma Shower, version 4). Two delivery scenarios in radiation therapy were simulated: "gating" treatment and 4D "image-guided" treatment. For each delivery scenario, one conformal plan and one Intensity Modulated Radiation Therapy (IMRT) plan were developed. A lesion in the left lung was modeled to investigate the impact of respiratory motion on radiation dose distributions. Based on target dose volume histograms (DVHs), it is demonstrated that it is important to use accurate "gating" to improve the dose distribution. The results also suggest that, during a 4D "image-guided" treatment delivery, monitoring of patient breathing pattern is critical. This study demonstrates the potential of using "standard" motion-simulating patient model for 4D treatment planning and motion management.