Patient-specific margins for proton therapy of lung

Lung cancer treatment presents a greater treatment planning and treatment delivery challenge in proton beam therapy compared to conventional photon therapy due to the proton beam's energy deposition sensitivity to the breathing-induced dynamic tissue density variations along the beam path. Four-dimensional computed tomography (4D-CT) has been defined as the explicit inclusion of temporal changes of tumor and normal organ mobility into an image series. It allows more accurate delineation of lung cancer target volumes by suppression of any breathing motion artifacts present in the CT images. It also allows analysis of the tumor's 3D spatial movement within a breathing phase cycle. The motivation for this study was to investigate dosimetric errors caused by lung tumor motion in order to find an optimal method of design for patient compensators and apertures for a passive scattering beam delivery system and treatment of the patient under free breathing conditions. In this study, the maximum intensity projection (MIP) method was compared to patient-specific internal margin designs based on a single breathing phase at the end-of inhale (EOI) or middle-of-exhale (MOE). It was found that MIP method provides superior tumor dose distribution compared to patient-specific internal margin designs derived from 4D-CT.