Experimental evaluations of the accuracy of 3D and 4D planning in robotic tracking stereotactic body radiotherapy for lung cancers

Purpose: Due to the complexity of 4D target tracking radiotherapy, the accuracy of this treatment strategy should be experimentally validated against established standard 3D technique. This work compared the accuracy of 3D and 4D dose calculations in respiration tracking stereotactic body radiotherapy (SBRT). Methods: Using the 4D planning module of the CyberKnife treatment planning system, treatment plans for a moving target and a static off-target cord structure were created on different four-dimensional computed tomography (4D-CT) datasets of a thorax phantom moving in different ranges. The 4D planning system used B-splines deformable image registrations (DIR) to accumulate dose distributions calculated on different breathing geometries, each corresponding to a static 3D-CT image of the 4D-CT dataset, onto a reference image to compose a 4D dose distribution. For each motion, 4D optimization was performed to generate a 4D treatment plan of the moving target. For comparison with standard 3D planning, each 4D plan was copied to the reference end-exhale images and a standard 3D dose calculation was followed. Treatment plans of the off-target structure were first obtained by standard 3D optimization on the end-exhale images. Subsequently, they were applied to recalculate the 4D dose distributions using DIRs. All dose distributions that were initially obtained using the ray-tracing algorithm with equivalent path-length heterogeneity correction (3D(EPL) and 4D(EPL)) were recalculated by a Monte Carlo algorithm (3D(MC) and 4D(MC)) to further investigate the effects of dose calculation algorithms The calculated 3D(EPL), 3D(MC), 4D(EPL), and 4D(MC) dose distributions were compared to measurements by Gafchromic EBT2 films in the axial and coronal planes of the moving target object, and the coronal plane for the static off-target object based on the gamma metric at 5%/3mm criteria (gamma(5%/3mm)). Treatment plans were considered acceptable if the percentage of pixels passing gamma(5%/3mm) (P-gamma<1) >= 90%. Results: The averaged P-gamma<1 values of the 3D(EPL), 3D(MC), 4D(EPL), and 4D(MC) dose calculation methods for the moving target plans are 95%, 95%, 94%, and 95% for reproducible motion, and 95%, 96%, 94%, and 93% for nonreproducible motion during actual treatment delivery. The overall measured target dose distributions are in better agreement with the 3D(MC) dose distributions than the 4D(MC) dose distributions. Conversely, measured dose distributions agree much better with the 4D(EPL/MC) than the 3D(EPL/MC) dose distributions in the static off-target structure, resulting in higher P-gamma<1 values with 4D(EPL/MC) (91%) vs 3D(EPL) (24%) and 3D(MC) (25%). Systematic changes of target motion reduced the averaged P-gamma<1 to 47% and 53% for 4D(EPL) and 4D(MC) dose calculations, and 22% for 3D(EPL/MC) dose calculations in the off-target films. Conclusions: In robotic tracking SBRT, 4D treatment planning was found to yield better prediction of the dose distributions in the off-target structure, but not necessarily in the moving target, compared to standard 3D treatment planning, for reproducible and nonreproducible target motion. It is important to ensure on a patient-by-patient basis that the cumulative uncertainty associated with the 4D-CT artifacts, deformable image registration, and motion variability is significantly smaller than the cumulative uncertainty occurred in standard 3D planning in order to make 40 planning a justified option. (C) 2013 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4794505]