Improving Lung Stereotactic Body Radiation Therapy Dose Conformity Using a Simple Noncoplanar Volumetric Modulated Arc Therapy Technique.

This study characterizes lung stereotactic body radiation therapy (SBRT) dose conformity for a noncoplanar volumetric modulated arc therapy (VMAT) technique. Retrospectively, 288 previously treated lung SBRT cases were replanned using a VMAT technique of 2 ipsilateral 180° arcs separated by 30° couch angles. Two objectives were used for optimization: a lower objective to achieve planning target volume (PTV) coverage and a custom normal tissue objective to steepen the dose gradient. The dose was calculated using Acuros. PTV coverage was 95%. Doses to the spinal cord, chest wall, esophagus, great vessels, heart, lungs, and trachea were evaluated. Conformity index (CI, isodose volume/PTV) values were recorded at the 10% to 100% isodose levels. CI50% results were benchmarked against the corresponding clinical plans and evaluated using the Wilcoxon signed-rank test. Linear regression was performed to characterize the relationship between dose conformity and the following PTV features: Hounsfield Units, volume, surface area, surface-to-volume ratio, and compactness. Compared with the clinical plans, the 2-objective VMAT plans demonstrated comparable or superior sparing of organs at risk with improvements in CI at the 10% to 100% isodose levels, all of which were statistically significant (P < .001). The average reductions in CI30% and CI50% were 3.5 and 0.63, respectively. Compared with the clinical plans, cases exceeding Radiation Therapy Oncology Group CI50% limits were reduced from n = 10 to 0 unacceptable and n = 78 to 20 acceptable variations. At CI30% to CI60%, regression showed that PTV Hounsfield Units and surface-to-volume ratio were significant (P < .001) predictors of dose conformity. An easily implementable VMAT technique achieved improved conformity across a broad range of lung SBRT cases and is now the standard at our institution. Further, dose conformity was characterized at different isodose levels with consideration of PTV features. Results from this study supplement historic clinical trial guidelines by providing more comprehensive and patient-specific goals for lung SBRT dose conformity.