Dosimetric Comparison of Different Prescription Modes in Lung Stereotactic Body Radiation Therapy

The purpose of this study was to compare the dose-volume statistics of stereotactic body radiotherapy (SBRT) for lung cancer between planning target volume (PTV): D95 and gross tumor volume (GTV): D99 dose prescriptions using Monte Carlo (MC) calculation. Plans for 183 patients treated between October 2010 and April 2013 were generated based on four-dimensional (4D) computed tomography (CT) under free breathing. A uniform margin of 8 mm was added to the internal target volume (ITV) to generate PTV. A leaf margin of 2 mm was added to the PTV. The plans were calculated with two different dose prescription methods: 40 Gy to cover 95% of the PTV (PTV prescription) and 44 Gy to cover 99% of the GTV (GTV prescription). A 6-MV photon beam was used. A dose-volume histogram (DVH) analysis was performed for dose to the GTV using PTV and GTV dose prescriptions. For each treatment plan, we evaluated the minimum dose to 99% of the GTV (D99). The D99 of GTV was 44.5 ± 1.9 Gy and 44.0 ± 0.0 Gy for PTV and GTV prescriptions, respectively. The dose to the GTV had wide variations with PTV prescription. We recommend that GTV based dose prescription should be used to standardize dose to the tumor and to achieve highly conformal dose distributions in SBRT for lung cancer.