Evaluation of Set-up Accuracy for Frame-based and Frameless Lung Stereotactic Body Radiation Therapy

Abstract

The purpose of this study was to evaluate the set up accuracy using stereotactic body frame and frameless immobilizer for lung stereotactic body radiation therapy (SBRT). For total 40 lung cancer patients treated by SBRT, 20 patients using stereotactic body frame and other 20 patients using frameless immobilizer were separately enrolled in each group. The setup errors of each group depending on the immobilization methods were compared and analyzed. All patients received the dose of 48∼60 Gy for 4 or 5 fractions. Before each treatment, a patient was first localized to the treatment isocenter using room laser s, and further aligned with a series of image guidance procedures; orthogonal kV radiographs, cone-beam CT, o rthogonal fluoroscopy. The couch shifts during these procedures were recorded and analyzed for systematic and random errors of each group. Student t-test was performed to evaluate significant difference depending on the immobilization methods. The setup reproducibility was further analyzed using F-test with the random errors excluding the systematic setup errors. In addition, the ITV-PTV margin for each group was calculated. The setup errors for SBF were 0.05±0.25 cm in vertical direction, 0.20±0.38 cm in longitudinal direction, and 0.02±0.30 cm in lateral direction, respectively. However the setup errors for frameless immobilizer showed a significant increase of -0.24±0.25 cm in vertical direction while similar results of 0.06±0.34 cm, −0.02±0.25 cm in longitudinal and lateral directions. ITV-PTV margins for SBF were 0.67 cm (vertical), 0.99 cm (longitudinal), and 0.83 cm (lateral), respectively. On the other hand, ITV-PTV margins for Frameless immobilizer were 0.75 cm (vertical), 0.96 cm (longitudinal), and 0.72 cm (lateral), indicating less than 1 mm difference for all directions. In conclusion, stereotactic body frame improves reproducibility of patient setup, resulted in 0.1∼0.2 cm in both vertical and longitudinal directions. However the improvements are not substantial in clinic considering the effort and time consumption required for SBF setup.