Clinical assessment and characterization of a dual tube kilovoltage X-ray localization system in the radiotherapy treatment room

Abstract

Although flat-panel kilovoltage X-ray imaging devices have been well tested for clinical use in diagnostic radiology, their use as a part of an image-guided radiation therapy (IGRT) system in a treatment room is new and requires systematic assessment. We used the Novalis IGRT system (BrainLAB, Feldkirchen, Germany) for the present study. The system consists of two floor-mounted kilovoltage X-ray tubes projecting obliquely onto two flat-panel detectors mounted on the ceiling. The system can automatically fuse two-dimensional localization images with three-dimensional simulation computed tomography images to provide positioning guidance. We evaluated these system parameters: • Overall performance of the IGRT system, including isocenter correlation between the IGRT system and the linear accelerator (LINAC) • Image quality of the system • Exposure received by patients for a pair of images • Linearity, uniformity, and repeatability of the system The Novalis system uses a daily isocenter calibration procedure to ensure consistency of isocenters between the IGRT and the LINAC systems. The localization accuracy was about 1 mm. We measured the relative modulation transfer function (RMTF) to quantify the spatial resolution of the imaging device, with f50 being 0.7-0.9 line pairs per millimeter. The maximal exposure of an image was 95 mR. We derived an empirical relationship between the exposure and the X-ray technical settings. The other parameters of the system were quantitatively measured and generally met the requirements. The IGRT system is safe and reliable for clinical use as a target localization device. The measured data can be used as a benchmark for a quality assurance procedure.