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Abstract
It has been proven that portal dosimetry can be derived from a mirror-based fluorescent EPID system by applying multiple kernels that are position dependent. The purpose of this study is to show that patient-specific IMRT/VMAT verification with a single kernel which is acquired from a series of output measurements of a few field sizes is feasible using a commercially available phosphor-screen-based geometric QA system. The optical scatter component in the RavenQA™ (LAP GmbH Laser Applications; Lüneberg, Germany) is corrected by deconvolution with a two-dimensional (2D) spatially invariant single optical scatter kernel (OSK). We assume that the OSK is a 2D isotropic point spread function that decreases as a function of distance from the scatter center. The OSK is determined by comparing output factors of various field sizes. We report on performance testing of the system using 12 intensity-modulated radiation therapy and three volumetric-modulated arc therapy cases. A single spatially invariant OSK can be employed, because the shapes of the OSK across the image plate are almost identical. The average 3%/3 mm gamma passing rate for 15 patients was 97.6%  ±  1.1%. The passing rate was  >95% for all patients. It is feasible to perform the patient-specific IMRT/VMAT verification with a single kernel using a commercially available phosphor-screen-based mechanical QA device in accordance with AAPM TG-142. It is also practical to implement since it only requires to measure the optical intensities of the field centers of several square fields, in order to obtain the OSK.

Author information

Lee M1, Ding K, Yi B.
1
Department of Radiation Oncology, University of Maryland School of Medicine, 22 S. Greene St., Baltimore, MD 21201, United States of America.