A single-optical kernel for a phosphor-screen-based geometric QA system (RavenQATM) as a tool for patient-specific IMRT/VMAT QA.
Authors of this article are:
Lee M Ding K Yi BY.
A summary of the article is shown below:
Purpose: 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. Methods: The optical scatter component in the RavenQA<sup>TM</sup> (LAP GmbH Laser Applications; Lüneberg, Germany) is corrected by deconvolution with a two-dimenstional (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 3 volumetric-modulated arc therapy cases. Results: 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. Conclusions: 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.
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