Abstract
Image‐guided radiation therapy using cone‐beam computed tomography (CBCT) is becoming routine practice in modern radiation therapy. The purpose of this work was to develop an imaging QA program for CT and CBCT units in our department, based on the American College of Radiology (ACR) CT accreditation phantom. The phantom has four testing modules, permitting one to test CT number accuracy, slice width, low contrast resolution, image uniformity, in‐plane distance accuracy, and high‐contrast resolution reproducibly with suggested window/levels for image analysis. Additional tests for contrast‐to‐noise ratio (CNR) and noise were added using the polyethylene and acrylic plugs. Baseline values were obtained from CT simulator images acquired on a Phillips Brilliance Big Bore CT simulator and CBCT images acquired on three Varian CBCTs for the imaging protocols most used clinically. Images were then acquired quarterly over a period of two years. Images were exported via DICOM and analyzed manually using OsiriX. Baseline values were used to ensure that image quality remained consistent quarterly, and baselines were reset at any major maintenance or recalibration. Analysis of CT simulator images showed that image quality was within ACR guidelines for all tested scanning protocols. All three CBCT systems were unable to distinguish the low‐contrast resolution plugs and had the same high‐contrast resolution over all imaging protocols. Analysis of CBCT results over time determined a range of values that could be used to establish quantitative tolerance levels for image quality deterioration. While appropriate for the helical CT, the ACR phantom and guidelines could be modified to be more useful in evaluating CBCT systems. In addition, the observed values for the CT simulator were well within ACR tolerances.PACS numbers: 87.57.Q‐, 87.57.qp, 87.57.C‐
Highlights
Image-guided radiation therapy (IGRT) using in-room cone-beam computed tomography (CBCT) is becoming routine practice in modern radiation therapy
Tolerance values for the above-mentioned image quality parameters are well established for CT and CT simulation.[1,2] image quality tolerances for CT cannot be used for CBCT
For routine image quality assurance (QA) of CBCT systems used for image guidance, most of the recommendations from the literature compare uniformity and CT number constancy/accuracy QA results to “baseline” QA results without providing quantitative tolerance levels[5,6,7,8,9] or provide relatively loose tolerance levels.[10,11,12] Most of the QA results and programs reported in the literature have used the Catphan phantom[5,9,10,11,12,13] (The Phantom Laboratories, Salem, NY), which is used for both acceptance testing and commissioning of most linac CBCT systems
Summary
Image-guided radiation therapy (IGRT) using in-room (or on-board) cone-beam computed tomography (CBCT) is becoming routine practice in modern radiation therapy. The ACR CT phantom was picked because it has wellestablished image quality tests and tolerances for conventional CT systems[1,14] and is quite easy to set up. These tests and tolerances provide a starting point for developing a routine QA program for CT and CBCT systems in a radiotherapy department based on one phantom
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