Gantry angle determination during arc IMRT: evaluation of a simple EPID-based technique and two commercial inclinometers

Pejman Rowshanfarzad,Peter M Mccowan,Mahsheed Sabet,Boyd M.C Mccurdy,Peter B Greer,Daryl J O'Connor

doi:10.1120/jacmp.v13i6.3981

Abstract

The increasing popularity of intensity‐modulated arc therapy (IMAT) treatments requires specifically designed linac quality assurance (QA) programs. Gantry angle is one of the parameters that has a major effect on the outcome of IMAT treatments since dose reconstruction for patient‐specific QA relies on the gantry angle; therefore, it is essential to ensure its accuracy for correct delivery of the prescribed dose. In this study, a simple measurement method and algorithm are presented for QA of gantry angle measurements based on integrated EPID images acquired at distinct gantry angles and cine EPID images during an entire 360° arc. A comprehensive study was carried out to evaluate this method, as well as to evaluate two commercially available inclinometers (NG360 and IBA GAS supplied in conjunction with popular array dosimeters Delta4 and MatriXXEvolution, respectively) by comparing their simultaneous angle measurement results with the linac potentiometer readouts at five gantry speeds. In all tested measurement systems, the average differences with the reference angle data were less than 0.3° in static mode. In arc mode, at all tested gantry speeds the average difference was less than 0.1° for the IBA GAS and the proposed EPID‐based method, and 0.6° for the NG360 after correction for the inherent systematic time delay of the inclinometer. The gantry rotation speed measured by the three independent systems had an average deviation of about 0.01°/s from the nominal gantry speed.PACS numbers: 06.30.Bp; 87.56.Fc; 87.56.‐v

Highlights

Intensity-modulated arc therapy (IMAT) is a novel form of radiotherapy treatment that allows the radiation dose to be delivered in one or two arcs.[1,2]204 Rowshanfarzad et al.: Gantry angle measurement during arc intensity-modulated radiotherapy (IMRT) essential to develop more sophisticated quality assurance (QA) programs that take all components that affect the accuracy of IMAT delivery into account.[2]One of the major considerations for implementation of new treatment techniques is verification of the predicted doses
In routine QA of linacs, a level is positioned on a flat surface of the gantry head close to the graticule and the gantry is rotated until the bubble settles at the center
In a study on linac gantry angles during arc treatments, cine images were acquired during IMAT deliveries using an electronic portal imaging device (EPID) with a specially designed phantom in the beam

Summary

Introduction

Intensity-modulated arc therapy (IMAT) is a novel form of radiotherapy treatment that allows the radiation dose to be delivered in one or two arcs.[1,2]204 Rowshanfarzad et al.: Gantry angle measurement during arc IMRT essential to develop more sophisticated QA programs that take all components that affect the accuracy of IMAT delivery into account.[2]One of the major considerations for implementation of new treatment techniques is verification of the predicted doses. In routine QA of linacs, a level is positioned on a flat surface of the gantry head close to the graticule and the gantry is rotated until the bubble settles at the center. Using this method, the gantry angle indicator can be checked only for cardinal angles, and the flatness of the surface usually remains unchecked.[9,10] Another method suggested for QA of the angle indicator is to perform a star shot on film at distinct static gantry angles and determine the angle based on the film setup.[9] This method is not suitable for testing in arc mode, and introduces the difficulties of film measurements and processing. The accuracy of gantry angles recorded in the header of these DICOM images was investigated by comparison to the angles derived by following the points of intersection of wires in each image.[12,13] It was found that the same angle may be repeated in headers of successive images due to the low frequency of angle readout update

Methods

Results

Conclusion