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A simple quality assurance test tool for the visual verification of light and radiation field congruent using electronic portal images device and computed radiography

Christopher F Njeh13*, Blas Caroprese2 and Pushkar Desai2

Author Affiliations

1 Texas Oncology Tyler, Radiation Oncology Department, 910 East Houston Street, Tyler 75703, TX, USA

2 East Texas Medical Center, Radiation Oncology Department, ETMC_Cancer Institute, 721 Clinic Drive, Tyler 75701, TX, USA

3 Physics Department, Queensland University of Technology, Brisbane, Australia

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Radiation Oncology 2012, 7:49  doi:10.1186/1748-717X-7-49

Published: 27 March 2012



The radiation field on most megavoltage radiation therapy units are shown by a light field projected through the collimator by a light source mounted inside the collimator. The light field is traditionally used for patient alignment. Hence it is imperative that the light field is congruent with the radiation field.


A simple quality assurance tool has been designed for rapid and simple test of the light field and radiation field using electronic portal images device (EPID) or computed radiography (CR). We tested this QA tool using Varian PortalVision and Elekta iViewGT EPID systems and Kodak CR system.


Both the single and double exposure techniques were evaluated, with double exposure technique providing a better visualization of the light-radiation field markers. The light and radiation congruency could be detected within 1 mm. This will satisfy the American Association of Physicists in Medicine task group report number 142 recommendation of 2 mm tolerance.


The QA tool can be used with either an EPID or CR to provide a simple and rapid method to verify light and radiation field congruence.

Quality assurance; Radiation field; Light field; Linear accelerator; Electronic portal images device; Computed radiography