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Development and implementation of an EPID‐based method for localizing isocenter
Journal article   Open access   Peer reviewed

Development and implementation of an EPID‐based method for localizing isocenter

Daniel E Hyer, Christopher J Mart and Earl Nixon
Journal of applied clinical medical physics, Vol.13(6), pp.72-81
11/2012
DOI: 10.1120/jacmp.v13i6.3965
PMCID: PMC5718539
PMID: 23149787
url
https://doi.org/10.1120/jacmp.v13i6.3965View
Published (Version of record) Open Access

Abstract

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter to an accuracy of less than 1 mm using the EPID (Electronic Portal Imaging Device). The proposed solution uses a collimator setting of 10 × 10 cm 2 to acquire EPID images of a new phantom constructed from LEGO blocks. Images from a number of gantry and collimator angles are analyzed by automated analysis software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180 ° collimator rotation to determine if the phantom is centered in the dimension being investigated. Repeated tests show that the system is reproducibly independent of the imaging session, and calculated offsets of the phantom from radiation isocenter are a function of phantom setup only. Accuracy of the algorithm's calculated offsets were verified by imaging the LEGO phantom before and after applying the calculated offset. These measurements show that the offsets are predicted with an accuracy of approximately 0.3 mm, which is on the order of the detector's pitch. Comparison with a star‐shot analysis yielded agreement of isocenter location within 0.5 mm. Additionally, the phantom and software are completely independent of linac vendor, and this study presents results from two linac manufacturers. A Varian Optical Guidance Platform (OGP) calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment and OGP calibration on a monthly basis. PACS number: 87.55.Qr
 linac phantom Winston‐Lutz Radiation Oncology Physics radiation isocenter radiocam Epidemiology

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