Creating a dynamic, multi-purpose correction for multiple geometries and field sizes to account for off-axis and asymmetric backscatter with varian portal dosimetry

Intensity modulated radiation therapy and volumetric modulated arc therapy are increasingly common in radiation therapy due to their benefits of target conformity and normal tissue sparing. Due to the complexities of plan delivery and the precision required, the dose delivered must be accurately measured for quality assurance (QA). One of the most efficient ways to perform patient-specific QA when using clinical linear accelerators (linac) is to use an electronic portal imaging device (EPID). Amorphous silicon (aSi) Electronic Portal Imaging Devices (EPIDs) are attached to the linac and can provide real-time feedback with spatial resolution on the order of sub-millimeter pixel size making them very favorable for QA. However, the response to radiation in the EPIDs is not similar to that in water or soft tissue, so beam intensity profile corrections must be used and output factors specific to the imager must be collected. Additionally, when radiation exits the imager, it will travel through the support arm made up of high density materials; this non-uniform backscatter will cause the EPID to detect differences that are dependent on position and field size. To meet this need Varian Medical Systems (Palo Alto, CA) has created a 2-dimensional Portal Dosimetry Pre-Configuration (PDPC) package, which uses a 15 x 15 cm2 matrix to correct for the asymmetric and off-axis response from the non-uniform backscatter; however, because this package is optimized for a 15 x 15 cm2 field size, it may cause over or under correction of the backscattered radiation. The aim of this project is to correct the imager response as a function of field size for both off-axis variations and imaging arm backscatter variations. These corrections would be application- facing corrections, allowing them to be more robust than current corrections applied beyond EPID dosimetry calibration.