Verification of the delivered patient radiation dose for non-coplanar beam therapy

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Kutuzov, Ivan
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Radiation therapy is applied in at least 50% of all cancer treatments in North America. Its purpose is to deliver a high dose of radiation to the tumor while limiting doses to surrounding normal tissues. This is a challenging task when organs-at-risk (OARs) are in close proximity to the tumor. This is why recently many researchers have investigated the opportunity of using non-coplanar beam arrangements to geometrically avoid critical structures. This approach can reduce irradiation of OARs, and potentially provide better dose conformity to the tumor. On the other hand, it leads to the necessity of more stringent quality assurance. In vivo dosimetry based on the analysis of megavoltage transmission images acquired with an Electronic Portal Imaging Device (EPID) is a powerful quality assurance method that validates the actual delivered 3D dose to the patient. Currently, CancerCare Manitoba has implemented an in vivo patient dose verification system using this approach, but it only works with conventional coplanar beam geometry. The purpose of this research was to add and validate new functionality to the previously developed dose verification system, allowing it to work with non-coplanar beam arrangements. The patient model in the existing Matlab (MathWorks, Natick, MA) program code was updated to enable modeling of non-coplanar treatment beams. Several non-coplanar plans were created in the clinically used treatment planning system (Eclipse, Varian Medical Systems). These test trajectories were delivered on a linear accelerator (Edge, Varian Medical Systems) using geometric phantom with the EPID deployed and acquiring transmission images. These images were then used to calculate the 3D dose distributions in the phantom using updated algorithm. The reconstructed dose distributions were compared to the corresponding reference dose distributions obtained from the treatment planning system. The chi-comparison test using 3%/3mm and 2%/2mm criteria between experimental and predicted dose matrices resulted in at least a 97.0% pass rate over the entire 3D dose distribution for all tested trajectories. This comparison shows EPID dose reconstruction as a promising method for in vivo patient 3D dose verification that can be used for quality assurance of complex non-coplanar treatments trajectories.
Medical physics, Radiotherapy, Clinical Dosimetry, in vivo dosimetry, EPID dosimetry, Non-coplanar treatment