Implementation of Quality Assurance Protocols for an Advanced Linear Accelerator with Volumetric Modulated Arc Therapy Features Using Electronic Portal Imaging Device (EPID)

  • C.P. Bhatt Department of Allied Sciences, Graphic Era (Deemed to be University), Dehradun, India
  • Kiran Sharma Department of Allied Sciences, Graphic Era (Deemed to be University), Dehradun, India
  • Manoj K. Semwal Army Hospital (Research and Referral), New Delhi, India
Keywords: Radiotherapy, Quality Assurance (QA), Volumetric Modulated Arc Therapy (VMAT), DynamicMultileaf Collimator(DMLC), ElectronicPortal Imaging Device (EPID)


To use Electronic Portal Imaging Device (EPID), an integral feature of a True Beam linear accelerator (linac) system, for implementing dosimetry based comprehensive Quality Assurance (QA) protocol needed for Volumetric Modulated Arc Therapy (VMAT) modality. Varian makesTrueBeam Version 2.0 linac system with Intensity Modulated Radiotherapy (IMRT), and VMAT treatment modalities were used in the study. The linac is equipped with a Multileaf Collimator (MLC) having 120 leaves (milleniumMLC) and an EPID (aS1000) having mega voltage photon (MV) detector system. The EPID has an active imaging area of 40 cm x 30 cm with 1024 x 768-pixel matrix with a pixel resolution of 0.39 mm. It is capable of capturing 14-bit images at 30 frames per second. We carried out the following QA tests using the EPID:i) Dynamic MLC (DMLC) dosimetry test ii) DMLC positional accuracy test (Picket Fence test) for fixed and rotating gantry modes iii) DMLC positional accuracy test during rotation with intentional errors iv) dose rate and gantry speed tests during RapidArc delivery and v) DMLC leaf speed test during RapidArc delivery. All the tests were analysed with Microsoft Excel application. Deviations of the EPID pixel values from known regions of interest during the various tests with respect to open fields were estimated for accuracy assessment. DMLC dosimetry tests showed a maximum deviation of 0.16 % with respect to reference condition at 0º gantry. The maximum positional accuracy of DMLC was found to be 0.28 mm for fixed gantry and 0.26 mm for rotating gantry. For varying dose rate and gantry speed, the average of the absolute value of all deviations Diff(x) was0.43. The MLC leaf speed variation during RapidArc resulted in the average of the absolute value of all Diff(x) of 0.20. Similar results have been obtained with a film based QA tests. The time taken in performing the above tests with EPID is far less as compared to the conventional methods. EPID based QA tests are reliable and quick. We believe that protocols developed for performing QA tests with EPID can replace the conventional methods of QA. EPID based QA will result in considerable time saving and thus helpful in increasing the patient throughput in a clinic. Also, the quicker and automated QA procedure based on EPID lends itself to better compliance and hence better treatment quality.


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