Implementation of Quality Assurance Protocols for an Advanced Linear Accelerator with Volumetric Modulated Arc Therapy Features Using Electronic Portal Imaging Device (EPID)
Keywords:Radiotherapy, Quality Assurance (QA), Volumetric Modulated Arc Therapy (VMAT), Dynamic Multileaf Collimator (DMLC), Electronic Portal Imaging Device (EPID).
To use Electronic Portal Imaging Device (EPID), an integral feature of a TrueBeam linear accelerator (linac)
system, for implementing dosimetry based comprehensive Quality Assurance (QA) protocol needed for
Volumetric Modulated Arc Therapy (VMAT) modality. Varian makes TrueBeam 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 (millenium MLC) and an EPID (aS1000)
having megavoltage 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 30frames
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 test 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) was 0.43. 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
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