(Monday, 7/13/2020) 3:30 PM - 4:30 PM [Eastern Time (GMT-4)]
Room: Track 5
Linac QA with detector arrays – Is the water tank dead
Presented by: Peter Balter, Ph.D.
Periodic quality assurance (QA) of a clinical linear accelerator (linac) is important to ensure that the deviations from baseline values acquired during commissioning, or from the treatment planning system (TPS) should not exceed the pre-defined tolerance levels. Beam output, energy and profile consistency checks are recommended for monthly and annually QA. Beam steering is typically done with a 3D water scanning system (3DWS) during initial acceptance testing, after major repairs and during annual QA.
Several studies have demonstrated that an ionization chamber array (ICA) can be used to measure changes in the energy of photon and electron beams with a higher sensitivity than can be achieved with percentage depth dose measurements in water tank.
Recent studies demonstrated that a 2D ICA can be used to steer photon and electron beams and achieve a resultant beam symmetry that matches that of a 3DWS, and also demonstrated that ICA can be used for acceptance and clinical verification of the Halcyon-Eclipse TPS static beam profile data without need for a 3DWS.
In the presentations, we will provide deep review of the strategies to achieve the clinical accuracy using ICA for both monthly and annual QA process of linac. These include good array calibrations and the use of proper metrics for symmetry and change in energy.
Learning Objectives:
1. To understand the validation of the array calibration to ensure that it does not introduce systematic errors.
2. To understand beam profile measurement with ICA and compared with in water scanning data, to establish energy monitoring procedures for both photon and electrons.
3. To understand that to achieve accurate results, the ICA should have good calibration, and the correct symmetry and energy metrics should be used.
Your Detector Array Has Arrived
Presented by: Sotiri Stathakis, Ph.D.
Multidimensional arrays have been adopted in radiation therapy clinics by medical physicists for various types of measurements. Common applications of such devices include patient-specific plan quality assurance (QA) measurements, periodic QA measurements for constancy checks of beam flatness and symmetry, as well as dose and beam quality constancy checks. The multidimensional arrays vary in terms of their size, detector types, and usage. The task group 312 is working towards creating a report to provide guidelines to the users on initial and ongoing performance assessments that should be performed to ensure proper functionality. The characteristics of the most common detector arrays will be provided along with their intended use. After reading this report, the user should understand how to assess the detector array’s performance and proceed with its use. Although several different detector systems can be used for patient QA as well as for beam constancy measurements (e.g. Electronic Portal Imaging Devices, film, transmission detectors, etc), the focus is solely on detector arrays that may be placed on top of the treatment couch. The report is not intended to provide a comprehensive review of machine quality assurance procedures or patient-specific IMRT QA.
Learning Objectives:
1.Understand how to assess the system’s performance
2.Testing guidelines for periodic QA of the system
3.Be familiar with the characteristics and use of the currently available systems.
Beam steering and monitoring photon and electron beam energies with ion chamber array Presented by: Song Gao, Ph.D.
Beam steering of a linear accelerator (linac) was traditional performed with a 3D water scanning system (3DWS) during annual quality assurance (QA). Our recent studies demonstrated that a calibrated 2D ion chamber array (ICA) can be used to steer photon and electron beams and achieve a resultant beam symmetry within 1% when re-measured in a 3D water scanning system (3DWS). Use of the ICA greatly speeds up the steering process because of its real-time feedback and reduces effort by eliminating the need to setup a 3DWS.
Photon and electron beam energy checks and profile consistency checks are recommended on a monthly basis. Several studies have demonstrated that an ICA can be used to measure changes in the energy of photon and electron beams with a higher sensitivity than can be achieved with percentage depth dose measurements in water scanning systems (3D or 1D).
This lecture will provide an overview of reliable and standard procedures of 2D ICA based beam steering with the real-time feedback and reduces effort by eliminating the need to setup a 3DWS; Use of the ICA greatly speeds up the monthly QA process for the energy monitoring and profile consistency checks of both photon and electron beams.
Learning Objectives:
1.To understand the use of ICAs to steer photon and electron beams.
2. To understand profile measurement with ICAs and how they relate to photon energy monitoring.
3.To understand electron beam energy monitoring using ICAs when combined with wedged shaped attenuators.
Funding Support, Disclosures, and Conflict of Interest: Dr. Balter has sponsored research agreements with Varian Medical Systems and Raysearch. These agreements are not related to any of the work presented in this session.
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