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Alternative Strategies for Linac Beam Verification Or Beam Data Collection Without Using a 3D Water Tank

N Knutson1*, J Wolthaus2*, M Barnes3*, (1) Washington University School of Medicine, St Louis, MO, (2) UMC Utrecht, Utrecht, Netherlands, (3) Calvary Mater Hospital Newcastle, Newcastle, Australia


(Wednesday, 7/17/2019) 11:15 AM - 12:15 PM

Room: Stars at Night Ballroom 4

1. Nels Knutson - Linac beam data collection: from 1-D to 3-D

Linear accelerator beam data measurement, verification, and commissioning, is essentially important in radiation therapy. Measurements are most commonly completed at multiple points in a water phantom with the ability to move a detector in the three Cartesian axes. These phantoms are commonly referred to as a 3D water tank. 3D tanks are the gold standard as they reliably acquire these scans. However; 3D tanks are expensive (as much as $100,000) and are often transported via a large moving truck or semi-trailer due to the size of the shipping container required to protect the equipment from damage. These costs impose a hardship on the user and in underdeveloped areas, may further limit the ability to adopt advanced treatment methodologies that can improve patient outcomes. Furthermore, the collection of beam data with a 3D tank can be arduous and time-consuming process in which errors in the collection will propagate across every calculation. If possible, simplifying this process and minimizing the potential for user errors would represent a large step forward in radiation therapy.
In this session we will look at work done to help rethink the collection of radiation therapy beam data measurement. Instead of using a traditional water tank we will discuss an alternative strategy using a simple smaller 1D tank with a scanning arm that moves only in depth and automated couch motions to collect beam data with the same accuracy as a 3D tank but with a significant reduction in complexity, cost, and equipment requirements.

Learning Objectives:
1. Learn how beam data collection can be completed with a 1D tank and automated couch motions.
3. Learn how these data can be used to validate your treatment planning system.
2. Learn how this compares to traditionally collected 3D tank data.

2. Jochem Wolthaus - Acceptance and comissioning of MRI-linacs with and without 3D scanning water tanks

With the clinical introduction of the MRI-linacs from ViewRay and Elekta, there is a need for accurate but fast alignment and dosimetric verification methods applicable in the presence of a magnetic field. Conventional measurement methods have to be adapted to take into account the effect on dose delivery due to the magnetic field. As a consequence, the use of the existing detectors should be reviewed.

3D water tanks give relatively high resolution 1D profiles at cardinal angles. However, setup of a water tank is very time-consuming in the MRI-linac and profile scanning is slow. 2D (or 3D) digital arrays are easier to use but generally have a low resolution, and at the point of measurement the full scatter conditions are not fulfilled. 2D Gafchromic film has a very high spatial resolution as well as high dose sensitivity and, using a proper phantom, does fulfill the full scatter conditions. However, retrieving absolute dose values from film requires careful handling and extensive image processing. Finally, point measurements (0D) don’t have any spatial information but since the physics and response is well understood, the accuracy of such measurements is very high.

The choice for alternative detectors depends on the measurement type, but needs to be at least as good as using a water tank. Tests to assess the geometrical alignment accuracy of the beam or MLC can be performed using film measurements of beam projections from opposing gantry angles or analyzing the projections from different gantry angles of well-defined objects (e.g. ball-bearings). Fast assessment of beam and dosimetric stability or reproducibility can be performed using digital 2D arrays. Measured profiles can be compared to reference profiles acquired during the acceptance and commission. In addition, comparison-models of 2D detector profile to a 3D full scatter water tank profile can be used to simulate water tank profiles.

2D film measurements can be used instead of the water tank for the collection of the beam data. In the commissioning of the clinical prototype MRI-Linac (used in our First-in-man study) beam data of different field sizes were acquired using a stack of films in a solid water phantom. For each field size, the set of 2D dose planes was then converted into a 3D dose cube. Subsequently, the 3D dose cubes were used in the beam modeling process of the TPS.

The presentation will provide an overview of the different measurement equipment and how to use them in the acceptance and commissioning as well as the QA of an MRI-linac. The pros and cons of the available detector types will be discussed.

Learning Objectives:
1. Understand the use of 2D film in geometrical alignment measurements in the MRI-linac
2. Understand the use of 2D film in the collection of beam data in the MRI-linac
3. Understand the use of 2D detectors in the verification of beam stability in the MRI-linac

3. Michael Barnes - Techniques for linac beam verification using EPID

Electronic portal imaging devices (EPIDs) now come standardly on all modern linear accelerators and have now been widely investigated as a high spatial resolution radiotherapy dosimeter. The use of EPID as the dosimeter for linac beam commissioning has the potential to standardise the commissioning process, which may reduce the prevalence of commissioning errors and also to improve the efficiency of the process resulting in new linacs treating patients sooner after install. This talk sets out a pathway towards universal use of EPID for commissioning and provides an overview of the literature on what has been done to date.

Learning Objectives:
1. To understand the advantages and limitations of EPID as a detector for linac beam commissioning.

Funding Support, Disclosures, and Conflict of Interest: Dr Knutson received research funding from the department of Radiation oncology at Washington University in St. Louis and from Varian Medical Systems for this work



Commissioning, Dosimetry, Acceptance Testing


TH- Radiation dose measurement devices: Development (new technology and techniques)

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