Volumetric Modulated Arc Therapy (VMAT) is a treatment modality where the irradiation field is rotated around the patient. During the irradiation, the Gantry speed, Leaf speed (MLC) and dose rate are continuously varied to adapt to the shape of the tumor and minimize the effect on the surrounding healthy tissues.
The VMAT QA analysis is based on the article by Ling et. al.[1]. An imaging device (EPID or film) rotates with the gantry. Different parts of the imaging device are irradiated with the same total number of MUs but under varying irradiation conditions:
- Variation of the gantry speed and the dose rate (corresponds to Test 2[1])
- Variation of the leaf speed and the dose rate (corresponds to Test 3[1])
Two images are acquired, one with a static open field and another with the VMAT plan providing a variation of dose rates. Both fields have the same dimensions. Analysis execution steps: - Normalize VMAT field to open field to account for flatness and symmetry of the beam.
- Define ROIs (the ROIs are placed in the middle of the irradiation zones) and calculate mean and standard deviation in each ROI.
- Optional: normalize the results to the overall mean.
Data acquisition and analysis
The figure below (left) shows the principle of data acquisition with an EPID rotating together with the gantry around the isocenter. The length scales of the images are given in mm and are projected to isocenter.
Two images are acquired, one with a static open field and another with the VMAT plan providing a variation of dose rates (Figure below, middle, and right). Both fields must have the same dimensions.
Left: principle of data acquisition, middle: open field, right: VMAT field.
Analysis steps:
- Read in open and VMAT field images.
- Extract information from file headers (if available):
- Pixel spacing
- SID and SAD
- Normalize VMAT field to open field to account for flatness and symmetry of the beam
- Detect center of field
Via image processing tools (same as for the EPID Plugin). This method is based on the detection of a rectangle in the open field images. See examples shown in the figure below.
Positive and negative contrast is treated automatically and the fields can also exceed the image in one direction. - Place ROIs and calculate mean and standard deviation in each ROI.
- Optional: normalize the results to the overall mean.
Examples of open fields and detected centers
Profiles of the open and VMAT fields are shown in the left figure below. Furthermore, vertical dashed lines mark the found field center position (red) and ROI positions (green). The right figure below shows the VMAT field after normalization to the open field. In this example, six different dose rates (from 125 to 440 MU/min) were irradiated. The ROIs are placed in the middle of the irradiation zones.Left: horizontal profiles of open and VMAT fields. Right: Normalized VMAT field with ROIs and applied MU/ min.
Workflow for VMAT QA Tests
Note: For detailed information on the myQA - Imaging QA – TestSetup/TestRun VMAT window, see Section 7.1, myQA Machines Reference Guide.
Setting up Baselines in Test Setup
Pre-conditions
The phantom is aligned to the center of the image. The EPID open field and MLC field images are measured. The images can be in DICOM (RTImage), .tiff, or .opg file format.
Note: The baselines and settings defined in Test Setup will be used for every execution in Test Run. However, they cannot be changed in Test Run.
Create a test for VMAT QA
In Test Setup,
- Click Edit in the Test Setup ribbon to enter the Edit mode.
- Select the desired machine in the Machines & Templates panel, and then in the Protocol Tree panel select a task in which the test is located.
- Click New in the ribbon.
- In the New Test Definition dialog, enter the required name of the test and, optionally, a description.
New Test Definition dialog
- Enter the name of the test and, optionally, a description.
- Select Category: MLC (for information display only).
- Select Result Type: MLC: VMAT / Dynamic MLC. Implementation is automatically selected the same.
- Click OK. The test is listed in the Protocol Tree and the Test Definition panel is displayed.
Test Setup workspace – Test definition panel
Define the test baseline
- In the Test Definition panel, click Open Module. The myQA – Imaging QA – TestSetup VMAT window
opens.MyQA - Imaging QA - TestSetup window
- In the Equipment setup section (1):
- Machine: Displays the machine selected in the beginning of this procedure.
- SID and SAD: Defult for both is 1000 mm. SID will be updated upon importing images.
- ROI model: Click the dropdown box and select a desired ROI set in the predefined ROI set list.
- Open / VMAT field: In this combo box, load the open field image file.
- Select the Normalization to 1.0 check box if desired.
- To view the imported images, select the corresponding tab on the top of the Image view (2). The display of an image can be adjusted with the tools in the Tools panel.
In the Profile chart section (5), the inline / crossline profiles of the open and VMAT fields are displayed. - The ratio of the two images is automatically calculated and displayed in the Ratio tab together with the ROIs. The center of the image is automatically identified. However, it can be relocated by drag-and-drop. The predefined ROIs will be automatically shifted. The ROI values can be viewed and edited in the Region of Interest panel.
- Once the isocenter and ROIs are properly defined, click the Process button in the Baseline values
section (4). The expected values are calculated and displayed along with the predefined the warning and fail tolerances.The ROIs mean values and standard deviations are shown by clicking the Show detailed results button.
- Inspect the tolerances. Modify them if necessary.
Note: The predefined expected values and tolerances are example values coming from test images, protocols or best practice and users should inspect these values. - Click OK to close the window. The reference images and the used configurations are saved and listed in the Test definition panel.
- By clicking the Reset configuration button, the test setup prcedure will be started from the beginning.
The configuration files are listed: - Click Set Active in the ribbon if the protocol is not yet set as an active protocol.
- Click Save in the ribbon.
Note: In this step, you can also create a test template first, and then copy the template into the machine protocol.
Executing VMAT Tests in Test Run
Please note that a previously defined test baseline will be used during execution, if such a baseline was previously defined in Test Setup.
Pre-conditions
The protocol is set to active in Test Setup.
Procedure
- In the Test Run workspace, select the corresponding protocol and task in the Agenda panel.
- In the content area, select the test and then click Start.
Test Run workspace – a VMAT task is selected (before tests are executed)
The myQA – Imaging QA – TestRun VMAT window opens. The equipment setup parameters,
normalization, expected values, and tolerances setup in the Test setup workspace are displayed. They cannot be modified in the Test run workspace. SID and SAD can be adapted to the actual setup. - In the Open / VMAT field combo box, load the open field image file.
- Check that the isocenter and ROIs are properly positioned. Click Process in the Baseline section. The
Actual values are calculated and compared to the specified tolerances. The result of the test (pass, fail, warning) will be set accordingly.Each ROI mean value and standard deviation are shown by clicking the Show detailed results button.
- Click Ok to close the myQA – Imagine QA – TestRun EPID window.
Test Run workspace – an EPID task is selected (after tests are executed)
- Click Finish in the ribbon and then finish in the Finish Task dialog. Afterwards, the test results will
appear in myQA Cockpit and a report can be created in the Test Repository page.Test Repository page with executed test list
References
[1] C. C. Ling, "COMMISSIONING AND QUALITY ASSURANCE OF RAPIDARC RADIOTHERAPY," Int. J. Radiation Oncology Biol. Phys., pp. 575-581, 2008.