Topics covered in this article include:

• Introduction
• Measurement Summary
• Instructions
• Available Tests
• Naming Convention
• Image Acquisition Suggestions
• Detailed Discussion of Measurements
• Sample Files

Introduction

VMAT tests 2 and 3 are tests of the VMAT (Volume Modulated Arc Therapy) system’s ability to deposit a constant dose while varying parameters such as dose rate, rotational velocity, and leaf speed.

They are based on the Ling et al. paper (Commissioning and Quality Assurance of RapidArc Radiotherapy Delivery System, Ling, C. Clifton et al.,International Journal of Radiation Oncology • Biology • Physics , Volume 72 , Issue 2 , 575 - 581)

• VMAT Test 2 demonstrates the ability to vary dose rate and gantry speed during RapidArc delivery.
• VMAT Test 3 demonstrates the ability to accurately vary leaf speed during RapidArc delivery.

For both test 2 and 3, an open field with a uniform dose is delivered to the EPID and captured as an image. Then strips with varying parameters are delivered to the EPID (7 strips for test 2, 4 for test 3). The strip image’s dose should closely match the dose on the open-field image in the central portion of each strip thus demonstrating the system’s ability to successfully deliver the desired dose under the full range of the RapidArc’s capabilities.

Measurement Summary

• VMAT Test 2: Corrected Readings
• VMAT Test 2: Reading Deviations
• VMAT Test 2: VMAT Test 2 Input Images
• VMAT Test 2: Reading Deviation Plot
• VMAT Test 2: VMAT Test 2 JSON Results
• VMAT Test 3: Corrected Readings
• VMAT Test 3: Reading Deviations
• VMAT Test 3: VMAT Test 3 Input Images
• VMAT Test 3: Reading Deviation Plot
• VMAT Test 3: VMAT Test 3 JSON Results

Instructions

Each VMAT test requires two images; one of which is an open field. The open field and strip image must be from the same plan set. If more than two image are uploaded at once, each set must have matching Series Instance UIDs (tag 0020,000E). VMAT test images must be DICOM files. If you have added the appropriate tests to your template, you will be able to select the test from the Type drop-down, as shown below.  

There are limited options for encoding the analysis into the filename, but this is not the the suggested method.  Alternatively, limited capabilities to manually identify planar images using DICOM tag values have been added to the image processing system. This is an extension of the existing naming convention system. The PatientID, StudyID, and SeriesDescription DICOM tags are checked and if the text "vmat2" or "vmat3" (case insensitive) is found the image set will be processed as an asymmetric field test series. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.

When imaging QA tests are added to templates an upload control will appear in the scheduled QA's data entry screen allowing the user to upload images for automated analysis.

To add files to the upload queue simply drag them from a Windows Explorer folder to the drag and drop folder and release them. Alternatively, by clicking on the Add Files button to the lower right of the control a windows file selection dialog will open and files can be selected for upload. Under either method, multiple files may be selected for upload at once. 

Both tests require a 2-file upload of the open field and strip image. 

If the automatically upload checkbox is checked (the default) then file uploading will start immediately as files are added.

If the automatically upload button is turned to off the file upload process must be started manually clicking the Start upload button on the lower right of the control. To clear the upload queue click the Clear button. 

Once file series have been uploaded they will be displayed below the upload control. 

To remove a series from the queue click the Cancel button beside the series. To start processing click the Start Processing button. A description for the image series can be added at this point. Click the Edit button next to the series. Type a description for the series into the text box that appears below Description and either click Save or press the enter key. The description can also be edited after the images have been processed. Descriptions will appear in the report with the analysis of the series. 

While files are being processed users may perform other tasks such as data entry.

Available Tests

The following tables show the tests to select in the template builder corresponding to the supported analyses.

Naming Convention

The files must be identified via the file name or DICOM tags. When using the file name to identify VMAT test files, the text vmat2 and vmat3, respectively, are required as part of the file names. Upper or lower case are acceptable. 

Limited capabilities to manually identify planar images using DICOM tag values have been added to the image processing system. This is an extension of the existing naming convention system. The PatientID, StudyID and SeriesDescription DICOM tags are checked and if the text "vmat2" or "vmat3" (case insensitive) are found the image will be processed as a VMAT Test 2 or VMAT Test 3 image. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.

Image Acquisition Suggestions

Images must be acquired using the appropriate plan files provided by Varian.

Detailed Discussion of Measurements

The analysis is done by obtaining a10 cm H x 0.5cm W ROI in the center of each strip centered around isocenter in the Y direction.

The same ROI's are obtained from the open field image.

The mean values pixel for each ROI on the strip image and the open field image are measured. The  calculated strip mean pixel values are used in the equations below. 

Calculate the Corrected Reading for each strip:

The purpose of tests is to see if the same dose is deposited for each of the strips under the different conditions for each one. The actual measure is the reading deviations. The test was meant to show more details so that the math could be done independently. The raw pixel value readings on the imager can vary quite a bit from month to month. So putting a tolerance on the corrected reading is generally not advised.
 

Calculate the Reading Deviation for each strip from Mcorr

We check if applying the scaling in the rescale slope and intercept tags in the DICOM files will help the deviation results (i.e., reduce the maximum deviation and the mean absolute deviation). If they do then we use the scaled image. If the rescale slope scaling is used it often flips the polarity of the image (i.e., the slope is negative) so to preserve the relationship between the images we multiply the ratio for the deviation by -1 if there is a negative rescale slope. 

Below is an example of the report generated from the tests:

Sample Files
 

Below are example files for use in testing: