Topics covered in this article include:
- Measurement Summary
- Available Tests
- Image Acquisition Suggestions
- Detailed Discussion of Measurements
- Example File
The flatness and symmetry from EPID routine segments the field from the background and finds the edges of the field at the point of maximum gradient along a given edge. Note that the use of FWHM as the field edge has been discontinued with the introduction of peaked FFF beams.
Currently the field sizes supported are 5, 10, 15, 20, and 25 cm at isocenter square fields.
Flatness measurements are made from the mean value of 4mm square ROIs centered on the cardinal axes at 80% of the detected field size compared against the mean value of a 4mm square ROI at the center of the field.
Symmetry is measured as a percentage difference between the left and right and top and bottom ROIs.
Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that the beam symmetry and flatness should be checked during monthly Quality Assurance (QA) in the Mechanical subsection of Table II.
The Total QA analysis software provides the following results:
The measurements are completely automated, requiring the user only to drag and drop the image set into the web-based software interface. A detailed report is created.
Beam flatness and symmetry images must be DICOM files and contain flat somewhere in the file name. 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 "flat" (case insensitive) is found the image set will be processed as a beam symmetry and flatness test series. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.
The recommended Total QA tests are the Annual Linac QA [TG-142 Table II]: flatness and symmetry tests.
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.
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.
Monthly Linac QA [TG-142 Table II]
Flatness and Symmetry Tests
Image Acquisition Suggestions
Below are the recommended steps to acquire the radiation/light field coincidence images:
- Set the jaws or MLC to the desired field size. The field size should be in increments of 5 cm.
- Acquire the first image.
- Repeat steps 2 and 3 as needed.
Detailed Discussion of Measurements
Below is an example of the report generated from the asymmetric field test:
Below are example files for a asymmetric field test series for use in testing: