Topics covered in this article include:

• Introduction
• Measurement Summary
• Instructions
• Available Tests
• Specifications to Create a Plan File
• Detailed Discussion of Measurements
• Example Files

Introduction

Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends verifying the positional accuracy of the Multileaf Collimator (MLC) for the four cardinal gantry angles in Table V. Multileaf collimation. The picket fence pattern is formed by radiation fields measured on an Electronic Portal Imaging Device (EPID) as all leaves are moved across the field of view at the same speed with several stops at measured intervals. Below is an example of a static picket fence test image:

Note: if the pattern is generated using 1.1 plan files then the standard picket fence tests should be used rather than the Ethos picket fence tests.

Measurement Summary

The analysis displays the following test results:

• Leaf position accuracy (uncorrected)
• Leaf position accuracy (rotation corrected)
• FWHM per leaf pair
• Max /Min FWHM per leaf pair
• Leaf Deviation Stats by Leaf Pair (uncorrected)
• Leaf Deviation Stats by Leaf Pair (rotation corrected)
• Estimated Rotation
• FWHM Summary
• Leaf Deviation Summary (rotation corrected)
• Leaf Deviation Summary (uncorrected)

The measurements are completely automated, requiring the user only to drag and drop the file into the web-based software interface. A detailed report is created.

Instructions

The test requires a DICOM image. It is necessary to use the file naming convention or to select "Ethos Picket Fence" in the Type field in the upload widget to identify the phantom. If using the filename to identify the image, use "ethospf" somewhere in the file name. Below is an example of selecting the image type in the upload widget:

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.

Available Tests

Specifications to Create a Plan File

Please use your manufacturers recommended plan file where available. If using an Elekta, please use the appropriate plan in the Example Plan File Downloads section.

Detailed Discussion of Measurements

The tests include results with and without rotation correction. Rotation correction is included because modern MLCs can position the leaves very accurately, however, the relative effect of any error in collimator rotation is quite pronounced. The results with rotation correction show the effects of leaf position errors with the rotational component removed.

Below is an example of the test output generated on a report:

Example Files

Below are example picket fence DICOM files for use in testing: