Topics covered in this article include:
- Measurement Summary
- Available Tests
- Specifications to Create a Plan File
- QA Settings
- Detailed Discussion of Measurements
- Example Files
The Hancock test was developed to test the accuracy of MLC leaf position on Elekta Linacs using EPID images. The EPID imaging system on the Elekta Linacs is fixed at a SID of 1600mm. Consequently, capturing all the leaves on a single EPID image is impossible. Historically, Elekta MLCs have required a minimum 10mm gap between the leaf banks. This requirement makes forming a traditional picket fence test pattern challenging.
The Hancock test overcomes these challenges by using two images. The first image is taken with the collimator at zero degrees rotation and covers leaf pairs 1 to 41. The second image is acquired with the collimator rotated 180 degrees and covers leaf pairs 40 through 80. The overlap at leaf pairs 40 and 41 between the two images is used to fix the position of the isocenter. All the subsequent leaf position measurements reference this isocenter position.
The Hancock test pattern has three strips on each image. The central strip is centered on the isocenter. The left and right strips are equidistant from the center strip at the pattern spacing distance.
The analysis displays the following test results:
- Leaf Position Deviation Plot (Uncorrected)
- Leaf Position Deviation Distribution (Uncorrected)
- Leaf Edge Deviation (Uncorrected)
- Leaf Position Deviation Plot (Rotation Corrected)
- Leaf Position Deviation Distribution (Rotation Corrected)
- Leaf Edge Deviation (Rotation Corrected)
- Deviation Summary (uncorrected)
- Deviation Summary (Rotation Corrected)
- Deviation Summary By Position (uncorrected)
- Deviation Summary By Position (Rotation Corrected)
- Leaf Edge Position w.r.t. Isocenter
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.
The test requires two DICOM or .HIS images. It is necessary to use the file naming convention or to select "Hancock Leaf Test" in the Type field in the upload widget to identify the images. If using the filename to identify the image, use "hancock" 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.
Specifications to Create a Plan File
Please use your manufacturers recommended plan file where available.
If any of the tests associated with the Hancock test are included in the schedule’s template then an the "Coordinate system" and "Angle Format in Filename" options will appear under the "Image Processing settings" section in the QA settings for the schedule:
The spacing between the strip centers in mm. The default spacing is 115mm. For example, if the strip spacing is 115mm, the left strip will be nominally centered at -115mm and the right strip at +115mm.
The nominal width of each strip in mm. The default width is 10mm. The pattern spacing and strip width combination define the nominal position of each strip edge. For example, if the pattern spacing is 115mm and the strip width is 10mm, then the left edge of the left strip is nominally at -120mm. The right edge of the left strip would then be at -110mm.
The nominal pixel size of the imager projected back to the isocenter. The physical size of the imager’s pixels is 0.4mm. The SID is nominally 1600mm. The nominal pixel size is then 0.4mm/pixelx1000mm/1600mm = 0.25mm. This pixel size may be adjusted to improve the test accuracy if the user has calibrated it.
The analysis will take Hancock images in both HIS or DICOM format. Because the images are difficult to distinguish, the two images must be specified by name in the settings. If the images uploaded do not match the names specified, the analysis will not work.
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.
Leaf Edge Deviation (Uncorrected and Rotation Corrected)
This is the deviation in mm of the detected leaf edge from the expected position based on the pattern spacing and strip width parameters. The spacing between the strip centers in mm. The default pattern spacing is 115mm. The default strip width is 10mm. The pattern spacing and strip width combination define the nominal position of each strip edge. For example, if the pattern spacing is 115mm and the strip width is 10mm, then the left edge of the left strip is nominally at -120mm. The right edge of the left strip would then be at -110mm.
The rotation-corrected results attempt to correct for detected minor collimator rotation (from the 0 and 180 positions) in the uncorrected results.
Leaf Position Deviation Plot (Uncorrected and Rotation Corrected)
These are plots of the deviation results above. Hovering over the individual readings will give details on the measurement and any applied tolerances.
Leaf Position Deviation Distribution (Uncorrected and Rotation Corrected)
This plot provides a visualization of the distribution of the deviations of each strip edge with a box and whisker plot. Hovering the mouse over each box will display statistics.
Deviation Summary (Uncorrected and Rotation Corrected)
The deviation summary provides statistics over the entire pattern.
Deviation Summary by Position (Uncorrected and Rotation Corrected
The deviation summary by position provides summary statistics by strip edge.
Leaf Edge Position w.r.t. Isocenter
The Leaf Edge Position w.r.t. Isocenter table provides the raw measured positions of the strips with respect to isocenter.
Below are example picket fence DICOM files for use in testing: