Topics covered in this article include:

• Introduction 
• Measurement Summary 
• Instructions 
• QA Settings
  • Coordinate System
  • Angle Information in Filenames or Series Descriptions
• Available Tests
• Image Acquisition Suggestions
• Detailed Discussion of Measurements
• Example Files

Introduction 

Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that the coincidence of radiation and mechanical isocenter should be checked against baseline during annual Quality Assurance (QA) in the Mechanical subsection of Table III. The coincidence of radiation and mechanical isocenters is evaluated by capturing the image of a BB phantom aligned to the lasers in a collimated radiation field measured on an Electronic Portal Imaging Device (EPID) at various gantry and couch angles. The center of the collimated field and the center of the BB are measured and compared. Ideally the measurement would be the same. Below is an example of a single image in a Winston Lutz series: 

The center of the collimated field and the center of the BB are measured and compared. Ideally the measurement would be the same. In the examples below Image #1 shows the centers that are almost identical while in Image #2 you can see a slight offset (note the positions of the red and blue + where the red + is the center of the BB and the blue + is the center of the field):

The Isocenter distance bar chart shows the offsets in each direction for each of the images in the set.

Displacements are calculated using either the gantry and table rotation tags in the DICOM header or encoded into the filename. 

The general reference for the Winston Lutz calculations is:

Low DA, Li Z, Drzymala RE. Minimization of target positioning error in accelerator-based radiosurgery. Med Phys. 1995;22(4):443‐448

Measurement Summary 

The analysis provides the following results:

• Coincidence of radiation and mechanical isocenter
• Coincidence of radiation and mechanical isocenter (image coordinates)
• Input images for Winston Lutz test 

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.

Instructions 

Winston Lutz images must be DICOM files. A minimum of three images are required. The table and gantry rotation must be included in the DICOM tags or encoded into the filename. The collimator rotation is optional.

In some cases, it may be necessary to assist the automatic image recognition with manual naming conventions. If the images are not recognized automatically please try adding "winstonlutz" 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 "winstonlutz" (case insensitive) is found the image set will be processed as a Winston Lutz series. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.

Gantry, table (couch), and collimator angles are typically obtained from the appropriate DICOM tags. In the event that the tags are missing or need to be overridden, the angles can be specified in the file name or series description tag. If the angles are specified in the file name this will take precedence over the values contained in the DICOM tags. See the QA Settings section for specifics.

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.

QA Settings

If any of the tests associated with the Winston Lutz 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:

Coordinate System

The three coordinate systems currently supported for visual display are Varian IEC, Varian, and IEC61217. The default is Varian IEC. For the purposes of the Winston Lutz analysis, the two main characteristics of the coordinate systems that are used in the calculation are the gantry angle and the table (couch) angle:

The collimator angle does not have a direct effect on the calculation. However, if the ‘cone’ is being formed by the jaws or MLC’s then taking two shots from the same gantry and table angle with opposite collimator angles can mitigate the effects of any jaw or leaf bank asymmetry.

 Notes:

• Where the user is designating the angles in the file name this is the coordinate system for the input angles and for the report output.
• If angles are pulled from the DICOM tags they are always IEC61217 but the results will be displayed using the coordinate system chosen.

Angle Information in Filenames or Series Descriptions

Many image sets exported from Elekta/ MosaiQ do not have the DICOM tags for the angles included. The work around for situations such as this, is to designate the angles in the filenames or series descriptions of the set. 

If angles are in more than one place the first one found with valid information will be used. The angle information is checked in the following order:

1. Filename encoding
2. Angles in DICOM file tags
3. Encoding in Series Description tag

To specify how angles are encoded, select either GC or GTC encoding in the "Angle Format in Filename" option in the schedule's QA settings.

The default is GC. If the naming convention is not used, the angles are pulled from the tags and this setting has no effect. Encoding the angles in the filenames takes precedence over the angles in the DICOM tags.

GC Encoding

GC stands for gantry and couch respectively. To specify the gantry and couch angles, add G, the gantry angle, C, and the couch angle value anywhere in the file name or series description. Adding the angles is not case sensitive but spaces or other characters are not allowed between the two angles. So, for example, you can have 'G0C90' or 'C145G180' but 'G0 C90' or 'C0_G270' will not work. 

GTC Encoding

GTC stands for gantry, table, and collimator respectively. To specify the angles, add G, the gantry angle value, T, the table angle value, C, and the collimator angle anywhere in the file name or series description. Adding the angles is not case sensitive but spaces or other characters are not allowed between the angles. The angles must be listed in the order gantry, table, and collimator. For example, G090T0C0 is legal while G090C0T0 is not.

Available Tests

Image Acquisition Suggestions

Below are the recommended steps to acquire the images:

1. Include at least three images in the set at various gantry angles. Optionally a 2D version can be done with two images but no dX,dY, dZ or 3D Magnitude results will be calculated.
2. The gantry should have at least one image in the set where the angle is not 0 or 180 so that there is motion in each axis.
3. The radiation field should be collimated using the MLC or a collimator.
4. The BB should not touch the edge of the radiation field.
5. The BB should be at the center of the image.

Detailed Discussion of Measurements

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

Example Files

Below are example files for a Winston Lutz test series for use in testing: