This article covers the following topics:

• Introduction 
• Measurement Summary 
• Instructions 
• Using Film for Collimator Starshots
• Available Tests 
• Specifications to Create a Plan File 
• DICOM Requirements to Differentiate Star Shots
• Detailed Discussion of Measurements 
• Example File 

Introduction 

Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that the collimator, couch, and the gantry rotation isocenters should be checked against baseline with ±1 mm tolerance during annual Quality Assurance (QA) in the Mechanical subsection of Table III. The Linac isocenter diameter is evaluated with "star-shot" pattern formed by radiation fields measured on an Electronic Portal Imaging Device (EPID) the collimator is rotated around the isocenter. Below is an example set of a star shot images: 

The software can combine the images into a single image:

Ideally the beams would all intersect in a single point but in practice the beams intersect geometrically in a small volume that looks more like a sphere or ellipsoid. The center of this sphere or ellipsoid is the radiation isocenter. Below is an example plot: 

A Hough transform on the image is used to extract the beams and angles. The three lines that would create largest circle are reported as the minimum tangent circle. The other lines will also go through that circle. The radius of the minimum tangent circle is the radius of the smallest circle that all the beam centers will touch or intersect.

The radiation isocenter may not coincide with the mechanical isocenter. Different beam energies may yield different isocenters. 

Measurement Summary 

The analysis performs the following measurements:

• Collimator rotation isocenter (starshot test)
• Minimum tangent circle plot (collimator starshot)
• Collimator Starshot Angle Detail Plot
• Collimator Starshot Angles

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 

Collimator Starshot files must be a DICOM file and contain the "starshot" within the file name or Starshot must be selected in the Type dropdown. The recommended beam width is 4 mm or larger as determined by the resolution in the file, however the analysis will attempt to process beams as small as 1 mm.

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.

Using Film for Collimator Starshots

The collimator starshot test may also be used for starshots that were acquired on film. Add coll somewhere in the filename to assign the results to the collimator starshot test.

Available Tests 

Below is a list of available tests.

Note: the Collimator Starshot Angles test requires the Beam Limiting Device Angle Tag (300A,0120) be present and contain a value.

Specifications to Create a Plan File 

When creating a plan file to generate the collimator star shot, the beam width, should be 4 mm or larger as determined by the resolution in the file and each beam must be distinct.  

DICOM Requirements to Differentiate Star Shots

The software distinguishes between the table and collimator star shots by examining the DICOM tags:

• BeamLimitingDeviceAngle
• GantryAngle
• PatientSupportAngle

If the BeamLimitingDeviceAngle (collimator angle) is the same for all the images AND the Gantry angles are all the same AND the PatientSupportAngle (couch angle) is NOT all the same then it's a Table. If it does not meet this test then it is a collimator.

Detailed Discussion of Measurements 

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

Example File 

Below is an example collimator star shot file set for use in testing: