myQA Machines - ISOTrack Plugin

General Description

The IsoTrack Plugin employs the Winston-Lutz test method to analyze the variation of isocenter positions present in the rotation of the gantry, collimator, and patient couch. Typically, a Winston-Lutz phantom is a small metallic ball (radiation opaque) that represents the planned target, which is surrounded by transparent plastic material. During measurements, the ball is aligned at the isocenter. Consequently, Winston-Lutz images show a radiopaque object at the isocenter surrounded by a radiation field, usually collimated by the treatment device used in the SRS / SBRT treatment (cone or MLC).

The IsoTrack Plugin analyses these images and the center of the object is compared to the center of the
radiation field, and the difference is the radiation field variance at that gantry, collimator, and couch angle. The AAPM Task Group Report 142 (2009) recommends that up to ± 1 mm deviation between the radiation and mechanical isocenter is acceptable for SRS / SBRT treatments.

Image acquisition

In general, any phantom designed for the Winston-Lutz test can be used, e.g., QUASAR™ IsoCenter Cube
from Modus Medical Devices Inc., Winston Lutz Phantom from Standard Imaging Inc. An example using the IBA cylindrical phantom is described in Section 8.4.1.

If multiple images are processed, their SIDs (Source to Image Distance) and DPIs (Dots per inch) must be the same.

See the table below for the Winston-Lutz image acquisition techniques and for which types of the
measurements they are suitable.

Note: The field size depends on the phantom used for the test. It should be big enough to contain the                      Winston-Lutz ball. However, it should not be so big that it also covers the irrelevant parts of the                      phantom, e.g., legs of the phantom, or any other object, e.g., the couch, which generates a pattern                  interfering the contour analysis.

Note: IsoTrack Plugin only supports pixel formats, grayscale 8-BBP, 16-BBP, and 32 BBP.

For 3D error analysis, it requires 4 images that fulfills the following requirements:

• Gantry angles are 0°, 90°, 180°, and 270°, respectively
• Collimator angles, SADs, SIDs, and DPIs are equal.
• Couch angles are 0°.

Tracked in myQA Cockpit

If the Show in myQA Cockpit option in myQA Platform is enabled, these test results are tracked in myQA Cockpit:

• Maximum 2D deviation
• 3D deviation if available

Test Setup: Create a Winston-Lutz Test Protocol

• In Test setup, click Edit in the ribbon.
• In the Machines & Templates panel, select the desired machine and then in the Protocol Tree panel, select or create a task.
• Click New in the ribbon. In the New Test Definition dialog, enter / select the following information, and then click OK.
  
  • Name: name of the test
  • Description: (optional) enter any description
  • Category: Mechanical (for information display only)
  • Result Type: Mechanical: Isocenter Check
  • Implementation: Winston Lutz (automatically loaded after selecting a Result Type)
  • Warning tolerance: value of warning tolerance
  • Error tolerance: value of error tolerance
• Click Set Active in the ribbon if the protocol should be run next.
• Click Save in the ribbonNote: You can create a test template first, and then copy the template into the machine protocol.

Test Run: IsoTrack Test Run Workflow

Note: Detailed information on IsoTrack Test Run workspace, see Section 8.1, myQA Machines Reference                   Guide.

• Click the Test Run button (1) and then the task (e.g., 2) to be excecuted.By default, the instructions is displayed. It can be hidden by clicking the Collaspe button (3) when
  starting an active task in the Test run workspace
• Click Load Images to open the dropdown list and select DICOM or TIFF according to the file format (see Section 8.1.1, myQA Machines Reference Guide).For 3D analysis, 4 images with 0°, 90°, 180°, and 270º, respectively must be loaded (see Section 8.1 for the complete requirements).
  For TIFF images, enter the SID value in the pop-up dialog during file-loading and define the gantry angles in the table of the image parameters manually after file-loading.
• Select the boxes of the images (1) to be tested.
• If desired, create an ROI for the analysis (Section 8.1.2, myQA Machines Reference Guide) (2) or adjust the contour threshold (3)
• Click the Calculate results button (4) to start the calculation.
• Inspect the results. To see the detailed results, click the Show detailed results (5) button (Section 8.1.5, myQA Machines Reference Guide). If necessary, change the parameters and re-do the calculation.
• Click Finish (6); the window display will change from testing to settings and results.

Example – Winston-Lutz Test on EPID Images

Phantom for Winston-Lutz Test

In this example, the images are taken with IBA Cylindrical Phantom equipped with a 6 mm steel ball insert. The material for the phantom is PMMA. Outer diameter: 13 cm, inner diameter: 2 cm, length: 13 cm. Engraving on surface indicates the center of the steel ball.

Due to the structure of the IBA Cylindrical Phantom, it is only suitable to be used with small field sizes, e.g., 2 cm × 2 cm or 3 cm × 3 cm.

IBA Cylindrical Phantom. 1: Steel ball with engraved surface (left) and IBA Cylindrical Phantom during
measurements (right)

Obtaining Winston-Lutz Images with EPID

The IBA cylindrical phantom is placed on the couch and aligned at the isocenter with the laser alignment
system of the treatment room. In this example, 4 EPID images with 6MV, 2 MU, and gantry angles at 0°, 90°, 180°, 270°, are captured with EPID with single-exposure mode. The images were saved as DICOM format.

Execute Winston-Lutz Image Analysis

• Create a protocol containing a Winston-Lutz test and set it as an active protocol in Test Setup.
• In Test Run, select the machine under Machines (1), and the activated Winston-Lutz test under Active Tasks (2).
• Click Load images (3) and then select DICOM since the data is DICOM file format in this example for
  detailed information of DICOM image import).

In the Import images dialog:
         Select File based.
         Click the Folder browse button (…), and then select the DICOM folder click OK
         Select the images to be imported by clicking the selection boxes and then click OK

• In Test Run, select the images to be analyzed (4) by clicking the check boxes in the first column.
• Make sure the gantry angles in the Gantry column (5) fulfilling the requirement if 3D analysis is required.
• Click Calculale resultsbutton (6).
• Inspect the results listed in the Dv (deviation) and Status (Passed / Warning / Failed / Undefined)
  columns, the Results table, and graphical illustrations on the right part of the window.
• Click Finish and the results will be displayed.