myQA SRS - Setting up the Device for Measurement

Preparation of TPS Plans for Verification in myQA Patients

A CT scan of the device setup, myQA SRS in a myQA Phantom, is required for calculating the dose TPS plan for the verification in myQA Patients. The dose plan to be verified is calculated on the CT images in the TPS. 

Note that origin of the measurement system should coincide with the origin of the CT data set (this is also the DICOM origin). In the TPS, the isocenter can be positioned precisely at the DICOM origin.

Acquire CT Image of the myQA SRS

Place the detector in the phantom (device-phantom assembly) and position it in the CT scanner using the alignment lasers:

• Position the device-phantom assembly in the center of the beam and align the effective point of measurement as indicated by the isocenter markings.
• The device-phantom assembly should be positioned so the central point of the measurement is at the CT origin.
• Point zero of the measurement system always coincides with point zero of the CT data set (the DICOM origin).
• In the TPS, the isocenter can be positioned precisely at the DICOM origin.
  
  1: Isocenter markings; 2: Offset markings (only myQA SRS Phantom^2)
• Scan the entire phantom. A CT slice thickness of 2.5 mm or smaller should be used with standard departmental scanning protocol for soft tissue (pelvis, abdomen) imaging.

Cross-Calibration with TPS / Chambers

• Import the CT scan into the TPS. 

• Create an external contour and perform mass density overrides of the following:

  • the phantom body (1) 

    

  • the device housing (2)
  • the detector area (3)

  Note: If required, add a support (couch) to the CT scan. 
  Example of possible mass density overrides:

  • myQA SRS Detector (ABS): 1.040 g/cm3
  • myQA SRS Phantom2 (RW3): 1.045 g/cm3
  • Detector area (High Z Material): N/A

  See Section 2.1 in myQA Patients Instructions for Use for more information on densities. 

  Note: Homogeneous density override is possible with RW3 density for the whole assembly.

• Verify the structures against the dimensions of the detector-phantom assembly (total height of the phantom and distance to isocenter).

  IMPORTANT NOTICE: PAY ATTENTION TO PHANTOM TYPE WHEN VERIFYING DIMENSIONS 
  Pay attention to phantom type when verifying dimensions. The dimensions of the different phantoms are available in Section 7.2.

• On the image, set a point on the plane that corresponds to the LINAC isocenter alignment marks on phantom surface and is shown on both coronal and transverse views. Label it.
• In the TPS, create an open reference field (it is recommended to use the same algorithm as the patient plan) at 0° and have the system calculate the dose using a grid size of 2.5 mm or smaller.
• Find the isocenter plane and export the dose. The isocenter is set up geometrically based on the depth and central chamber position in the TPS image.

For cross-calibration with a calibration chamber:

• Align the phantom on the couch.
• Place the chamber in the chamber insert of the phantom.
• Attach the chamber to an electrometer, measure the background, and pre-irradiate the chamber.
• Irradiate an open field (dimensions and MU according to open field in TPS) and write down the chamber reading.
• Apply all necessary correction factors to the chamber reading 

• Repeat the cross-calibration 3 to 4 times in order to ensure that the reading is stable. 

  Note: If not done by the electrometer, apply the KTP correction and calculate the dose. Verify the calculated                  measured dose matches the dose calculated by the TPS.

  Please note, due to artifacts on the detector layer in the coronal view, you may need to adjust the window and level of the CT to filter out the artifact.
  
  Illustration of Correlation of the alignment markers on the myQA SRS Phantom2 (left) and the effective point of measurement (isocenter) on the images of myQA SRS detector in the TPS (right)

• Create an open reference field (it is recommended to use the same algorithm as the patient plan) at 0° and have the system calculate the dose using a grid size of 2.5 mm or lower.
• Find the isocenter plane and export this dose plane that will be used for the verification in myQA Patients. The isocenter is set up geometrically based on the depth and central pixel position in the TPS image. 

• For cross checking, export the isocenter dose as the TPS reference dose. Insert a chamber into the phantom and measure the dose in the 10 cm × 10 cm field. Compare the TPS reference dose with the measured reference dose. 

  The procedure for performing detector calibrations are detailed in Section 4.3.2 in myQA Platform Instructions for Use.