Equipment Setup
For the measurement of angular correction factors, we recommend that you use a set of static fields with incident angles between 0° and 360° with an angular resolution of 10° (0° - 80°) and 2° to 3° (80° - 110°, with measurement at 90°), for each photon energy used.
- Field size:
- myQA SRS: 10 cm × 10 cm and 5 cm × 5 cm for LUTs with CAX correction
- MatriXX: 10 cm × 10 cm for LUTs without off-axis correction and 24 cm × 24 cm or LUTs with off-axis correction
- Alignment: Align the detector center IN the phantom to the isocenter of the beam as defined by room lasers, beam crosshair, and radiation field precisely
IMPORTANT NOTICE: ENSURE THE MEASUREMENT DATA IS SYMMETRIC
Before starting LUT measurements, ensure that phantom and the detector are aligned with the beam isocenter axis and beam symmetry is within the specifications. It is also essential to check the symmetry in the gun-target direction before the off-Y-axis measurements. Proper alignment and symmetric response can be confirmed by data comparison of 2 opposite angles, e.g., 330° and 30°.
- For dose rate, avoid low dose rates of less than 50 MU per minute.
- Treatment table attenuation must not be included in the look up table
WARNING: PLAUSIBILITY CHECK OF DIFFERENT SYMMETRIC CHAMBERS
The user should perform a plausibility check of chambers to ensure symmetry.
IMPORTANT NOTICE: EXCLUDE TREATMENT TABLE FROM CALCULATIONS AND MEASUREMENTS
The treatment table is not supposed to be accounted during the LUT creation and measurements between 100° and 260° require repositioning and rotation of the detector.
Note: CREATING CUSTOM LUTS FOR CUSTOM SETUPS
If using a different phantom / custom setup with RW3 plates, or if the beam quality is outside of the range of the BQI in the default LUT tables, it is recommended to create a custom LUT.
Preparation of the Calculation Data
- Use the same CT dataset of the QA phantom as that will be used for the patient plan verification with the MatriXX or myQA SRS detector.
- Use the same structures and density overrides in the phantom dataset as that will be used for patient plan verification.
- Create and calculate same number of beams and field sizes as needed for measurements ( as described in Section 5.1) for each energy for which LUT will be created.
Example of the calculation for the 10 cm × 10 cm beams between 0° and 45° on the miniPhantom R with density overrides. Mean dose to the structure encompassing 9 central ionization chambers was taken as the reference for the LUT creation (blue curve on the DVH).
- It is important to use high resolution dose calculation grid ( 2 mm × 2 mm × 2 mm or below) and if
applicable low MC uncertainty ( below 1% per beam) - Extract calculated doses using points of interest or region of interest dose functions from the TPS for each beam
LUT Creator Workflow
- To open the LUT Creator dialog, click Equipment Setup in the page navigation, then click LUT Creator in the Equipment Setup ribbon.
myQA Platform > Equipment Setup page
- In the LUT Creator dialog, enter the parameters for the header information (1, see more information in Section 4.1, myQA Platform Reference Guide).
- Tick the Off CAX: 10cm X and/or 10cm Y box (2) if the off-CAX correction is desired (see more
information in Section 4.1, myQA Platform Reference Guide). If none of these boxes are ticked, the
central axis correction factors will be calculated.LUT Creator dialog. 1: For Header information, 2: 10 cm off CAX correction, 3: Table for entering gantry angles and measured and calculated doses
- Enter the gantry angle, measured/calculated doses (based on the TPS calculation) in the table (3). The gantry angle can be up to 359° with the highest resolution of 1°. The LUT factor will be calculated and displayed automatically once all three values are entered.
- Click Add new gantry angle to add a new gantry angle
- Click the Delete icon
to delete the row of this gantry angle
- IMPORTANT NOTICE: MEASUREMENTS FOR ONLY GANTRY ANGLE 0° TO 180°
LUT factors are created symmetrically. The measurements only need to be done for
gantry angle 0° to 180°; the factors of 180° to 360° are mirrored those of 0° to 180° - Click the Export LUT button; the Save as … browser will open. Save the LUT in a desired location. Upon exporting the LUT, each chamber will have its own factor per gantry angle.
By default, the filename will be the same as the LUT Name. To use the created LUT, import it to myQA Patients in the Measurement Configuration dialog (see Section 6.1).
A LUT is composed of a set of correction factors matrix per angle. It is exported as a csv file with the
format that is required for the verification measurements in myQA Patients.
Saving / Loading a LUT for Editing
The current LUT can be saved to a *.IbaLutPrj file by clicking the Save to temp file button. The default filename is “YYYY-MM-DD_New LUT.IbaLutPrj”. This file can be loaded for editing later by clicking the load from temp file button.
Importing LUTs
LUTs can be imported into myQA Patients:
- Connect the device to myQA Patients
- Go to the Measurements page and click the Settings icon to open the Measurement configuration
- Go to the Gantry angle correction section of the Calibrations tab.
- If the new LUT has the same name as an existing LUT in the software (i.e., an update to an existing
LUT) the existing LUT must be deleted from myQA SW. From the Lookup table drop-down list, select the correct LUT and click the delete button.This removes the old LUT file.
- Click the Import button and select the correct *.csv file.
- Select the updated LUT from the Lookup table drop-down list and close the window.
CAUTION: ENSURE THE CORRECT LUT IS USED
The LUTs in the software are specific to the detector type and energy, and some are enhanced with the machine type. The use of an incorrect LUT may lead to erroneous QA results.