6 Section 2. Operation
• To generate calibration factors for FFF beams. Repeating the last two steps of the VMAT
array calibration process under an FFF beam updates the calibration factors for FFF
fields.
Flattening Filter Free (FFF) Beams
Do not use a flattening filter free (FFF) beam during Standard or VMAT array calibrations. For FFF
(e.g., 6 FFF) array calibrations, perform calibration with flattening filter equivalent (e.g., 6 MV) open
fields, and then perform a MapPHAN Factor calibration to update the calibration factors for FFF
fields. See
Array Calibration Modes on page 5.
Array Calibration Procedure
1 In the SNC Patient menu, select Setup > Calibrate Array.
2 Follow the initial setup instructions in the Array Calibration dialog box and then click Begin.
3 In the Measurement Parameters dialog box, select one of the following Array Cal Modes:
• Standard (IMRT). Rotational (VMAT), or MapPHAN Factor
• Beam energy
• MapPHAN type (None, 5 cm, or 10 cm).
A MapPHAN accessory is required to use the ‘VMAT’ and ‘MapPHAN Factor’ array calibration
modes. See
Measurement with MapPHAN Accessory on page 10.
4 Click OK and follow the on-screen instructions.
Recalibration Interval
Absolute Dose Calibration
The absolute dose calibration procedure consists of collecting a MapCHECK 3 measurement
under the conditions at which dose delivered to the central detector is known and entering the
value of that known dose in the calibration wizard. SNC Patient software correlates the counts
collected by the device during the calibration to the known dose entered by the user, establishing
the absolute dose calibration factor.
Dose Rate
The radiation response of MapCHECK 3 diodes varies by up to ±1.5% with change in dose rate
(100 MU/min to 1400 MU/min). Therefore, dose calibration should be performed at the dose rate
that is similar to that at which QA measurements will be performed. This is especially important
for flattening-filter-free (FFF) energy modes, for which the range of available dose rates is wide.
For best results, it may be necessary to create more than one dose calibration for the same energy
mode - e.g., 6 MV FFF low dose rate calibration for VMAT measurements and 6 MV FFF high dose
rate calibration for IMRT or machine QA measurements. Performing the absolute dose calibration
at a dose rate that is significantly different from that of QA measurements can result in an error in
absolute dose reported by MapCHECK 3.
Absolute Dose for Static Deliveries
MapCHECK 3 is designed to collect measurements with the beam normal to the front surface of
the device (e.g., machine QA measurements or collapsed-angle IMRT QA). A MapPHAN is not
required to collect these types of measurements.
Determine the Known Dose
Recommended Method
1 Set up a water-equivalent phantom in a commissioned treatment planning system.
CAUTION: Annual array calibration is recommended but not required. Test array
calibrations annually and more frequently if the instrument is heavily used.
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