Absolute Dose for Static Deliveries 7
2 Create a beam with a field size of 10 cm x 10 cm. For machines that do not support 10 cm x
10 cm field size, use machine-specific reference field size.
3 Ensure the build
up over the isocenter accounts for the additional intrinsic buildup over
MapCHECK 3 detector plane (1.5 g/cm
2
).
For example, if dose calibration me
asurement will be performed with 5 cm of total water-
equivalent buildup, set the isocenter at the depth of 5 cm for dose calculation, with 3.5 cm
water equivalent buildup placed on top of MapCHECK 3 for the measurement.
4 In
clude at least 3 cm of water-equivalent backscatter material downstream from isocenter.
5 Se
t prescription to deliver 100 monitor units for 1 fraction.
6 Calcu
late plan and record dose at isocenter.
7 I
f desired, verify calculated dose by performing an ion chamber measurement under the same
conditions.
Alternate Methods
•Acquire a CT image of MapCHECK 3 with desir
ed buildup and import into TPS in lieu of
creating a water-equivalent phantom.
• Calculate known dose using a hand calculation
or second-check software. The accuracy of
this method greatly depends on the fidelity of the PDD, TPR, and output factor data and any
conversions, interpolation, or corrections applied.
• Measure the known dose directly using a calibrated f
armer-type ion chamber. Apply all
necessary corrections to minimize measurement uncertainty.
Deliver Absolute Dose Calibration Measurement
1 Set up MapCHECK 3 such that the detector plane is positioned at the isocenter plane.
2 A
lign the anterior surface of the MapCHECK 3 to lasers or field cross-hair so that the center
detector is located at isocenter.
3 Pla
ce additional buildup to match the total water-equivalent thickness used when calculating
the known dose. For the example above, place 3.5 cm of water-equivalent buildup on top of
the MapCHECK 3 surface, for a total depth of 5 cm to the MapCHECK 3 detector plane.
4 Se
t the field size to 10 cm x10 cm (or machine-specific reference field size used for
determining the known dose).
5 In
the SNC Patient toolbar, click Load Array Calibration , select the array calibration that
corresponds to the absolute dose cal
ibration energy, and then click Open. The default path
to the array calibration files is C:\SNC\SNC Patient\factors\<serial number>.
6 In th
e SNC Patient menu, select Setup > Calibrate Dose and follow the on-screen
instructions to collect and save the absolute dose calibration measurement. See Figure 2-1
on page 8.
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