Fluke Biomedical 37-705 - Manual (Page 31)

VeriDose QC Phantom
General Description
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3-3
flatness and symmetry should be within the approximate tolerances as specified by AAPM guidelines
(TG-40).
Method 2
Calibrate all the detectors at the same time by aligning the QC Phantom's central diode with the central
ray. Set the 20 x 20 cm field size and align to the field outline on the QC Phantom. Set the distance to
100 cm SSD to the phantom surface and place build-up material equal to or greater than Dmax depth
over the entire phantom. Irradiate the phantom to a known calibration dose. Repeated exposures are
recommended for dose averaging, with a maximum cumulative dose of less than 1000 cGy. Note that this
method will yield a reference value of 0.0%, for flatness and symmetry, to be used only as QA baseline
values. These baseline reference values for flatness and symmetry will be relative to the actual values at
the time of calibration, therefore, special attention must be paid to setting correct tolerance levels. Initial
values for flatness and symmetry must be factored into the pass/fail criteria of the beam to conform to
AAPM guidelines (TG-40).
Calibrate each detector (channel), one at a time, positioned at isocenter. A typical exposure of 100 - 200
cGy at a dose rate of 100 to 300 cGy/min is adequate for calibration. Upon completion of the calibration
process, several additional exposures should be made in the normal measurement mode in order to verify
the calibration. To print the results, select the Flatness/Symmetry report on the VeriDose V.
3.6 Flatness and Symmetry Measurements
Position the VeriDose QC Phantom flat on the treatment table. Position the central axis chamber at
isocenter and align the X and Y crosshairs with the laser positioning system. Collimate the field size to 20
x 20 cm using the borders on the QC Phantom as a guide. To make measurements, follow the procedure
outlined in the VeriDose V Measure Modes - Dose section of the manual. A Flatness and Symmetry
Report may be printed by selecting the appropriate report in the Measure Reports menu and pressing the
ENTER key.
3.7 Depth Dose check
Position the VeriDose QC Phantom on its edge with Y + positioned closest to the gantry. Additional build-
up may be placed against the sides of the phantom. Position the central axis chamber at isocenter and
align the X and Y crosshairs with the laser positioning system. Collimate the field size to 20 x 20 cm. To
make measurements, follow the procedure outlined in the VeriDose V Measure Modes - Dose section of
the manual. A Depth Dose Report may be printed by selecting the appropriate report in the Measure
Reports menu and pressing the ENTER key.