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Megavoltage Linac Electrons

Radiotherapy calibrations index:

Cross-calibration in a megavoltage electron beam

This service is aimed at parallel plate ionisation chambers of the type used to measure electron beams. The chamber is calibrated in a high energy electron beam against a thimble chamber which has been calibrated in 60Co, and the parallel plate chamber response is determined at the user qualities by interpolating energy correction factors from TRS-398. In the future, the cross-calibration will be replaced with a direct calibration which will result in a lower uncertainty. In the meantime the cross-calibration procedure is a suitable step for clinics wanting to calibrate electron chambers without having to set up a calibrated thimble chamber in their own linacs and follow the procedure in TRS-398.

Details of the beam used for the cross-calibration are given in Table 1.

Table 1: Megavoltage photon beams available for calibration at ARPANSA

Beam1 R50 Rate2 Pulse rate
18 MeV 7.13 g/cm² 400 MU/min 200 Hz

1 Nominal accelerating potential
2Linac output rate. The linac delivers approximately 1 cGy/MU at dmax in an isocentric setup (100 cm source-detector distance) with a 10 cm x 10 cm field.

Extract from the linac electron calibration report

The results from a typical calibration report are shown in Figures 1 and 2. The calibration coefficient for the chamber under test is determined for one electron energy. The chamber calibration coefficients at other energies is determined using calculated correction factors for the given chamber type in TRS-398. At present, only chambers which are included in TRS-398 can be extrapolated using this method.

Excerpt from a calibration certificate for a Farmer-type chamber. To find kQ for a user beam, a simple quadratic equation is usedFigure 1: Example results of an electron beam cross-calibration for a PTW 34001 Roos chamber.

 

Excerpt from a calibration certificate for a Farmer-type chamber. To find kQ for a user beam, a simple quadratic equation is usedFigure 2: Example of the interpolation of the results from the cross-calibration to the user beam qualities, specified by R50. This interpolation is provided by ARPANSA using the correction factors from TRS-398 and the R50 values provided by the user.

 

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