Measurement of backscattered radiation from secondary collimator jaws into the beam monitor chamber from a dual energy linear accelerator
Abstract
AIM: The photons and electrons backscattered from the upper and lower secondary collimator jaws give rise to a significant increase in the ion charge measured by the monitor chamber, and this increase varies between different accelerators. We have studied the effect of backscatter into the monitor chamber at 6 MV and 18 MV photon energies for the linear accelerator Clinac-1800.
MATERIALS AND METHODS: The variation of the output factor was first studied for variable asymmetric fields of fixed field size defined by variable right upper collimator jaw together with a variable Cerrobend block low melting point alloy, with constant lower jaw position. Output measurements were carried out at Dmax in a polystyrene phantom at the geometric center of the asymmetric field. The backscatter radiation effect was also analyzed applying a pair of 20 x 20 x 7.5 cm Cerrobend alloy blocks with 6.0 mm diameter pin-holes in the center aligned telescopically with the field defining light beam. The ion chamber with build-up cap was placed at 25 cm behind the second pin-hole. The in air measurements were made varying upper and lower collimators individually and together. For comparison, a similar study was conducted in a Theratron-780C cobalt-60 beam.
RESULTS: In the asymmetric field experiment it was noticed that till the collimator jaw crosses the midline, the output factor is almost constant for 6 and 18 MV photon beams. For extreme field asymmetry, the decreases in output factor were 3.2% and…
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