DOSIMETRIC PROBLEMS AT LOW MONITOR UNIT SETTINGS FOR SCANNED AND SCATTERING FOIL ELECTRON-BEAMS

Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for d ifferent types of linear accelerators. It is observed that for the sca ttering foil units, the dose/MU is a smooth function of MU for all bea m energies. Discrepancies in dose/MU are highest at the lowest MU. Sig nificant variation (5%-245%) in dose linearity is observed among vario us linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units test ed. On the contrary, for the scanned beam, there is no predictable pat tern as dose/MU is random in nature and varies with time and beam ener gy. The maximum dosimetric error is observed for the highest energy be am where the beam width is most narrow. Using film, the beam uniformit y was noticed to be very poor at low MU and high energy for scanned be ams. The beam uniformity and dose linearity are random at low MU due t o the random nature of the scan cycle. Under the adverse conditions, t he deviation in dosimetric parameters was observed up to 200 MU.