Dose characteristics of a new 300kVp orthovoltage machine

The dose characteristics of a relatively new type of orthovoltage machine which displays different beam qualities compared to other orthovoltage machines has been studied. Surface dose, depth dose and dose profiles have been measured with various ionization chambers as well as diodes and thermoluminescent dosemeters. Profiles produced by the 300kVp x-ray machine's fixed and variable collimators show no significant differences. A slight depth dose build up effect of 2% is seen over the first 1mm with the variable collimator at 250kVp but this is not seen with the fixed collimators. Surface charge is increased to 110%, normalised to 100% at 0.3mm depth in solid water when the front perspex plate of the 10x10cm 50cm FSD fixed collimator is removed. The use of a magnetic field placed directly under the fixed collimator to sweep away any electrons produced has shown that the extra charge and thus dose is caused mainly by electrons produced from the fixed collimator material. The percentage surface charge was reduced from 110% to 102% at 50cm FSD with the magnetic field energised.