The impact of automatic wedge filter on photoneutron and photon spectra of an 18-MV photon beam

The effect of an automatic wedge filter on photon and photoneutron spectra of a medical linac was evaluated using the Monte Carlo method. The head of an Elekta SL75/25 was simulated using the MCNPX Monte Carlo code. The photon and photoneutron spectra for open and wedged beams were calculated at the iso-centre with a source to axis distance of 100 cm. For a wedged beam, the neutron fluence was from 3.84 to 7.2 times higher for field sizes from 10 x 10 to 30 x 30 cm2. The neutron fluence is decreased with field size for open beams and is increased with field size for wedged beams The photon beam spectra became harder and the mean energy was 6% higher for a wedged beam, which led to a 4% increase in relative depth dose and a better skin sparing effect. The results here recommend that the higher photoneutron fluence of the wedged beam should be taken into account in patient dosimetry and shielding calculations. (authors)