The Influence of Oblique Incidence Electron Beams to Beam Specification Parameters

The changes of beam specification parameters along the normal central axis of electron beams with increasing gantry angle had been measured. The electron beams were produced by Varian Clinac2100C with energies of 6, 9, 12, 16, and 20 MeV. This work was performed using Wellhoffer water phantom provided with two small cylindrical IC-15 chambers. It was found that the oblique incidence beams cause the d(max) depth, R-p, R-85 at normal central axis shifted to skin surface. For all energies, the changes of gantry angle from 0 to 20 shift the d(max) depth in the range of 2.0% up to 17.73%, RP in the range of 2.02 up to 10.78%, and R-85 in the range of 8.93 up to 9.37%. The percentage ionization along the diagonal fanline was also measured. The results indicated that for lower energy electron beams (6 MeV, 9 MeV, and 12 MeV) the diagonal fanline was still located at the penumbra region ( 80%-20% isodose line), whereas for higher energies (16 MeV and 20 MeV) these diagonal fanline were in radiation field area particularly for the gantry angle of 15 degrees and 20 degrees.