AbstractID: 7735 Title: Head-scatter factor for circular fields of Megavoltage Electron beams
To calculate monitor unit for electron beams, it is important to determine the incident electron fluence, φe, which varies due to
scattered electrons from the accelerator head, the applicator, the insert, and the air gap. This variation is characterized by the headscatter factor, H, defined as the ratio of φe with and without the block, for the same electron energy, cone size (c), and source-todetector distance (SDD). The phantom scatter factor, PSF, defined as the ratio of output factor in water at dmax and H, is only a
function of radius and electron energy. In comparison, H is a function of electron energy, radius of cutout, SDD, as well as the cone
size. H measured in air agreed with H measured in water at surface to within 1.5% for all electron energies (6 – 22 MeV). The range
of variation of H decreases with increasing electron energies, 11% to 5% for 6 to 22 MeV. H is independent of cone size for c > 15
cm, probably because of the same secondary collimator positions. Unlike photon beams, which is almost SDD independent, H for 1cm radius field varied with SDD by 70% to 12% for 6 and 22 MeV, respectively. This variation is caused by the scattering in the air
gap and reduces with increasing radius, saturating for r > 5 cm. In conclusion, H can be easily measured in air using a diode detector,
which accounts for the variation of φe for beam radius, cone size, electron energy, and SDD.