AbstractID: 6888 Title: Misalignment of electron and light fields at off-axis positions
Due to electron scattering in the head of the machine and the intervening air column, an attribute
which gives rise to the virtual electron source, the electron radiation field size, defined as 50% of the
central axis dose, is larger than the field light indicates. This divergence appears to increase with field size
and extended distance. More generally, each individual radiation field edge is shifted away from the
central axis with respect to its light projection in an amount increasing with its off-axis distance, making
the electron field shifted outward with respect to the light field. Radiation and light field alignment is
further complicated by non-divergent electron cutouts.
To quantify this shift, we used cutouts with 2x2 cm2 apertures in a 25x25 cm2 cone located 5, 10,
and 15 cm away from the central axis in conjunction with 6, 12, and 20 MeV electrons generated by a
Varian 2100C. Film measurements at SSDs up to 115 cm showed that the discrepancy between the
lateral borders of the light and radiation fields could be as high as 8 mm, depending on the beam energy,
off-axis position, and SSD. The geometrical model based on the difference in the light and virtual
electron source positions correctly predicts the direction but underestimates the magnitude of the shift.
Since the light field is typically used for abutting an electron field with another radiation field and
for positioning an ion chamber for an output measurement, these variations in the radiation field edge
should be considered.