Energy and field size dependence of polarity

METHODS

Energy dependence of polarity error was investigated at d_max
and SSD=95 for 6, 8, 10, 12, 15 and 18 MeV electron beams
from Phillips SL-25 LINAC. Field size dependence of polarity
effect was investigated at dmax and SSD=95 cm using 6x6,
10x10, 14x14 and 20x20 cm² standard electron cones for 6,
8, 10, 12, 15 and 18 MeV electron beams from Phillips SL-25
LINAC.

RESULTS

Polarity effect decreased as electron beam energy increased.
While polarity error was 0.52% for high energies, it increased
up to 4.5% for low energies. Moreover, polarity error increased
as field size increased for the same electron energy.

CONCLUSION

In conclusion, especially for low energies and wide field sizes,
polarity error is measured and used in absolute dose calculations
for parallel plate chambers.

Keywords: Field size; polarity effect; energy; electron beam