TURKISH JOURNAL OF ONCOLOGY 2010 , Vol 25 , Num 2
Energy and field size dependence of polarity
Hilal ACAR
İstanbul Üniversitesi Onkoloji Enstitüsü, Klinik Onkoloji Anabilim Dalı, Radyasyon Onkolojisi Bilim Dalı, İstanbul OBJECTIVES
The aim of this study was to investigate polarity effects of electron beams with different energies.

METHODS
Energy dependence of polarity error was investigated at dmax 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