TITLE:
The Impact of Small Field’s Off-Set on Output Factor in Intensity Modulated Radiation Treatment
AUTHORS:
Hulya Ozdemir, Nina Tuncel, Adem Unal Kizildag
KEYWORDS:
Small Field, Off-Set Fields, Total Output Factor, In-Air Output Factor, Intensity Modulated Radiation Treatment
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.6 No.4,
November
23,
2017
ABSTRACT: In intensity modulated radiation treatment (IMRT) planning, the use of asymmetrically collimated fields that are placed on central axis or its off-set is mostly
required. Output is the main topic discussed today for extremely small and/or severe irregularly shaped fields. The air scatter
data are involved directly or indirectly in obtaining the output.
Despite the fact that extensive data have been published in many studies to provide a guide on the
magnitude of output factor for clinical
accelerators, there are very few data reviewed about output factor
in-air or phantom for off-set fields. This study was aimed to investigate the impact
of these conditions for small fields. This study was conducted in Elekta Synergy linear accelerator which produces 6 MV
X-ray energy. The in-air output factor
(Sc) has been measured by CC04 ion chamber with brass-alloy “build-up” cap
and Dose-1 electrometer, and the total output (Scp) measurements
were carried on at dose maximum depth in phantom by the same chamber and Thermoluminescence dosimeter (TLD) for 1 - 10 cm2 fields. The
all measurements at center of isocenter and off-set fields
at three directions (X2, Y1, Diagonal) were done. By decreasing field size from
10 to 2 cm2 at isocenter, the Sc value using CC04 was decreased to
5.4% and Scp using CC04 and TLD to 14.5% and 11% respectively. By
increasing off-set value, the Sc and Scp values were increased in all directions comparing to central
fields. The maximum increase was obtained in Y1 direction for Sc and
Scp. TLD results for Scp is slightly higher than CC04. The dosimetric properties
of small fields and their off-set should be evaluated and modelled
appropriately in the treatment planning system to ensure accurate dose
calculation in Intensity Modulated Radiation Treatment.