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.
Highlights
In radiation therapy (RT), the reliability of treatment planning system that based on factors or model is affected by many factors
The concept of in-air output ratio [6] [7] [8] [9] is characterized by the variation in the incident photon fluence per monitor unit according to collimator settings, which is known as collimator scatter factor [10] or head scatter factor (Sc) for photon [11] [12]
The geometric uncertainty related to field aperture arrangement in linear accelerators (LINAC) and detector position in water phantom were less than 1 mm totally
Summary
In radiation therapy (RT), the reliability of treatment planning system that based on factors or model is affected by many factors These may include such phenomena as scattered photons by structures in the linear accelerator head (head-scatter), backs catered photons and electrons into the monitor chamber (monitor backscatter), and the partially obscuring of the X-ray source by the collimators (source-obscuring effect) especially in very small fields [1]. The X-Y jaws and MLC with unidirectional motion that play a major role in field shaping constitute the moving structures of LINACs. The concept of in-air output ratio [6] [7] [8] [9] is characterized by the variation in the incident photon fluence per monitor unit according to collimator settings, which is known as collimator scatter factor [10] or head scatter factor (Sc) for photon [11] [12]. The underestimation of output factors due to the increase of lateral electron disequilibrium and the volume effect of detectors is expected [13] [14] [15]
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