AbstractID: 8553 Title: The small field output factor as a sensitive indicator for megavoltage photon
beam Monte Carlo modeling
Accurate estimation of the energy and radial intensity distributions of the incident electron beam is
necessary for Monte Carlo linac simulation. In a recent study1 the in-air off-axis factor has been suggested
in order to estimate the radial electron-beam distribution incident on the target. However, this radial
distribution is directly related to the focal spot size, which could be estimated by the small field output
factor in air or in phantom. When the collimator setting goes small enough to start blocking the x-ray
source, the output factor drops sharply. This source-obscuration effect2 could be utilized as a sensitive
indicator of the radial electron-beam distribution. The small field output factors in phantom for the
Siemens Primus 6MV photon beams were measured down to the 1x1 cm2 field sizes and compared with
the calculated ones from the BEAM simulation with varying the FWHM of Gaussian distribution for the
incident electron beams. While changing the FWHM from 0.32cm to 0.1cm, the output factor for 1x1 cm2
fields changed sharply from 0.377 to 0.620. However, the changes in the depth dose curves were
negligible within 1%. Since the calculated output factors are very much sensitive to the FWHM, the
inaccuracy due to the measurement and the collimator position could be compensated. The small field
output is also important to the Monte Carlo based planning for intensity modulated radiation therapy
(IMRT), which often contains small irregular segments.
1
Sheikh-Bagheri et al. Med. Phys. 29(3), (2002)
2
Zhu et al. Med. Phys. 22(6), (1995)
Supported by the Korean Ministry of Science and Technology