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Radiation Protection Dosimetry Advance Access published online on September 29, 2009
Radiation Protection Dosimetry, doi:10.1093/rpd/ncp204
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

THE IMPACT OF AUTOMATIC WEDGE FILTER ON PHOTONEUTRON AND PHOTON SPECTRA OF AN 18-MV PHOTON BEAM

Seyyed-Mostafa Ghavami1, Asghar Mesbahi2,3,* and Ehsan Mohammadi4

1 Radiology Department, Paramedical School, Tabriz University of Medical Sciences, Tabriz, Iran
2 Medical Physics Department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran
3 Radiation Therapy Department, Imam Hospital, Tabriz, Iran
4 Radiation Therapy Department, Imam khomeini Hospital, Tehran, Iran

* Corresponding author: asgharmesbahi{at}yahoo.com

Received June 22, 2009, amended September 4, 2009, accepted September 8, 2009

The effect of an automatic wedge filter on photon and photoneutron spectra of a medical linac was evaluated using the Monte Carlo method. The head of an Elekta SL75/25 was simulated using the MCNPX Monte Carlo code. The photon and photoneutron spectra for open and wedged beams were calculated at the isocentre with a source to axis distance of 100 cm. For a wedged beam, the neutron fluence was from 3.84 to 7.2 times higher for field sizes from 10 x 10 to 30 x 30 cm2. The neutron fluence is decreased with field size for open beams and is increased with field size for wedged beams The photon beam spectra became harder and the mean energy was 6 % higher for a wedged beam, which led to a 4 % increase in relative depth dose and a better skin sparing effect. The results here recommend that the higher photoneutron fluence of the wedged beam should be taken into account in patient dosimetry and shielding calculations.


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