Radiation Protection Dosimetry Advance Access originally published online on September 17, 2007
Radiation Protection Dosimetry 2008 128(3):359-362; doi:10.1093/rpd/ncm421
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The effect of field modifier blocks on the fast photoneutron dose equivalent from two high-energy medical linear accelerators
1 Department of Medical Physics, Tarbiat Modares University, PO Box 14115-151, Tehran, Iran
2 Gamma Irradiation Center, Atomic Energy Organization of Iran, End of Kargare Shomali, PO Box 11365-3486, Tehran, Iran
3 Department of Medical Physics and Engineering, Esfahan University of Medical Sciences, Hezarjarib, Esfahan, Iran
4 Department of Radiology Technology, School of Paramedicine, Iran University of Medical Sciences, Hemmat Highway, Tehran, Iran
5 National Radiation Protection Department, Iranian Nuclear Regulatory Authority, PO Box 14155-4494, Tehran, Iran
* Corresponding author: bijanhashemi{at}yahoo.com
Received February 22, 2007, amended July 23, 2007, accepted July 31, 2007
High-energy linear accelerators (linacs) have several advantages, including low skin doses and high dose rates at deep-seated tumours. But, at energies more than 8 MeV, photonuclear reactions produce neutron contamination around the therapeutic beam, which may induce secondary malignancies. In spite of improvements achieved in medical linac designs, many countries still use conventional (non-intensity-modulated radiotherapy) linacs. Hence, in these conventional machines, fitting the beam over the treatment volume may require using blocks. Therefore, the effect of these devices on neutron production of linacs needs to be studied. The aim of this study was to investigate the effect of field shaping blocks on photoneutron dose in the treatment plane for two high-energy medical linacs. Two medical linacs, a Saturn 43 (25 MeV) and an Elekta SL 75/25 (18 MeV), were studied. Polycarbonate (PC) films were used to measure the fluence of photoneutrons produced by these linacs. After electrochemical etching of the PC films, the neutron dose equivalent was calculated at the isocentre and 50 cm away from the isocentre. It was noted that by increasing the distance from the centre of the X-ray beam towards the periphery, the photoneutron dose equivalent decreases rapidly for both the open and blocked fields. Increasing the energy of the photons causes an increase in the amount of photoneutron dose equivalent. At 25 MeV photon energy, the lead blocks cause a meaningful increase in the dose equivalent of photoneutrons. In this research, a 30% increase was seen in neutron dose contribution to central axis dose at the isocentre of a 25 MeV irregular field shaped by lead blocks. It is concluded that lead blocks must be considered as a source of photoneutron production when treating irregular fields with high-energy photons.