title: Measurement of photoneutron dose produced by wedge filters of a high energy linac using polycarbonate films.
abstract: Radiotherapy represents the most widely spread technique to control and treat cancer. To increase the treatment efficiency, high energy linacs are used. However, applying high energy photon beams leads to a non-negligible dose of neutrons contaminating therapeutic beams. In addition, using conventional linacs necessitates applying wedge filters in some clinical conditions. However, there is not enough information on the effect of these filters on the photoneutrons produced. The aim of this study was to investigate the change of photoneutron dose equivalent due to the use of linac wedge filters. A high energy (18 MV) linear accelerator (Elekta SL 75/25) was studied. Polycarbonate films were used to measure the dose equivalent of photoneutrons. After electrochemical etching of the films, the neutron dose equivalent was calculated using Hp(10) factor, and its variation on the patient plane at 0, 5, 10, 50 and 100 cm from the center of the X-ray beam was determined. By increasing the distance from the center of the X-ray beam towards the periphery, the photoneutron dose equivalent decreased rapidly for the open and wedged fields. Increasing of the field size increased the photoneutron dose equivalent. The use of wedge filter increased the proportion of the neutron dose equivalent. The increase can be accounted for by the selective absorption of the high energy photons by the wedge filter.
authors.affiliation: Department of Medical Physics, Tarbiat Modares University, Al-Ahmad and Chamran Cross, Tehran, Iran.
authors.forename: Seyed Mehdi
authors.initials: SM
authors.lastname: Hashemi
authors.forename: Bijan
authors.initials: B
authors.lastname: Hashemi-Malayeri
authors.forename: Gholamreza
authors.initials: G
authors.lastname: Raisali
authors.forename: Parvaneh
authors.initials: P
authors.lastname: Shokrani
authors.forename: Ali Akbar
authors.initials: AA
authors.lastname: Sharafi
authors.forename: Falamarz
authors.initials: F
authors.lastname: Torkzadeh
mesh_terms: D009502:Neutrons; D010315:Particle Accelerators; D011108:Polymers
chemical_list: D011108:Polymers; C017902:lexan