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.
This report presents status of external and internal dose assessment of workers and introducing the structure of National Dose Registry System of Iran (NDRSI). As well as types of individual dosemeters in use, techniques for internal dose assessment are presented. Results obtained from the International Atomic Energy Agency intercomparison programme on measurement of personal dose equivalent H(p) (10) and consistency of the measured doses with the delivered doses are shown. Also, implementation of dosimetry standards, establishment of quality management system, authorisation and approval procedure of dosimetry service providers are discussed.
The effect of pre-irradiation annealing in an oven at 240 degrees C/10 min (oven anneal) and in a TLD reader at 6 degrees C s(-1), 10 s dwell time at a maximum temperature 240 degrees C (reader anneal), on the glow curve structure of a home made sample of LiF:Mg,Cu,P powder has been investigated. It is shown that on oven annealing the glow peak areas of peaks 2 and 3 increase and that of peak 4 decreases, while on reader annealing the area of peak 4 increases and those of peaks 2 and 3 effectively decrease. Values of fading during a post-irradiation storage of 6 months were measured to be 26% and 12% for samples undergoing oven and reader annealing respectively. To reduce the effect of fading several pre-heats at temperatures of 100, 110, 120 and 130 degrees C for 10 min were investigated and 120 degrees C/10 min was chosen as the optimal condition for pre-heat. It is also observed that when imposing pre-heat thermal treatment on this dosimeter or storing the irradiated dosimeter the height of peak 4 effectively increases while the integral of TL signals remains constant.
The X-ray body scanner (BS) is going to find common use as a body-checking equipment at the entrance borders of countries, to find illicit drugs or forbidden items which have been hidden inside the body cavities, or attached to the body parts of the passengers. Considering the tissue weighting factors of the sensitive organs, the total effective dose due to the scanning by the system was estimated to be 3.8 µSv per scan. The ambient dose equivalent rates within the distance range of 300-40 cm from the X-ray generator were measured to be 4.5 up to 50 µSv h(-1). It is concluded that, in general, BS systems could be a safe device for the operators and people who are being scanned. But using such systems should be justified for everybody and special care should be taken for children and pregnant ladies.
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