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(PP235) The study of low-dose accelerator based neutrons for bystander effects.
Liu, Zhengfeng*,1, Prestwich, William1, Byun, Soo Hyun1, Mothersill, Carmel1, McNeill, Fiona1, Seymour, Collin1, 1 Medical Physics and Applied Radiation Sciences, Hamilton, Ontario, Canada
ABSTRACT- Bystander effects have been well observed following both alpha and gamma irradiation of many cell lines. Neutrons provide another high LET radiation. Nevertheless, no information is available regarding neutron induction of the bystander effects. Such information is essential for a precise evaluation of the risks of environmental radiation and radiation therapy regimens. The 3MV Van de Graff accelerator at McMaster University accelerator laboratory is extended to a neutron irradiation facility for low-dose bystander effects research. In order to determine the neutron fluence rate, a long counter and activation foils have been used as monitors. Meanwhile, the interactions of neutrons with the monitors have been simulated using a Monte Carlo N Particle (MCNP) code. Since accelerator neutron beam is a mixed field including gamma-ray and neutron, we should differentiate the doses for the bystander effects from the two radiations. A tissue equivalent proportional counter (TEPC) filled with propane based tissue equivalent gas simulating a 2 m diameter tissue sphere has been investigated to estimate the neutron and gamma absorbed doses. The measurement system is described including the radiation source, detector, and pulse processing electronics. Evaluation and interpretation of microdosimetric spectra of gamma and neutron fields have been systematically studied. Furthermore, the absorbed dose from gamma rays has been compared with the result from an ionization chamber (Farmer dosimeter type 2570). On the other hand, we need to know the dose threshold of the bystander effects from gamma radiation. As long as we can keep gamma contamination below the threshold, the observed bystander effects should be caused solely by neutrons. We aim to find out the dose threshold of gamma radiation for the bystander effects. Ionizing radiation (60Co) doses 0.10 mGy and 7 mGy were given to a human skin cell line to measure bystander effects. It was found that the existence of bystander effects (medium transfer) when the gamma dose is 7 mGy and no significant cell killing caused by the bystander effect when the dose is less than 0.10 mGy. This result may suggest there is a dose threshold for the bystander effects for this cell line between 0.1 and 7 mGy.
Key words: bystander effects, neutron, gamma, dose threshold
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