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Neutron contamination of 10 MV X-rays: its relevance to treatment room door and maze design

¹ Medical Physics Department, Guy's and St Thomas' NHS Foundation Trust, St Thomas' Hospital, London SE1 7EH, ² Medical Physics Department, Royal Sussex County Hospital, Brighton, East Sussex BN2 5BE, UK

Correspondence: Dr Peter Rudd, Medical Physics Department, Guy's and St Thomas' NHS Foundation Trust, St Thomas' Hospital, Lambeth Palace Road, London SE1 7EH, UK. E-mail: peter.rudd{at}gstt.sthames.nhs.uk

10 MV X-rays produced by linear accelerators are commonly used in radiotherapy. This paper demonstrates that neutron leakage cannot be neglected at 10 MV when direct access doors are used or when short mazes, typically less than 7 m in length, are employed. Measurements and calculations were made in terms of the operational quantities and personal and ambient dose equivalents, and the use of neutron fluence or contentious issues, such as effective dose and radiation weighting factors, were avoided. Neutron leakage from the head of an Elekta Precise linear accelerator was measured and compared with published data. The neutron protection afforded by a direct access door was considered and the use of borated polyethylene to remove thermal neutrons was found to be unnecessary. Extensive measurements were made in two treatment room mazes using survey meters of the Andersson-Braun and Leake type. Acceptable agreement was found between the individual instrument readings and calculations. However, one meter unexpectedly appeared to both respond to photons and over-respond at high neutron dose rates.