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Radiation Protection Dosimetry Advance Access originally published online on March 28, 2008
Radiation Protection Dosimetry 2008 130(4):403-409; doi:10.1093/rpd/ncn065
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

The measurement of photoneutron dose in the vicinity of clinical linear accelerators

J. C. Rivera1, R. C. Falcão1,2,* and C. E. deAlmeida1

1 LCR/UERJ, R. São Francisco Xavier, Pav. Haroldo Lisboa da Cunha, Sala 136, Térreo, Rio de Janeiro, RJ, Brazil
2 Comissão Nacional de Energia Nuclear, R. Gal. Severiano 90, sala 416, Botafogo, 22294-900 Rio de Janeiro, RJ, Brazil

* Corresponding author: rossanafalcao{at}uol.com.br

Received October 4, 2007, amended January 29, 2008, accepted February 6, 2008

In Brazil, the replacement of rather old cobalt and cesium teletherapy machines with high-energy (E > 10 MV) medical linear accelerators (linacs) started in the year 2000, as part of an effort by the Ministry of Health to update radiotherapy installations. Since then, the contamination of undesirable neutrons in the therapeutic beam generated by these high-energy photons has become an issue of concern when considering patient and occupational doses. The walls of the treatment room are shielded to attenuate the primary and secondary X-ray fluence, and this shielding is generally considered adequate also to attenuate neutrons. However, these neutrons are scattered through the treatment room maze and might result in a radiological problem at the door entrance, an area of high occupancy by the workers of a radiotherapy facility. This paper presents and discusses the results of ambient dose equivalent measurements of neutron using bubble detectors. The measurements were made at different points inside the treatment rooms, including the isocentre and the maze. Several radiation oncology centres, which are users of Varian Clinac or Siemens machines, have agreed to allow measurements to be taken at their facilities. The measured values were compared with the results obtained through the semi-empirical Kersey method of neutron dose equivalent calculation at maze entrances, with reported values provided by the manufacturers as well as values published in the literature. It was found that the measured values were below the dose limits adopted by the Brazilian Regulatory Agency (CNEN), requiring no additional shielding in any of the points measured.


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