Original Article

The effect of the carbon fibre insert for the Varian Exact™ couch on the attenuation and build-up of high energy photon beams

ML Wilson^a1 c1, WP Colley^a1 and AW Beavis^a1a2a3

^a1 Queen’s Centre for Oncology and Haematology, Castle Hill Hospital, Cottingham, UK

^a2 Department of Computer Science, University of Hull, Cottingham Road, UK

^a3 Faculty of Health and Wellbeing, Sheffield-Hallamshire University, Broomfield Road, Sheffield, UK

Abstract

Carbon fibre couch inserts are widely used in external beam radiotherapy to provide rigid and lightweight patient support. Carbon fibre is often perceived to be essentially radiotranslucent implying that it does not interfere with the radiation beam. However, there is evidence in the literature which suggests that this perception may not be appropriate, particularly at oblique angles of incidence. Furthermore, there is evidence indicating that the use of carbon fibre significantly reduces the skin sparing effect. In this study, the radiation attenuation and surface dose enhancement characteristics of the carbon fibre insert for the Varian Exact^TM couch have been investigated. It was found that attenuation increased significantly with increasing angle of incidence, resulting in in-phantom dose reductions of up to 6% at 6 MV and 4% at 15 MV. It has been shown that it is possible to model couch attenuation on a commercial treatment planning system (Elekta CMS XiO) by including the carbon fibre insert in the planning computed tomography (CT) dataset. Finally, the carbon fibre insert was found to significantly increase skin dose to the patient. The skin dose was approximately three times as large when the couch insert was added to 6 and 15 MV photon beams. However, even with this substantial increase it is highly unlikely that the skin tolerance dose will be exceeded.

(Online publication November 23 2010)

Keywords

• Carbon Fibre;
• treatment couch;
• attenuation;
• skin dose

Correspondence:

^c1 Correspondence to: ML Wilson, Radiation Physics Department, Queen’s Centre for Oncology and Haematology, Castle Hill Hospital, Cottingham, HU16 5JQ, UK. E-mail: mark.wilson@hey.nhs.uk