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Practical considerations for reporting surface dose in external beam radiotherapy: a 6 MV X-ray beam study

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Abstract

In this study, we assessed the accuracy of surface doses determined by direct measurement and treatment planning system (TPS) calculations, relative to benchmark Monte Carlo (MC) doses calculated at 70 μm for a 6 MV, 10 × 10 cm clinical radiotherapy beam. In a homogeneous phantom with both open and fixed wedged fields, we found that the relative dose measured with an Attix chamber underestimates the MC calculated surface dose by 2.9 %, while the relative dose measured with EBT2 Gafchromic film overestimates the MC surface dose by 0.9 %. There was a significant over-response of up to 20 % in doses calculated at <2 mm depth with the Eclipse analytic anisotropic algorithm (AAA) compared to corresponding MC doses for an open field. This drops to <2 % at 2 mm depth. In a heterogeneous phantom, EBT2 film overestimates relative dose by up to 3.1 % compared to the MC calculated surface dose. The AAA relative dose calculated in a heterogeneous phantom at 2 mm depth agrees to within 1.5 % with the MC doses calculated at the same depth, but overestimates the MC surface dose (at 70 μm) by up to 2.5 %. Our results suggest that TPS doses evaluated near the surface be reported with a depth that should be at least 2 mm and this should be taken into consideration in the planned target volume for treatments where surface dose is a constraining factor. Our study demonstrates the usefulness of EBT2 film for measuring surface dose: under homogeneous conditions, the effective point of measurement of EBT2 film can be considered equivalent to the clinical skin depth of 70 μm.

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Acknowledgments

The authors acknowledge the Department of Radiation Oncology, Royal Prince Alfred Hospital, for access to clinical facilities used for the experimental measurements and the University of Sydney for computational resources used for the MC simulations. The authors also thank Dr. Xue Yang for her computational technical support, Professors Anatoly Rozenfeld, Tomas Kron and David Thwaites for valuable discussions, Dr. Susan Carroll for clinical advice on breast radiotherapy and Varian Medical Systems for providing information on the Eclipse TPS and technical specifications for the linear accelerator used in the MC model. This study was financially supported by the National Breast Cancer Foundation of Australia (www.nbcf.org.au).

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  1. Institute of Medical Physics, School of Physics, University of Sydney, NSW, 2006, Sydney, Australia

    J.-H. Kim, R. Hill & Z. Kuncic

  2. Department of Radiation Oncology, Royal Prince Alfred Hospital, Sydney, Australia

    R. Hill

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Correspondence to J.-H. Kim.

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Kim, JH., Hill, R. & Kuncic, Z. Practical considerations for reporting surface dose in external beam radiotherapy: a 6 MV X-ray beam study. Australas Phys Eng Sci Med 35, 271–282 (2012). https://doi.org/10.1007/s13246-012-0145-1

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