A comparison of surface doses for very small field size x-ray beams: Monte Carlo calculations and radiochromic film measurements

Abstract

Stereotactic radiosurgery treatments involve the delivery of very high doses for a small number of fractions. To date, there is limited data in terms of the skin dose for the very small field sizes used in these treatments. In this work, we determine relative surface doses for small size circular collimators as used in stereotactic radiosurgery treatments. Monte Carlo calculations were performed using the BEAMnrc code with a model of the Novalis Trilogy linear accelerator and the BrainLab circular collimators. The surface doses were calculated at the ICRP skin dose depth of 70 μm all using the 6 MV SRS x-ray beam. The calculated surface doses varied between 15 and 12 % with decreasing values as the field size increased from 4 to 30 mm. In comparison, surface doses were measured using Gafchromic EBT3 film positioned at the surface of a Virtual Water phantom. The absolute agreement between calculated and measured surface doses was better than 2.0 % which is well within the uncertainties of the Monte Carlo calculations and the film measurements. Based on these results, we have shown that the Gafchromic EBT3 film is suitable for surface dose estimates in very small size fields as used in SRS.

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Acknowledgments

Computational resources and services used in this work were provided by the High Performance Computing and Research Support Unit, Queensland University of Technology, Brisbane, Australia. Also, we’d like to acknowledge that Dr S. B. Crowe’s contribution to this work was supported by the Australian Research Council through Linkage Grant No. LP110100401.

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Correspondence to J. E. Morales.

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Morales, J.E., Hill, R., Crowe, S.B. et al. A comparison of surface doses for very small field size x-ray beams: Monte Carlo calculations and radiochromic film measurements. Australas Phys Eng Sci Med 37, 303–309 (2014). https://doi.org/10.1007/s13246-014-0260-2

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Keywords

  • Stereotactic radiosurgery
  • SRS
  • Surface dosimetry skin dose
  • Monte Carlo calculations
  • Radiochromic film
  • EBT3