Evaluation of surface doses and effect of air gaps under bolus during external photon beam radiotherapy

Document Type : Original Paper

Authors

1 Department of Medical Physics, Graduate School of Nuclear and Allied Sciences, University of Ghana, Accra, Ghana

2 Radiation Oncology Physics Department, Center International de Cancerologie de Lomé, Lome, Togo

3 Department of Nuclear Safety & Security, Graduate School of Nuclear and Allied Sciences, University of Ghana, Accra, Ghana

Abstract

Introduction: Existence of air gaps under bolus has the potential to alter dose administered during external photon beam radiotherapy. Hence there is the need to ascertain the extent to which doses are altered as a result of the presence of these air gaps. The purpose of this study was to evaluate the implications of air gaps under bolus on surface dose (Dsurf) during external photon beam radiotherapy.

Materials and Methods: Using GafChromic EBT3 films and a Roos chamber, Dsurf measurements were taken for various photon beam energies (6 MV and 15 MV), bolus-to-surface distances and field sizes. Dose verification was performed for the above mentioned beam energies on a RANDO phantom by measuring doses at depth of maximum dose (dmax) using a diode.

Results: The introduction of the bolus for 6 MV and 15 MV photon beam energies as well as for the various field sizes increased Dsurf. Using Roos chamber for 5×5 cm2 field dose measurement, Dsurf increased by 61% and 72% for 6 MV and 15 MV photon beams respectively with the introduction of the bolus. As the bolus-to-surface distance increased, Dsurf recorded for small field sizes (5×5 cm2 and 10×10 cm2) decreased. Dsurf measured using Roos chamber for 5×5 cm2 field reduced by 21% and 33% with a 5 cm air gap for 6 MV and 15 MV respectively. However, for larger field sizes (15×15 cm2 and 20×20 cm2), the change in surface dose as a result of change in air gap was negligible.

Conclusion: Dsurf is largely unaffected by the presence of air gaps for large field sizes (above 10×10 cm2). However, for smaller field sizes, the introduction of air gaps decreased Dsurf for all selected beam energies.

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Iranian Journal of Medical Physics

Articles in Press, Accepted Manuscript
Available Online from 27 August 2023

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History

  • Receive Date: 16 February 2023
  • Revise Date: 21 August 2023
  • Accept Date: 27 August 2023

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