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Evaluation of the effects of bolus air gaps on surface dose in radiation therapy and possible clinical implications

University of British Columbia

In clinical radiotherapy, treatments are frequently delivered using photons with energies in the Megavoltage range. The advantage of such beam energies is that the majority of dose is deposited deeper within the patient, with the depth of maximum dose being up to several centimetres beyond the patient’s skin. This effect allows radiotherapists to target tumours on organs such as the prostate. However, there are times when larger doses must be deposited to the near-skin region. In these cases, a layer of ”substitute tissue” called a bolus is applied, to shift dose towards the patient’s skin. Given the natural contours of the human body, it is difficult for a bolus to achieve perfect contact with the patient and air gaps are often present between the applied bolus and the patient’s surface. Such air gaps have the potential to disturb the distribution of surface dose. In this thesis we present an investigation into the magnitude of the effects of bolus-surface air gaps on 6 MV photon beam surface dose. Using a combination of ionization chamber measurements, film dosimetry, and Monte Carlo simulations, we establish that surface dose is significantly reduced in the presence of an air gap. The observed reduction in dose increases as the distance between the bolus material and the phantom surface increases, and is more severe at smaller field sizes. By examining simulated and experimentally measured surface-dose-profiles, we demonstrate that bolus-surface gaps alter the shape of the dose distribution near the field boundary. We find that surface dose is reduced near the inside of the field edge, with a corresponding increase in dose in the region outside of the defined field. We propose that this effect is caused by low-energy electrons that are generated within the bolus material, near the field edges, but are then scattered outside of the treatment region when passing through the air gap. As such we would recommend that care be taken to reduce the size of any air gaps to below 1 cm, especially in cases with weaker treatment beams and where the PTV is located near to at-risk organs.

Text

2018-08-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/67038

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/