Kelleter, L; Zhen-Hong Tham, B; Saakyan, R; Griffiths, J; Amos, R; Jolly, S; Gibson, A; (2019) Technical note: Simulation of dose build-up in proton pencil beams. Medical Physics , 46 (8) pp. 3734-3738. 10.1002/mp.13660.

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Abstract

Purpose: The purpose of this study is to characterise the magnitude and depth of dose build-up in pencil beam scanning proton therapy. Methods: We simulate the integrated depth-dose curve of realistic proton pencil beams in a water phantom using the the Geant4 Monte Carlo toolkit. We independently characterise the electronic and protonic components of dose build-up as a function of proton beam energy from 40 to 400 MeV, both with and without an air gap. Results: At clinical energies, electronic build-up over a distance of about 1mm leads to a dose reduction at depth of the basal layer (0.07 mm) by up to 6% compared to if no build-up effect were present. Protonic build-up reduces the dose to the basal layer by up to 16% and has effects at depths of up to 150 mm. Secondary particles with a mass number A > 1 do not contribute to dose build-up. An air gap of one metre has no significant effect on protonic build-up but reduces electronic build-up below 1%. Conclusions: Protonic and electronic dose build-up are relevant for accurate dosimetry in proton therapy although a realistic air gap reduces the electronic build-up to levels where it can be safely neglected. We recommend including electrons and secondary protons in Monte Carlo-based treatment planning systems down to a predicted range of 10-20 µm in order to accurately model the dose at depths of the basal layer, no matter the size of the air gap between nozzle and patient.

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