Technical Note: Simulation of dose buildup in proton pencil beams

Abstract

Purpose The purpose of this study is to characterize the magnitude and depth of dose buildup 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 Geant4 Monte Carlo toolkit. We independently characterize the electronic and protonic components of dose buildup as a function of proton beam energy from 40 to 400 MeV, both with and without an air gap. Results At clinical energies, electronic buildup over a distance of about 1 mm leads to a dose reduction at depth of the basal layer (0.07 mm) by up to 6% compared to if no buildup effect were present. Protonic buildup 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 buildup. An air gap of 1 m has no significant effect on protonic buildup but reduces electronic buildup below 1%. Conclusions Protonic and electronic dose buildup are relevant for accurate dosimetry in proton therapy although a realistic air gap reduces the electronic buildup 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 μ m 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.

Determination of surface dose in pencil beam scanning proton therapy

“…400 µm and for 220 MeV protons of approx. 1100 µm . This proves that the settings used here in TOPAS were well comparable with the previous publications and further support our experimental data.…”

“…The maximum range of the electrons was 0.8 mm in C2F4 and 1.5 mm if converted to tissue equivalent depth. In general, an exact range limit of an electron buildup can hardly be defined due to the overlap with the nuclear buildup and was found mostly empirically …”

“…First, there is a short‐range electron build‐up downstream of the interface between air and patient and second, a several centimeters long nuclear buildup occurs . The nuclear part consists mainly of secondary protons, which have different ranges depending on their energy . Recently published experimental measurements demonstrated Monte Carlo codes as useful tools to separate these effects in the buildup …”

“…The nuclear part consists mainly of secondary protons, which have different ranges depending on their energy . Recently published experimental measurements demonstrated Monte Carlo codes as useful tools to separate these effects in the buildup …”

Comment on: “Technical note: Simulation of dose buildup in proton pencil beams” [Med Phys. 46(8): 3734–3738 (2019)]

“…1 Medical Physics recently published an article by Kelleter et al who also performed simulations of the dose buildup effects of proton pencil beams. 2 Kelleter et al compare their findings to ours and claim, "In contrast to what was found by Pfuhl et al we do not see evidence that particles with A > 1 contribute to the buildup effect on the short or the long length scale." This sentence is misleading to the reader, which is why we would like to clarify the issue in this letter.…”