Neutrons in proton pencil beam scanning: Parameterization of energy, quality factors and RBE

The biological effectiveness of neutrons produced during proton therapy in
inducing cancer is unknown, but potentially large. In particular, since neutron
biological effectiveness is energy dependent, it is necessary to estimate,
besides the dose, also the energy spectra, in order to obtain quantities which
could be a measure of the biological effectiveness and test current models
and new approaches against epidemiological studies on cancer induction after
proton therapy. For patients treated with proton pencil beam scanning, this
work aims to predict the spatially localized neutron energies, the effective
quality factor, the weighting factor according to ICRP, and two RBE values,
the first obtained from the saturation corrected dose mean lineal energy and
the second from DSB cluster induction.
A proton pencil beam was Monte Carlo simulated using GEANT. Based
on the simulated neutron spectra for three different proton beam energies
a parameterization of energy, quality factors and RBE was calculated. The
pencil beam algorithm used for treatment planning at PSI has been extended
using the developed parameterizations in order to calculate the spatially
localized neutron energy, quality factors and RBE for each treated patient.
The parameterization represents the simple quantification of neutron
energy in two energy bins and the quality factors and RBE with a satisfying
precision up to 85 cm away from the proton pencil beam when compared
to the results based on 3D Monte Carlo simulations. The root mean square
error of the energy estimate between Monte Carlo simulation based results
and the parameterization is 3.9%. For the quality factors and RBE estimates
it is smaller than 0.9%. The model was successfully integrated into the PSI
treatment planning system.
It was found that the parameterizations for neutron energy, quality factors
and RBE were independent of proton energy in the investigated energy range
of interest for proton therapy. The pencil beam algorithm has been extended
using the developed parameterizations in order to calculate the neutron energy,
quality factor and RBE.