Assessment of secondary neutrons in particle therapy by Monte Carlo simulations

Abstract

Objective: The purpose of this study is to estimate the energy and angular distribution of secondary neutrons inside a phantom in hadron therapy, which will support decisions on detector choice and experimental setup design for in-phantom secondary neutron measurements. Approach: Dedicated Monte Carlo simulations were implemented, considering clinically relevant energies of protons, helium and carbon ions. Since scored quantities can vary from different radiation transport models, the codes FLUKA, TOPAS and MCNP were used. The geometry of an active scanning beam delivery system for heavy ion treatment was implemented, and simulations of pristine and spread-out Bragg peaks were carried out. Previous studies, focused on specific ion types or single energies, are qualitatively in agreement with the obtained results. Main results: The secondary neutrons energy distributions present a continuous spectrum with two peaks, one centred on the thermal/epithermal region, and one on the high-energy region, with the most probable energy ranging from 19 MeV up to 240 MeV, depending on the ion type and its initial energy. The simulations show that the secondary neutron energies may exceed 400 MeV and, therefore, suitable neutron detectors for this energy range shall be needed. Additionally, the angular distribution of the low energy neutrons is quite isotropic, whereas the fast/relativistic neutrons are mainly scattered in the down-stream direction. Significance: It would be possible to minimize the influence of the heavy ions when measuring the neutron-generated recoil protons by selecting appropriate measurement positions within the phantom. Although there are discrepancies among the three Monte Carlo codes, the results agree qualitatively and in order of magnitude, being sufficient to support further investigations with the ultimate goal of mapping the secondary neutron doses both in- and out-of-field in hadrontherapy. The obtained secondary neutron spectra are available as supplementary material.

A Monte Carlo study on the secondary neutron generation by oxygen ion beams for radiotherapy and its comparison to lighter ions

“…The values obtained for the most probable energies in the TE and FR regions are reported in table 1, along with the maximum energy of the FR region of each spectrum computed as the energy where the intensity drops to < 1% of the maximum, and the total fluences for the TE and FR regions. As reported, the value of the maximum energy increases when increasing the energy of the primary beam, easily reaching or even surpassing the value of the initial energy per nucleon of the primary ions, which was already observed in the case of helium and carbon ions in the previous study (Vedelago et al 2022).…”

“…To reproduce the location z max of the maximum of the Bragg peaks in the previous study (Vedelago et al 2022), oxygen ions 16 O with initial energies of {222.51; 332.52; 514.82} MeV u −1 corresponding to » z max {7.5; 15.0; 30.0} cm were simulated. The depth-dose profiles were computed by subdividing the water phantom in slabs of 45.0 × 45.0 × 0.1 cm 3 and integrating the absorbed dose in them.…”

“…The geometry and simulation parameters used are based on a previous study (Vedelago et al 2022). For completeness and self-consistency, they are briefly described here.…”

“…Different radiation transport and nuclear interaction models available in the MC codes play a key role, since changing models or comparing different MC codes may deliver significantly different results (De Saint-Hubert et al 2022, Griffin et al 2022). Even with differences in the absorbed dose below 0.5% and 1.6% respectively for pristine and spread-out Bragg peaks (SOBP), differences of up to 44% and 64% were observed in the fast/ relativistic (FR) region of the secondary neutron spectra (Vedelago et al 2022). When considering the thermal/ epithermal (TE) region as well as in the FR region, differences of up to 57% in the total neutron production yield were reported (Robert et al 2013).…”

Monte-Carlo techniques for radiotherapy applications II: equipment and source modelling, dose calculations and radiobiology

“…Monte-Carlo techniques have been extensively used to model clinical proton and heavy ion beams and their interactions and deposition of dose in patient geometries. [30][31][32][33] Zarifi et al used the GATE code to study the depth dose characteristics of mono-energetic proton pencil beams of energies 5-250 MeV in water and obtain the energy-range relationship. Further, the stopping powers of the proton pencil beams in a water phantom were compared to data from the NIST standard reference database.…”