Neutron Production in The Scattering S250 Mevion Proton-Therapy System Nima Tatari

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Neutron Production in The Scattering S250

Mevion Proton-Therapy System

Nima Tatari

Washington University in St. Louis, Department of Physics

n.tatari@wustl.edu

November 16, 2022

Abstract

Purpose: Secondary radiation in a radiation therapy environment is considered a cause for post-treatment side eﬀects including

secondary carcinogenesis. In a proton-therapy setup, neutrons deliver a considerable extra dose to the patient. Various measurements

and calculations of neutron dose suggest a wide range of values for this quantity. Based on work with a Mevion S250 passive scatter-

ing proton-therapy system, this paper provides a comparison between the previously published values and analyzes the Monte Carlo

calculated neutron dose from the present work using the neutron cross-section data. The purpose is to provide a range for the conver-

gence of several neutron-absorbed dose data points (e.g. 3.94 mSv/Gy in a Bonner sphere measurement and 0.4 mSv/Gy in a SWENDI

measurement) in previous works of other authors.

Methods: The analysis is based on the neutron spectrum in the regions of thermal to evaporation energies. Data for the neutrons

is obtained by the Geant4 Monte Carlo simulation toolkit for particle transport. A water phantom of dimensions 35 ×35 ×35cm3is

positioned at the center of the treatment room (i.e. the isocenter) modeling the human’s body. The calculated spectra are compared to

the previous measurements. Tables of neutron cross-sections are provided to explain the spectral behavior of the neutrons.

Results: Several items in the room contribute diﬀerently to the dose absorbed by the human body. These are scored separately and

presented in pie charts. The calculated total amount of the absorbed neutron dose values are provided along and perpendicular to the

beamline, ranging from 1 to 20 mSv/Gy depending on how close to the neutron source the data is obtained.

Conclusions: Matching the calculated neutron dose data with the previously measured values within a range below 1% provides

further evidence for the accuracy of the measurement method, i.e. extended range Bonner sphere (ERBS). The percentage contribution

of the internally produced neutrons to the total absorbed dose was studied by varying the beam energy. We concluded that the reducible

part of the neutron dose absorbed to the body, i.e. the externally produced neutrons, comprises less than 50% of the total.

Keywords: Proton-therapy, Mevion, Secondary radiation, Neutron absorbed dose.

1 Introduction

A study of the neutron production in a single-room Mevion

S250 passive scattering proton-therapy system is performed. The

proton-therapy system has been in operation at Washington Uni-

versity in St. Louis since 2013 and was decommissioned in 2020.

Geant4 Monte Carlo toolkit has been used to calculate neutron

ﬂuence spectra from various nozzle components. Each ﬂuence

spectrum is discussed based on the cross-sections of the materi-

als contributing to secondary neutron production.

2 Materials and methods

Several parameters in the simulation consist of the geometrical

dimensions of the room, synchrocyclotron, and nozzle compo-

nents. The nozzle includes a ﬁrst scatterer, a range modulation

wheel, the main range shifter, a second scatterer, a beam collima-

tion system, a brass aperture, and a range compensator. A water

phantom models the patient’s body. The geometry and physics

models in the simulation are discussed in the following section.

2.1 Geometry

Modeling a geometry in Geant4 is based on the dimensions, com-

positions, and densities of the objects in the environment of ra-

diation transport. The treatment room, synchrocyclotron, beam-

delivery system, and water phantom deﬁne the geometry, each

part contributing to the total neutron production based on dif-

ferent physical processes. The neutrons from the room are cre-

ated by previously generated neutrons in the nozzle. The con-

tribution of the protons that are lost in the synchrocyclotron’s

extraction stage to the production of secondary neutrons is cal-

culated. Figures 1 (a,b,c) show the geometry of the room con-

taining the proton-therapy system. The whole vault is modeled

with a 13.5×15 m2ﬂoor and with a 15 m height.

Figure 1: a) 3D view, b) top view, and c) side view of the treatment room geom-

etry design in the Geant4 model of the Mevion S250 scattering proton system.

The components of the nozzle are shown in Figure 2. The ﬁrst

scatterer is positioned after the beam extraction point from the

synchrocyclotron.

The scattering proton machine is modeled with a primary

mono-energetic proton beam of 250 MeV and is spread laterally

to get an even dose throughout a speciﬁc area and depth. Rota-

arXiv:2210.12602v2 [physics.med-ph] 15 Nov 2022

Figure 2: Nozzle components in a Geant4 model of Mevion S250 scattering pro-

ton system.

tion of the RMW superposes the modulated proton beams with

diﬀerent energies. The result is a spread-out Bragg peak (SOBP),

with a uniform proton dose curve, inside the water phantom.

Broadening of the proton beam in the model is achieved

through the second scatterer. The unwanted part of the beam is

trimmed by the collimation system which is modeled by highly

attenuating materials such as stainless steel and brass with large

stopping power. The collimation system is modeled with a

stainless-steel cylinder with a 3 cm thickness, 9.75 cm inner ra-

dius, and 36 cm height. A similar structure with a thickness of 2

cm, 15 cm inner radius, and 52.5 cm height was used for the col-

limator with the same material. In addition to the outer cylinder,

the collimator contains two inner rings at its entrance and exit

sides. Each ring has an inner radius of 12.5 cm with the same

material as the outer collimator. A brass aperture with a thick-

ness of 6.84 cm is placed next to the collimator. The center of

the brass aperture contains a square opening.

During the actual treatment, the ﬁne-tuning of the output

beam energy is done by a patient-speciﬁc lucite compensator that

varies the depths of the beam at diﬀerent positions to shield the

healthy tissues from the proton beam. The compensator is mod-

eled with a lucite cylinder of 5 cm height placed as the last step

of the nozzle.

2.2 Geant4 Monte Carlo simulation toolkit

Geant4 (GEometry ANd Tracking) has been developed by CERN

as a general-purpose Monte Carlo (MC) simulation software for

particle transport through matter. Two fundamental aspects of

Geant4 are handling the geometrical shapes and tracking various

particles through objects made of diﬀerent materials. Detailed ra-

diation transport calculations are performed by the toolkit for that

purpose. The building blocks of Geant4 simulations are G4Step

objects from which information can be stored. Particle transport

involves a 3D simulation of the interactions of particles irradi-

ated through materials with speciﬁc geometries. In this work,

the primary particle source is protons.

3 Results and discussion

The proton PDDs and neutron spectra are benchmarked with

measured data. The neutron ﬂuence spectra calculated by Geant4

form the basis of the presented analysis. The properties of the

spectra are described based on the neutron cross-section data ob-

tained from ENDF datasets [1].

Protons form a well-deﬁned dose deposition peak as they

travel through matter, due to the stopping power. The simula-

tion PDD is obtained by emitting a primary proton beam with

250 MeV energy, 2 MeV energy bandwidth, and 1 mm spot size.

Figure 3: Matching the Geant4-calculated pristine percentage depth dose to the

experimental data. Both PDDs correspond to the ﬁrst step of the range mod-

ulation wheel. The benchmarked parameters include the thickness of the range

shifter (aﬀecting the range in water), the energy width of the primary proton beam

(aﬀecting the range straggling), and the spot size of the initial beam (aﬀecting the

entrance dose height).

Each range modulation step delivers a speciﬁc proton beam

energy to the isocenter with a corresponding pristine peak depth.

Rotation of the range modulation wheel leads to a simultane-

ous superposition of the proton beams with various energies, and

depending on the design of the angular extent of each step that

corresponds to their weights, leads to a spread-out Bragg peak or

SOBP.

The main characteristics of an SOBP are range and modula-

tion. The range is deﬁned as the distance from the surface of

the water phantom to the distal 90% proton dose. Modulation is

deﬁned as the distance from the proximal 95% dose to the distal

90% [2]. Speciﬁcally, for the treatment setup of the whole brain

the range of 17 cm and modulation of 16 cm has been used in the

work of Howell et al. [2]. The same SOBP has been simulated

in this work for benchmarking the neutron calculations.

Simulation of an SOBP requires a linear combination of sev-

eral pristine peaks with diﬀerent ranges. Each pristine peak is

obtained by positioning a step of the range modulation wheel in

the primary proton beam path. The ﬁnal SOBP has a 17 cm range

and 16 cm modulation. The ﬁrst 15 pristine peaks delivered by

the 15 are used as the base functions in a singular value decompo-

sition (SVD) method to obtain the composition weights. Figures

4 and 5 show the normalized pristine peaks and the SOBP with a

range of 17 cm, respectively.

Figure 4: Normalized proton pristine peaks corresponding to 15 steps of RMW.

To obtain the agreement between the transverse proton dose

proﬁles, the parameters that deﬁne the shape of the second scat-

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NeutronProductioninTheScatteringS250MevionProton-TherapySystemNimaTatariWashingtonUniversityinSt.Louis,DepartmentofPhysicsn.tatari@wustl.eduNovember16,2022AbstractPurpose:Secondaryradiationinaradiationtherapyenvironmentisconsideredacauseforpost-treatmentsideeectsincludingsecondarycarcinogenesis.Ina...

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Neutron Production in The Scattering S250 Mevion Proton-Therapy System Nima Tatari

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