Project in PHYS291

Detection of secondary particles in proton therapy using Monte Carlo simulations

Eidi Helland - spring 2019

Introduction

- Project description

For my project work I am going to make a program that collects large data files generated from Monte Carlo simulations in FLUKA. The simulations consist of a proton beam with energies of 100, 160, 200 and 230 MeV interacting with a water phantom. From these interactions, secondary particles (such as neutron, photons and protons) are produced. In order to detect these particles and their quantities, scoring detectors are implemented in the simulations. The design used in the simulations is shown in Figure 1. The results will be used to analyse some quantities (production, energy and angular distribution) for secondary particles. The secondary particles that will be investigated in this project are neutrons and photons. The large data files that are generated will be analysed and visualized through ROOT.

Figure 1: Design of the set up with the water phantom implemented in Monte Carlo Simulations.
The figure illustrates that secondary neutrons produced along the beam in the water phantom can be
converted to protons through a converter(20cm x 20cm). The converter is followed up by two scoring detectors.

- Theory

Radiotherapy is an important method in cancer treatment with the aim to irradiate and kill cancer cells through energy deposition, while minimizing the dose to healthy tissue. With proton therapy, a large fraction of the energy is deposited right before the end of the range, resulting in a so-called Bragg peak. If there are uncertainties regarding the range and one cannot predict where the proton beam will stop, severe dose deposition may occur. This can lead to much higher dose to healthy tissue and the tumour not receiving required dose. In order to control the proton beam range, detection of produced secondary particles have been suggested and developed. When a proton beam travel through a medium, secondary particles are produced through inelastic collision with the nuclei, if the primary protons have enough energy. By detecting these secondary neutrons, range verification for the proton beam may be accomplished. This is because the production of secondary neutrons shows a correlation with the particle range. However, other secondary particles (like photons) may interfere with the measurements of the neutrons.

Method (the code)

In the simulations, the number of primary protons were set to 1E8 for both the production and energy distribution. The simulations for the energy distribution is in the boundary between the area before the converter (air) and the converter (see Figure 1). For the angular distribution, the number of primary protons were set to 5E5.

The scripts used for this project are:
- Neutron production as a funcion of depth
- Energy distribution for produced neutrons
- Angular distribution for produced neutrons

- Photon production as a function of depth
- Energy distribution for produced photons
- Angular distribution for produced photons


To see the input files used in the scripts, click her.
The zip file for all the scripts can be found here.

Results

- Production distribution of the particles as a function of depth

Plot 1 shows the production as a function of depth for neutrons, while plot 2 shows the production for photons.
Plot 1: Neutron fluence per primary proton as a function of depth for the four different initial energies.


Plot 2: Photon fluence per primary proton as a function of depth for the four different initial energies.


- Energy distribution of the particles produced

Plot 3 shows the energy distribution for neutrons produced and plot 4 shows the energy distribution for photons.
Plot 3: Neutron fluence per primary proton as a function of energy. The energy distribution for the produced neutrons is illustrated for the four different inital energies. The plot on the right has a logarithmic scale in order to show the lower energy range better.


Plot 4: Photon fluence per primary proton as a function of energy. The energy distribution for the produced photons is illustrated for the four diffeent initial energies.


- Angular distribution of the particles produced

Plot 5 shows the angular distribution for neutrons produced in the water phantom, while plot 6 shows the angular distribution for photons.
Plot 5: Angular distribution for neutrons produced in the water phantom for the four different energies. Direction cosines are shown for the direction along the beam axis (x, solid line) and for the y- and z-axes (overlapping dashed and dotted lines, respectively), perpendicular to the direction of the inital beam.


Plot 6: Angular distribution for photons produced in the water phantom for the four different energies. Direction cosines are shown for the direction along the beam axis (x, solid line) and for y- and z-axes (dashed and dotted lines, respectively), perpendicular to the direction of the inital beam. The lines are frequently overlapping.


Discussion/conclusion

The plots for the production distribution shows that more neutrons are produced than photons. This can be benefical for detection of neutrons. Yet, there are produced photons that may interfere with the neutron detection. The production for both particles depends on the inital energy of the proton beam. For neutrons the maximum fluence is around 5 cm for 100 MeV, 12 cm for 160 MeV, 18 cm for 200 MeV and 22 cm for 230 MeV. For photons the maximum production is 8 cm for 100 MeV, 15 cm for 160 MeV, 21 cm for 200 MeV and around 25 cm for 230 MeV.

The energy distribution for neutrons illustrates that most of the produced neutrons are at the lower energy range. The neutron fluence decreases for higher energy range. For photons, the energy distribution is between 5.5 and 16 MeV, where the maximum fluence has energies around 7 MeV. The energy distribution for both neutrons and photons are depending on the initial energy of the proton beam. However, the dependence of initial energy seems greater for neutrons. This is because the energy distribution for photons at all energies (except from 100 MeV) are somewhat overlapping (see plot 4).

The angular distribution for neutrons illustrates that most of the neutrons are emitted in forward direction (x-direction). For the directions perpendicular to the proton beam (y- and z direction), the angular distribution were symmetric. For photons, the angular distribution illustrates that the photons produced in the water phantom are emitted somewhat uniformly in all directions. The number of particles increases with the energy of the primary proton beam.

References

Smeland Ytre-Hauge, K., et al,. The NOVO project neutrons detection for real-time range verification in proton therapy, A Monte Carlo feasibility study, 2018.

Acknowledgements

I would like to thank Boris Wagner and Ladislav Kockback for their help and tips for making this project possible. I would also like to thank Kristian Smeland Ytre-Hauge for helping me create this project and his guidance.