Year

2011

Degree Name

Masters of Research - Science

Department

Centre for Medical Radiation Physics - Faculty of Engineering

Abstract

Proton therapy boasts numerous advantages over photon techniques including highly conformal dose distribution with a smaller integral dose, a reduction of dose to normal tissue and no exit dose. However, despite these positive properties, protons have the ability to produce unwanted dose outside the primary radiation eld that has the potential to produce secondary cancer in the patient. This undesired dose is a result of primary particle interactions (either with beam modifying devices or inside the patient) producing secondary particles such as neutrons. Secondary neutrons pose a threat to patients as it provides additional dose outside the treatment site. Nowadays there is a strong need for a detector that is capable of distinguishing the dose delivered by the primary proton beam and the associated secondary neutron eld. Therefore an investigation of a detector system that can classify particles in mixed radiation elds is very benefcial for out-of-field proton therapy studies.

This thesis evaluates the suitability of sicicon PIN diodes and ΔE-E detectors for use in proton therapy measurements by means of dedicated Geant4 simulations.

In the GEANT4 simulation study a 150 MeV proton beam was modelled nor- mally incident upon a lucite phantom. The physics interactions used in the simulations were chosen based on previous work by Jarlskog and Paganetti [50], which evaluated the most suitable Geant4 physics models for Monte Carlo simulations of proton therapy.

Monte Carlo simulations of silicon PIN diodes were performed to observe the behaviour of secondary neutrons out-of-field originating from the primary protons beam. In particular such studies were performed to calculated displacement Kinetic Energy Released per unit Mass (KERMA) as a function of the distance from the edge of the primary field. The displacement KERMA is determined by investigating the non-ionising energy loss (NIEL) response of the silicon detectors outside the primary field.

The shape of the spectra detailing the secondary neutron energy fluence remains the same for all three lateral detector positions. The fluence and energy of the secondary neutorns was seen to decrease as the distance from the incident beam increases. Displacement KERMA was also found to reduce with increasing distance from the field edge. The enery of the secondary neutrons decreases, which should ultimately result in an increased displacement KERMA. Instead the displacement KERMA decreases as a consequence of the diminishing neurton fluence out of field.

A monolithic silicon ΔE-E telescope detector was also investigated in this thesis. This detector system offers crucial two-dimensional information on the linear energy transfer (LET) of the particle. It also provides identi cation of particles based on energy deposition collected in coincidence throughout the ΔE and E stages of the detector, giving the ΔE-E telescope detector an advantage over other detectors. This detector is ideally implemented in proton therapy as it has the unique ability to identify the different components of a mixed hadronic field. This is particularly important when studying the risk of second cancer in tissue and organs that lie outside the gross treatment volume. This thesis aimed to validate the GEANT4 application by performing preliminary simulations and qualitatively comparing the results with experimental results carried out at Loma Linda Medical Centre USA by A. Wroe.

Results obtained by the Monte Carlo simulations show high energy deposition around the area of the Bragg Peak resulting from incident particles possessing an increased energy transfer in this region. The coincidence plots produced also display an area of low energy secondary neutrons that are a result of primary interactions within the lucite phantom.

This work shows a good qualitative agreement between results of the GEANT4 simulation and the reference results. A quantitative analysis goes beyond the scope of this thesis and is recommended to be performed in the near future.

The validation work, stated in this thesis, will allow quantifying the accuracy of the results deriving from the simulation. It will also allow use of this GEANT4 application to characterise the behaviour of the ΔE-E telescope out-of- eld, as this have never before been done

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