Monte Carlo simulations of neutron based in-vivo range verification in proton therapy – characterstics of proton-induced secondary particles
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Radiation therapy is an important treatment modality within cancer treatment, with the aim to deliver a high dose to the target volume while sparing the surrounding tissue. Protons have favourable characteristics of depositing the treatment dose more conformal compared to conventional photon therapy, motivating further development and improvement within proton therapy. Despite the beneficial qualities of proton radiation, uncertainties in the proton beam range prevents full exploitation of proton therapy’s potential to reduce dose to healthy tissues compared to photon therapy. Detection of secondary neutrons created in the patient through nuclear interactions, has been proposed as a method for monitoring the proton beam range during treatment, as there has been shown a correlation between the spatial neutron production distribution and the beam range. However, other generated secondaries may interfere with the secondary neutron detection. The amount and distribution of different secondaries which may reach the proposed neutron detector has not yet been investigated. The overall objective of this thesis was therefore to use Monte Carlo (MC) simulations to quantify the production of secondary radiation, including protons, prompt gamma-rays, neutrons and alpha particles. This knowledge is essential for estimating interference from the different secondary particle species on the measurements of secondary neutrons for the purposes of range monitoring. The production of secondary radiation was examined for two cases: a water phantom and a clinical treatment plan for a patient. MC simulations were conducted with the FLUKA MC simulation package. The water phantom was simulated with clinically relevant monoenergetic proton beams of 100, 160, 200 and 230 MeV, and the clinical treatment plan imported to FLUKA had beam energies between 93-197 MeV. A hydrogen-rich material, converting neutrons to protons, and two particle detectors were implemented in FLUKA. A clear resemblance between the results from the water phantom and the patient treatment plan was seen. The results indicated that secondary protons had the highest production rate, followed by secondary neutrons and prompt gamma-rays. Alpha particles were observed to have little potential relevance on the neutron measurements. Towards the converter surface, secondary protons had the highest reduction compared to the production rate and prompt gamma-rays and neutrons were the dominating secondaries at the converter surface. The proportion of protons reaching the converter and their energy was found sensitive to the phantom size, indicating that the ratio between the secondaries can vary for each patient case, with more protons reaching the converter for smaller patients. Possible shielding methods in order to reduce potential noise from unwanted secondary radiation in the detector can be adding a layer of lead or tungsten prior to the converter surface, in order to stop and absorb protons and to some degree gamma-rays, while secondary neutrons can traverse and convert to detectable protons.
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