Proton therapy quality assurance: characterisation of PIN diodes for neutron dosimetry by means of Geant4 simulations

The secondary neutron radiation field can produce a harmful effect and eventually a secondary cancer in patients undergoing proton therapy treatment. PIN silicon diodes are investigated at the Centre for Medical Radiation Physics (CMRP), of the University of Wollongong, to monitor the dosimetric effect of the neutron field associated with proton therapy. In a protontherapy passive scattering beam line, the final target-shaped collimator, a brass layer with thickness varying usually between 2 cm and 8 cm, produces an undesired neutron field, deriving from proton hadronic interactions, and incident on the patient. In order to calibrate PIN diodes as neutron radiation monitoring detectors for proton therapy quality assurance, it is necessary to characterise the response of the detector in a phantom, varying the thickness of the brass layer. The aim of this thesis is to characterise the response of the PIN diode in a proton field typical of prostate cancer treatment, varying the brass layer thickness. The project has been done by means of Geant4 simulations. The response has been characterised in-field, for an incident pencil proton beam with energy 190 MeV, along the beam central axis, at different depths along the Bragg Peak, with a brass layer thickness varying between 0 cm and approximately 4 cm. The results of this work show that increasing the brass layer thickness reduces the depth of the Bragg Peak in the phantom as expected. Increasing the brass layer thickness produces a higher number of neutrons incident on the patient. The numbers of neutrons then decreases with depth on the central axis of the proton beam due to elastic and inelastic scattering, and capture reactions.The response of the PIN diode is dominated by the proton beam in-field as expected up to the Bragg Peak.