Samenvatting

When treating cancer by radiation therapy it is always the goal to irradiate the tumour as much as possible and to spare the healthy tissue around it. For this a new type of radiation therapy called Volumetric Modulated ARC Therapy (VMAT) has been developed. During VMAT treatment the radiation machine rotates around the patient delivering focused beams of radiation to the tumour. Because the radiation can come from multiple angles the radiation dose to healthy tissue is reduced while the radiation dose to the tumour is increased. Because VMAT uses focused radiation bundles it is required to have detectors that can measure dose in a point. The diamond detector is a promising detector for this type of detection.
Before the detector can be used for measurements it has to be characterized. This includes characterisation for the dependence of the orientation of the detector, the temperature dependence of the detector, the energy dependence of the detector, the long-term stability of the detector and the pre-irradiation of the detector. For all characterisations except the characterisation for energy dependence the detector was radiated using a Cobalt-60 source.
The characterisation for the orientation of the detector was done by irradiating the detector under various angles. This characterisation is split into the axial angles and radial angles. The dependence of the angles has been investigated with the detector in air as well as the detector submerged in 5 cm of water. The characterisation for the temperature dependence of the detector has been determined by placing the detector in a phantom of water. The detector was radiated while in water of 10 °C, 20 °C and 30 °C. The energy dependence of the detector was determined by radiation the detector with an X-ray tube. The amount of charge measured per Gray of radiation was calculated for mean photon energies of 51 keV, 69 keV, 86 keV, 105 keV and 1,25 MeV. The long-term stability of the detector was investigated by performing the exact same measurement multiple times over the course of the internship and then comparing the results. The pre-irradiation dose required to stabilize the signal from the detector has been determined by not radiating the detector for multiple days and then doing a measurement without pre-irradiation. By calculating how long it takes to stabilize the signal from the detector the amount of Gray needed to pre-irradiate it can be calculated.
Measurements showed that the detector has a large dependence on orientation. Axial angle orientation showed a maximum of 1,4% deviation in measured dose in air and a maximum of 1% deviation in measured dose in water. The dependence on axial angle showed a maximum of 31,4% deviation in measured dose in air and 4,3% in water. Results show that the amount of dose measured deviates a maximum of 3% when the temperature of the water is 10 °C above or below the standard temperature of 20 °C.
It was determined that over the range of 51 keV to 1250 keV photon energy the response of the detector deviates 14%. Lastly the amount of pre-irradiation needed until the detector signal is stable was calculated. The average amount of dose needed to pre-irradiate the detector is 25 gray and with the Cobalt-60 source this equals 45 minutes of pre-irradiation.
The conclusion of the characterisation of the detector is that it can only be used for measurements if the medium around the detector provides enough build-up. When this is not the case the deviation of the response of the detector is too large.