Pulse-Resolved Dosimetry Using CVD Diamond Detectors and Applications in Advanced Radiotherapy Treatment

A pulse-by-pulse dosimetry system based on diamond material was developed for the purpose of point dose measurements that require high spatial and temporal dose resolutions during VMAT technique delivery. In collaboration with the University of Bristol, a front-end electrometer with a high sample-rate was developed to measure individual pulses generated from the therapeutic linear accelerator (LINAC).One of the main drawbacks of using diamond detectors in the dosimetry field is the non-linear characteristic of the diamond response against the radiation dose-rate (∆). The dose-rate dependency of several diamond detectors was investigated, including the commercial diamond detector known as microDiamond and a purchased prototype diamond detector known as DD4. Two methods were employed in varying the dose-rate, either by varying the pulse-rate of the LINAC or varying the distance between the radiation source and the detector. The ∆ values obtained for the microDiamond detector were 0.9985 ± 0.0005 and 0.9870 ± 0.0024 based on the first and second method, respectively. For the DD4 detector, the ∆ value obtained based on the second method was 0.996 ± 0.025, which was close to the value of 0.99 ± 0.03 reported in previous work for DD4.The dosimetry system based on LINAC has successfully measured the dose for each pulse up to the rate of 400 Hz. The signal response from the DD4 detector was analysed and the models of the dose deposited were reconstructed based on the data obtained from the treatment planning system (TPS) and the LINAC logfile. Both models indicate dose differences of less than 0.5 % compared to the measurement. Cumulative dose as a function of time and gantry angle-to-agreement (∆t/∆g) were evaluated using a gamma-like analysis to determine the percentage of gamma passing rates (%GP). The %GP for cumulative dose evaluation against the time of 5%/1s was 100 % for both TPS and LINAC logfile compared to the measurement, and above 95 % with 3 %/1s criterion for both evaluated parameters. The %GP for cumulative dose evaluation against the gantry angle with 5 %/40 and 3 %/40 achieved above 95 %for both comparisons. The evaluation was also conducted for the instantaneous dose but with a high failure in %GP. With improvements in the front-end and using a newly installed Elekta Agility, a treatment plan was created for an anonymised prostate cancer patient using VMAT technique delivery in a phantom. The signal response from the DD4 detector was analysed using a more sophisticated method where the signal response was separated from the background (BG) signal for individual pulses. The result of the investigation shows the change in BG as a function of the pulse repetition frequency (PRF). The analysis also revealed that the average dose per pulse increases at higher PRF. A comparison of the cumulative dose measured by the DD4 detector and the commercial independent dose calculation tool known as RadCalc reveals the dose per control point (CP). Gamma-like analysis showed that there was an excellent agreement between the measured and intended cumulative dose delivered with %GP, was 100 % for a 1 %/2s criterion.