A Procedure to Verify the Accuracy of Delivery of Prescribed Radiation Doses in Radiotherapy
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In New Zealand there are currently no regular external audits to verify the full treatment chain in radiotherapy. This thesis reports on a project to devise such an audit procedure suitable to assess the accuracy of the delivery of prescribed radiotherapy doses to patients over the full treatment process. The National Radiation Laboratory (NRL), regulatory authority, will use the method developed to conduct biennial audits of all radiotherapy centres.
A commercial chest phantom with a MOSFET dosimetry system was provided for this project. The MOSFETs were commissioned and their characteristics determined, namely reproducibility, energy dependence and angular dependence. The MOSFETs were also tested in a clinical environment with the phantom. Measurements were carried out to test the MOSFET capabilities in both lung and soft tissue in the phantom. Two plans were devised for the audit process, a straightforward one with two parallel opposed beams and a more complex one involving lung tissue and wedges. These plans were designed to test the entire treatment planning and delivery process.
It was found that each MOSFET detector needed to be individually calibrated. Reproducibility was found to have an average standard deviation of 2% on standard sensitivity and 1.2% on high sensitivity. The angular dependence of the detectors showed that when the MOSFET was rotated by 90 degrees to the beam axis a drop in response of 3% was observed with 6 MV. The energy dependence factor was constant within uncertainty for all MOSFETs.
Overall, the MOSFET and phantom dosimetry system was determined to be suitable for the audit. The measurements with phantom showed that doses in high dose regions could be determined accurately. The greatest variation from the Treatment Planning system dose to the measured dose was 6%. The trial runs of the audit in two New Zealand radiotherapy centres showed that the procedure created is able to find discrepancies within the desired 5%, recommended by the ICRU, in the prescribed dose to the phantom.