Room: Exhibit Hall | Forum 4
Purpose: To develop a mailed audit system for external beam radiation therapy, traceable to the Canadian National Metrology Institute, with an overall uncertainty comparable to calibrated secondary standard reference chambers. The program will serve as an independent check of dose delivery in megavoltage photon and electron beams, for both standard (i.e., TG-51) and non-standard fields (e.g. IAEA TRS-483).
Methods: Alanine was chosen as the dosimeter to be used for the audit system. Influence quantities and uncertainty components related to alanine dosimetry were systematically investigated, including dose linearity, energy dependence, environmental sensitivity, and signal extraction at low doses.
Results: The biggest challenge was signal-to-noise at doses below 5 Gy and this was addressed by using a template based subtraction algorithm to remove unwanted base-line components in the low dose domain and by permanently ï¬?xing a reference sample to the cavity for signal normalization to account for environmentally induced signal changes. Linearity at the Â± 0.2% level was observed for doses in the range 5-100 Gy. The energy dependence for MV photon beams relative to Co-60 was measured and found to be consistent with literature data. A hermitically-sealed dosimeter holder was developed to mitigate the effect of environmental changes during a transport and irradiation. Trial audits were carried out in clinical 6 MV and 10 MV beams with an overall measurement uncertainty of 0.7 %. Some anomalies were observed below 5 Gy but above this limit, the agreement between clinically stated dose and that measured using alanine dosimeters was better than 0.2%.
Conclusion: The work to date has successful confirmed the use of alanine as a mailed audit dosimeter for external beam radiotherapy and work is continuing to develop and validate a national audit system.