A New Method for the Measurement of Reference Dosimetric Quantities in Electron Beams
B. R. Muir1*, M. R. McEwen2, D. W. O. Rogers1, (1) Carleton University, Ottawa, ON, (2) National Research Council, Ottawa, ONTU-A-BRB-11 Tuesday 8:00:00 AM - 9:55:00 AM Room: Ballroom B
Purpose: To present and confirm the accuracy of a new method to extract data for reference dosimetry in electron beams from depth-ionization measurements.
Methods: Depth-ionization curves are measured in water using NRC reference ion chambers and several plane-parallel chambers in 4 to 18 MeV electron beams from the Elekta Precise clinical linear accelerator. To extract useful information from depth-ionization results, the accuracy of setting the chamber position must be established. For this purpose, differences in the depth at which the chamber's ionization reading falls to 50% of its maximum value are used to derive EPOM values relative to those of reference chambers which have been previously established (Lacroix et al., Med. Phys. 37 (2010) 4331). After validation of positioning accuracy, relative ion chamber perturbation factors are determined using the ratio of readings to those of reference ion chambers as a function of mean electron energy, which changes smoothly with depth.
Results: Repeatable results for EPOM at the 0.1 mm level indicate little uncertainty from positioning. Different chambers of the same type give EPOM results that differ by less than 0.2 mm, suggesting insignificant chamber-to-chamber variability. Values of EPOM at I50 determined for 4 to 18 MeV beams do not exhibit significant variation with beam energy. For well-behaved chambers, relative perturbation factors at the same mean electron energy are consistent regardless of the incident beam energy. Relative perturbation factors for the same chambers repeated using a different reference chamber after several days are unchanged within 0.3%. Relative perturbation factors for the NACP and Roos chambers agree with published data.
Conclusions: These measurements show promising results for the determination of reference dosimetry data from depth-ionization curves. Ultimately, these relative measurements will be compared against the NRC primary standard calorimeter to get absolute perturbation factors that will prove valuable for updated dosimetry protocols.