Room: Karl Dean Ballroom A1
Purpose: To demonstrate the equivalence of the quality of ion chamber calibrations performed with beams from a cobalt-60 irradiator and research and clinical linear accelerators with provenance spanning decades.
Methods: The National Research Council of Canada has provided cobalt-60 absorbed dose ion chamber calibrations since the late 1990s. A similar service was established in 2007 for ion chamber calibrations using photon beams from the Elekta Precise linac, installed in 2002. These measurements have also been performed on a 1968 Vickerâ€™s research accelerator and an Elekta Synergy linac with an Agility multi-leaf collimator, installed at NRC in 2016. Calibrations are performed with chambers set up in a water phantom under reference conditions as specified by the TG-51 protocol. In linac beams, the transfer of absorbed dose calibration coefficients traceable to the primary standard water calorimeter is performed using secondary standard ion chambers with demonstrated stability at the 0.1 % level.
Results: The stability of linac output over the course of a two hour period directly impacts the ability to accurately transfer the standard and this stability is at the 0.1-0.2 % level for the three machines investigated. The stability of linac calibration coefficients is better than 0.15 % over a period of 10 years. The results obtained on all three accelerators are nearly identical and well within uncertainties when comparing absorbed dose calibration coefficients and beam quality conversion factors as a function of the beam quality specifier, %dd(10)x.
Conclusion: Traditional Co-60 sources used for absorbed dose calibration are all very similar and the equivalence of therapy-level irradiators has been demonstrated. However, the situation is not so clear for ionization chamber calibrations from a linear accelerator. This investigation establishes the long-term stability of linac-based calibrations and demonstrates the equivalence of calibrations obtained with three electron linear accelerators generations apart.