AbstractID: 9654 Title: ’Extension of the AAPM TG-51 protocol to incorporate
non-reference calibration conditions’
The current calibration protocol AAPM TG-51 based on the absorbed dose requires calibration measurements under reference
conditions (10x10cm2 square field at 100cm source-to-surface distance). For some radiotherapy systems (e.g., helical tomotherapy,
radiosurgery using cobalt sources, robotic systems) this requirement cannot be met and calibration has to be performed under nonreference experimental conditions. In order to solve this problem, a general methodology is proposed by extending the existing AAPM
TG-51 protocol with inclusion of the measured-to-reference conversion factor, kmr. Calculation of this factor is dependent on the basic
dosimetric quantities, like stopping power ratios, mass attenuation coefficients and various chamber correction factors. Where
measurements are not feasible, accurate Monte Carlo modeling is required. The extension of the protocol is illustrated on the case of
the helical tomotherapy radiation unit, where the closest measurement conditions compared to the reference conditions are a 10x5cm2
field size at the 85cm source-to-surface distance. It was calculated that the kmr factor for this conditions is close to 1.000
(0.997±0.005). In addition, the deviation of the measurement conditions from the reference conditions results in the change of the
quality conversion factor kQ for approximately 0.995-0.998, depending on the ionization chamber used.
Research supported by TomoTherapy Inc.