A Monte Carlo derived TG-51 equivalent calibration for helical tomotherapy
Helical tomotherapy (HT) requires a method of accurately determining the absorbed dose under reference conditions. In the AAPM's TG-51 external beam dosimetry protocol, the quality conversion factor, k Q, is presented as a function of the photon component of the percentage depth-dose at 10 cm depth, %dd(10) x, measured under the reference conditions of a 10 × 10 cm 2 field size and a source-to-surface distance (SSD) of 100 cm. The value of %dd(10) x from HT cannot be used for the determination of k Q because the design of the HT does not meet the following TG-51 reference conditions: (i) the field size and the practical SSD required by TG-51 are not obtainable and (ii) the absence of the flattening filter changes the beam quality thus affecting some components of k Q. The stopping power ratio is not affected because of its direct relationship to %dd(10) x. We derive a relationship for the Exradin A1SL ion chamber converting the %dd(10) x measured under HT "reference conditions" of SSD=85 cm and a 5 × 10 cm 2 field-size [%dd(10) x[HT Ref]], to the dosimetric equivalent value under for TG-51 reference conditions [%dd(10) x[HT TG-51]] for HT. This allows the determination of k Q under the HT reference conditions. The conversion results in changes of 0.1% in the value of k Q for our particular unit. The conversion relationship should also apply to other ion chambers with possible errors on the order of 0.1%.