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, kQ, 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 x 10 cm2 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 kQ 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 kQ. 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 x 10 cm2 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 kQ under the HT reference conditions. The conversion results in changes of 0.1% in the value of kQ for our particular unit. The conversion relationship should also apply to other ion chambers with possible errors on the order of 0.1%.