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%.