Determination of the correction factors for different ionization chambers used for the calibration of the helical tomotherapy static beam

Background and purpose: To determine the machine-specific correction factors for three commercial ionization chambers (Exradin A1SL, PTW Semiflex and PTW PinPoint) to be applied in the dose calibration of a helical tomotherapy (HT) unit. Materials and methods: Machine-specific reference (msr) conditions for HT unit involved a 10 × 5 cm ² (IEC x × y) radiation field at 85 cm SSD with the ionization chamber (IC) positioned at 10 cm depth in Virtual Water. Each msr correction factor was determined using the formalism proposed by the AAPM Task Group 148 (i.e. k _{Q[HT TG-51]}) and was compared to direct calibration of the ICs against radiochromic films, calibrated in absolute dose at our institute, following the formalism proposed by the IAEA-AAPM joint committee (i.e. k ^Qmsr,Q ^fmsr, ^fref). Results: TPR _20,10 values under HT msr conditions were 0.608, 0.608 and 0.615 corresponding to %dd(10) _{x[HT Ref]} of 0.606, 0.606 and 0.613 for A1SL, Semiflex and PinPoint ICs, respectively. Based on these findings, k _{Q[HT TG-51]} determination resulted 0.998 ± 0.001, 0.998 ± 0.001 and 0.991 ± 0.002 for A1SL, Semiflex and PinPoint ICs, respectively. The k ^Qmsr,Q ^fmsr, ^fref correction factors obtained through direct calibration were 1.000 ± 0.010, 1.004 ± 0.007 and 0.998 ± 0.006 for A1SL, Semiflex and PinPoint ICs, respectively. Conclusions: Direct calibration of ICs versus radiochromic films provided correction factors equivalent to those obtained following the TG-148 formalism. This study showed that ICs calibrated for conventional linear accelerators can be used for the calibration of the static beam delivered by a HT unit taking into account the particular reference conditions.