Technical note: Consistency of IAEA's TRS-483 and AAPM's extended TG-51 protocols for clinical reference dosimetry of the CyberKnife M6 machine

BackgroundWhile IAEA's TRS-483 code of practice is adapted for the calibration of CyberKnife machines, AAPM's TG-51 is still the protocol recommended by the manufacturer for their calibration. The differences between both protocols could lead to differences in absorbed dose to water during the calibration process. PurposeThe aims of this work are to evaluate the difference resulting from the application of TG-51 (including the manufacturer's adaptations) and TRS-483 in terms of absorbed dose to water for a CyberKnife M6, and to evaluate the consistency of TRS-483. MethodsMeasurements are performed on a CyberKnife M6 unit under machine-specific reference conditions using a calibrated Exradin A12 ionization chamber. Monte Carlo (MC) simulations are performed to estimate kQmsr,Q0fmsr,fref$k_{Q_{\mathrm{msr}},Q_0}<^>{f_{\mathrm{msr}},f_{\mathrm{ref}}}$ and kvol$k_{\text{vol}}$ using a fully modeled detector and an optimized CyberKnife M6 beam model. The latter is also estimated experimentally. Differences between the adapted TG-51 and TRS-483 protocols are identified and their impact is quantified. ResultsWhen using an in-house experimentally-evaluated volume averaging correction factor, a difference of 0.11% in terms of absorbed dose to water per monitor unit is observed when applying both protocols. This disparity is solely associated to the difference in beam quality correction factor. If a generic volume averaging correction factor is used during the application of TRS-483, the difference in calibration increases to 0.14%. In both cases, the disparity is not statistically significant according to TRS-483's reported uncertainties on their beam quality correction factor (i.e., 1%). MC results lead to kQmsr,Q0fmsr,fref=1.0004 +/- 0.0002$k_{Q_{\mathrm{msr}},Q_0}<^>{f_{\mathrm{msr}},f_{\mathrm{ref}}}=1.0004\pm 0.0002$ and kvol=1.0072 +/- 0.0009$k_{\text{vol}}=1.0072\pm 0.0009$. Results illustrate that the generic beam quality correction factor provided in the TRS-483 might be overestimated by 0.36% compared to our specific model and that this overestimation could be due to the volume averaging component. ConclusionsFor clinical reference dosimetry of the CyberKnife M6, the application of TRS-483 is found to be consistent with TG-51.