Dose verification and calibration of the Cyberknife® by EPR/alanine dosimetry

Standard calibration protocols in radiotherapy (AAPM TG-51 and IAEA TRS 398) recommend a 10 × 10 cm2 irradiation field at a distance of 100 cm. However, the specific geometry of the Cyberknife machine enables only circular radiation beams with a maximum diameter of 6 cm field size at a distance of 80 cm from the source which prevents from a direct use of these protocols. Thereby, specific calibration and verification protocols are required.Within this framework, the Laboratoire National Henri Becquerel and the Centre Oscar-Lambret decided to study two points of interest: i) the feasibility of the calibration of the Cyberknife beam; ii) the verification of the delivered dose during a treatment with a very small diameter beam using Electron Paramagnetic Resonance (EPR)/alanine dosimetry. Alanine dosimeters are suitable for this work thanks to their small size, their integrating capability, and their low energy dependence as a function of both dose rate and beam energy in the radiotherapy dose range.The results of the dose rate calibration using EPR/alanine are very good, their deviations compared to the dose rate measured in the Cyberknife reference conditions are small (about 0.5%).The doses delivered during a treatment, measured within a 2% standard uncertainty, showed less than 4% deviation from the doses calculated by the treatment planning system. So, a rather good agreement is found between the measured and theoretically delivered doses. Nevertheless, the smaller the collimator diameter is, the larger the deviation is. This problem could be due to the evaluation of the output factor correction which is difficult to achieve for such small fields or to the Treatment Planning System algorithm itself and in particular the profile modelling.