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This work investigated the absorbed dose to water rate under reference conditions in a Cyberknife VSI system using radiochromic films EBT3 and MD-V3 and three ionization chambers: an Exradin A12 and two FC65P Welhöfer Scanditronix with different serial numbers. The correction factor, , ,was studied using a Varian iX linac and the Cyberknife system. The measurements in the Varian iX were performed in a 10 × 10 cm 2 field, 10 cm depth in liquid water at 90 cm and 70 cm SSD and in a 5.4 × 5.4 cm 2 field, 10 cm depth at 70 cm SSD to simulate the Cyberknife conditions. In the Cyberknife system, measurements were performed using ionization chambers and both film types at 70 cm SSD and 10 cm depth in its 6 cm diameter reference field.The results indicate that , ,is independent of the dosimeters and the evaluation methods. Maximum differences of 0.22% -0.55% (combined uncertainties of 1.22% -1.98%, k = 1) are obtained on , ,whereas discrepancies of 2.08% -2.09% (combined uncertainties of 1.87% -2.13%, k = 1) are observed using the Cyberknife system. Given the agreement between detectors and the combined standard uncertainties, the data from Varian iX could be considered the most accurate and consequently a weighted average factor of 0.902 ± 0.006 could be used for the Cyberknife VSI system reference field. Within measurement uncertainties, the absorbed dose rate measured in the Cyberknife VSI system reference field was found to be independent of the dosimeters used. These results suggest that the absorbed dose measured at a point within a given field size should be the same, regardless the dosimeter used, if their dosimetric characteristics are well known. This highlighted the importance of performing dosimetry by controlling all parameters that could affect the dosimeter response. One can conclude that radiochromic

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Purpose: This paper aims to measure the absorbed‐dose‐rate in a CyberKnife unit reference‐field (6cm diameter) using three ionization chambers (IC) following the new IAEA/AAPM formalism and Gafchromic film (MD‐V3‐55 and EBT3) protocol according to our work reported previously. Methods: The absorbed‐dose‐rates were measured at 90cm and 70cm SSD in a 10cmx10cm field and at 70cm SSD in a 5.4cmx5.4cm equivalent to 6cm diameter field using a linac Varian iX. All measurements were performed at 10cm depth in water. The correction factors that account for the difference between the IC response on the reference field and the CyberKnife reference field, k_(Q_msr,Q)^(f_msr,f_ref), were evaluated and Gafchromic film were calibrated using the results obtained above. Under the CyberKnife reference conditions, the factors were used to measure the absorbed‐dose‐rate with IC according to the new formalism and the calibrated film was irradiated in water. The film calibration curve was used to evaluate the absorbed‐dose‐rate in the CyberKnife unit. Results: Difference up to 2.56% is observed between dose‐rate measured with IC in the reference 10cmx10cm field, depending where the chamber was calibrated, which was not reflected in the correction factor k_(Q_msr,Q)^(f_msr,f_ref) where variations of ~0.15%‐0.5% were obtained. Within measurements uncertainties, maximum difference of 1.8% on the absorbed‐dose‐rate in the CyberKnife reference field is observed between all IC and the films Conclusion: Absorbed‐dose‐rate to water was measured in a CyberKnife reference field with acceptable accuracy (combined uncertainties ~1.32%‐1.73%, k=1) using three IC and films. The MD‐V3‐55 film as well as the new IAEA/AAPM formalism can be considered as a suitable dosimetric method to measure absorbed‐dose‐rate to water in small and non‐standard CyberKnife fields used in clinical treatments However, the EBT3 film is not appropriated due to the high uncertainty provided (combined uncertainty ~9%, k=1) This work is partially supported by Conacyt grant 127409 and PAPIITUNAM grant IN10581