The aim of the current work was to present the performance evaluation procedures implemented at our department for the commissioning of a G4 CyberKnife system. This system consists of a robotic manipulator, a target-locating system and a lightweight 6-MV linac. Individual quality assurance procedures were performed for each of the CyberKnife subsystems. The system was checked for the mechanical accuracy of its robotic manipulator. The performance of the target-locating system was evaluated in terms of mechanical accuracy of both cameras' alignment and quality assurance tests of the x-ray generators and the flat-panel detectors. The traditional linac 6-MV beam characteristics and beam output parameters were also measured. Results revealed a manipulator mechanical mean accuracy of approximately 0.1 mm, with individual maximum position uncertainties less than 0.25 mm. The target-locating system mechanical accuracy was found within the acceptance limits. For the most clinically used parameters in the CyberKnife practice, e.g. 100-120 kV and 50-200 ms, kV and exposure time accuracy error were measured as less than 2%, while the precision error of the kV was determined as less than 1%. The acquired images of the ETR grid pattern revealed no geometrical distortion while the critical frequency f50 values for cameras A and B were calculated as 1.5 lp mm(-1) and 1.4 lp mm(-1), respectively. Dose placement measurements were performed in a head and neck phantom. Results revealed sub-millimeter beam delivery precision whereas the total clinical accuracy of the system was measured equal to 0.44 +/- 0.12 mm, 0.29 +/- 0.10 mm and 0.53 +/- 0.16 mm for the skull, fiducial and Xsight spine tracking methods, respectively. The results of this work certify the G4 CyberKnife SRS system capable of delivering high dose distributions with sub-millimeter accuracy and precision to intracranial and extracranial lesions. Moreover, total clinical accuracy of the investigated G4 system was found to be improved for the skull and fiducial tracking methods and was comparable for Xsight spine tracking method compared with the earlier generation of the instrument.

A descriptive analysis was made in terms of the related radiation induced acute and late mucositis and xerostomia along with survival and tumor control rates (significance level at 0.016, bonferroni correction), for irradiation in head and neck carcinomas with either 2D Radiation Therapy (2DRT) and 3D conformal (3DCRT) or Intensity Modulated Radiation Therapy (IMRT). The mean score of grade>II xerostomia for IMRT versus 2-3D RT was 0.31 ± 0.23 and 0.56 ± 0.23, respectively (Mann Whitney,P<0.001). The parotid-dose for IMRT versus 2-3D RT was 29.56 ± 5.45 and 50.73 ± 6.79, respectively (Mann Whitney,P=0.016). The reported mean parotid-gland doses were significantly correlated with late xerostomia (spearman test, rho = 0.5013,P<0.001). A trend was noted for the superiority of IMRT concerning the acute oral mucositis. The 3-year overall survival for either IMRT or 2-3DRT was 89.5% and 82.7%, respectively (P=0.026, Kruskal-Wallis test). The mean 3-year locoregional control rate was 83.6% (range: 70–97%) and 74.4 (range: 61–82%), respectively (P=0.025, Kruskal-Wallis). In conclusion, no significant differences in terms of locoregional control, overall survival and acute mucositis could be noted, while late xerostomia is definitely higher in 2-3D RT versus IMRT. Patients with head and neck carcinoma should be referred preferably to IMRT techniques.