Stability results over one year period for UNIQUE and Clinac iX. Error bars refer to one standard deviation. A) Output (weekly check): percentage dose deviation from reference. At week n.42, machine output was re-tuned for both machines according to institutional protocols. B) MU stability (monthly check): ion chamber reading ratio for a fixed number of MU (100) between delivery at a fixed dose rate (100, 200, 300, 400, 500, 600 MU/min) and reference reading at 300 MU/min. C) MU linearity (monthly check): ratio between ion chamber reading at fixed number of MU (5, 10, 50, 100, 200, 300 MU) with respect to the same for reference of 100 MU. Measurements are relative to a fixed dose rate of 300 MU/min.