Note: Gating Characteristics of an Elekta Radiotherapy Treatment Unit Measured with Three Types of Detector Short title: Gating Characteristics of an Elekta Radiotherapy Treatment Unit

The characteristics of an Elekta Precise treatment machine with a gating interface were investigated. Three detectors were used: a Farmer ionisation chamber; a MatriXX ionisation chamber array and an in-house, single pulse-measurement ionisation chamber (IVC). Measurements were made of dosimetric accuracy, flatness and symmetry characteristics and duty cycle for a range of beam-on times and gating periods. Results were compared with a standard un-gated delivery as a reference. For all beam-on times, down to 0.5 s, dosimetric differences were below ±1 % and flatness and symmetry parameter variations were below ±1.5 %. For the shorter beam-on times the in-house detector deviated from the other two detectors, suggesting that this device should be used in conjunction with other detectors for absolute dosimetry purposes. However it was found to be useful for studying gated beam characteristics pulse by pulse.