Non-clinical medical linear accelerator for dosimetry research.
The Varian Truebeam is a state-of-the are medical linear accelerator. At Danish hospitals, similar accelerators are used for treatment of cancer patients (external beam radiotherapy). The Truebeam at DTU Health Tech is only for non-clinical work (i.e. no patients are treated here). We use the accelerator for dosimetry research and calibrations.
The rig used for irradiation of ionization chambers and other dosimeters in MV photon beams. The Ionization chamber position can be moved using and a motorized xyz stage with sub-micron resolution, The control box and its joystick is shown to the right in the photo above. The small rack to the right also contains a Keithley 2700 ohm meter for reading 8 thermistors used in the set-up plus a reference resistor.
Accelerator model: Truebeam from Varian Medical Systems.
Version: 2.0
Photon energies with flattening filter: 4, 6, 10, 15, and 18 MV
Photon energies without flattening filter (FFF): 6 and 10 MV
Electron energies: 6, 9, 12, 15, 16, 18, and 20 MeV
MLC: Millennium 120 leaf MLC (40 center pairs of 5 mm width, and 20 peripheral pairs of 10 mm width).
The accelerator iso-center is found before calibrations, and the distance from a reference plate is measured. The iso-center is transferred to the calibration rig using optical methods.
Calibrations are carried out with the water tank front surface 100.0 mm from the iso-center. This distance is realized as shown below using a calibrated 100.0 mm gauge block. One end of the block is positioned at the optical iso-center using an xyz motorized stage.The other end of the block is used for zeroing an indicator dial mounted on the accelerator head. When the water tank later is brought into position (see photos below), the water tank distance is then adjusted such that the indicatior once again shows zero. We therefore know that the water tank is at a 100.0 mm distance from the iso-center. A small adjustment can be applied, if a density-equivalent distance of 10 g/cm2 is required.
For TPR20.10 measurements, we use a 200.0 mm gauge block. All linac calibrations are made relative to an external monitor chamber (PTW 7862) mounted in a special fixture on the accelerator head. The temperature of the monitor camber is measured using thermistors on both sides of the chamber.
The water tank surface has here been adjusted just that the indicator dial is back at zero.
Ionization chambers can be positioned with high precision at the iso-center line with or without sleeves. The two black manual xy-stages shown in the photo below contain cross-hair that have been adjusted to be on the iso-center line at the start of the calibration session. During the calibration session, these two cross-hairs facilitate verification that the Leica NA724 telescope remains on the optical iso-center.
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