Varian TrueBeam

Varian TrueBeam STx Linear Accelerator

Varian TrueBeam STx Linear Accelerator during commissioning phase

TrueBeam is the new model of medical linear accelerator from Varian Medical Systems. It was cleared by the FDA in December 2009. There was operating prototype at Memorial Sloan-Kettering Cancer Center. The installations of TrueBeam around US started in Spring of 2010.

There are two varieties of this linac – TrueBeam and TrueBeam STx. The STx model is intended for stereotactic use and has HD MLC. The older model of SRS linac is Novalis Tx.

A new feature for TrueBeam is the availability of the so-called FFF (Flattening Filter Free) mode. The photon beams usually have a metal filter on their path to make photon fluence uniform (or flat). With proliferation of accurate treatment planning algorithms and Intensity-Modulated Radiation Therapy (IMRT) the uniform fluence is not a concern. When the filter is removed from the photon beam, the intensity increases by a factor of 2 for 6 MV photons and by a factor of 4 for 10 MV photons.

The Clinac models have a maximum dose rate of 6 Gy/min and Novalis models 10 Gy/min. The maximum dose rate for TrueBeam in FFF mode are 14 Gy/min for 6 MV and 24 Gy/min for 10 MV beams. The filtered modes of TrueBeam still have 6 Gy/min maximum dose rate.

Linear accelerators produce radiation in pulses, up to 360 beam pulses per second. The FFF mode does not change the number of pulses, but the dose per pulse increases significantly. As a result, a special attention should be paid to ion chamber collection efficiency during commissioning. The table below shows dose per pulse for TrueBeam photon beams.

Nominal Photon Beam Energy Maximum Dose Rate (Gy/min) Dose per beam pulse (mGy/pulse)
6X 6 0.28
10X 6 0.28
15X 6 0.56
6X FFF 14 0.65
10X FFF 24 1.11

To access ion chamber collection efficiency, it is convenient to have a water phantom scanning system, such as IBA Dosimetry BluePhantom that allows to set different bias voltages for signal and reference chamber.

TrueBeam Specifications

Photon energies with flattening filter: 6, 10, 15
Photon energies in flattening-filter-free (FFF) mode: 6, 10
Electron Energies: 6, 9, 12, 16, 20
Max dose rates (MU/min) 6, 10, 15 – 600, 6 FFF – 1400, 10 FFF – 2400, electrons – 1000

Cone-beam CT (CBCT)
kV Imaging

kV X-Ray Tube (GS-1542) Performance Specifications

  • Focal spot size: small 0.4 mm; large 1.0 mm
  • kV rating: maximal 18 kW for small filament; maximal 57.5 kW for large filament
  • Inherent filtration: 2.7 mm on the exit window of the kV tube

Electronic imaging device (EPID)

  • Detector area and pixel pitch. In the imaging plane of a detector, active detector dimensions are 30.1 cm x 40.1 cm, and pixel pitch in each dimension 0.392 mm x 0.392 mm
  • Field of view at isocenter: 30.1 cm x 40.1 cm
  • Size of largest image: 768 pixels x 1024 pixels

TrueBeam Linacs in New York

Varian website