TrueBeam beamlines above S/N 1250 are so-called Continuous Energy (CE) beamlines. The visual identification is easy: besides the "CE" sticker, CE-beamlines are red:

CE beamline of TrueBeamSTx S/N1275
(CE-beamline of TrueBeam STx S/N 1275)

With non-CE beamlines, the red color is a little darker, and more blueish (magenta-like).

What is TEE™?

Everybody knows that the optimal photon energy for a certain treatment plan is never accurately determined. For instance, head & neck patients are always planned with 6 MV photons, whereas for pelvic cases, often the higher photon energy (15 MV, 18 MV) is chosen, based on "tradition" and "experience".

However, it would be quite a coincidence if for a certain h&n planning case the optimal photon energy1 would just be the preselected 6 MV. It is equally possible, and actually much more likely, that the optimal energy2 will be different from 6 MV. Maybe it is 7.1 MV, we would never know. This is where TEE comes into play.

With TEE, the free parameter "photon energy" in the treatment planning process is subjected to optimization. If "TEE" is selected as field energy (see sidebar), the Progressive Resolution Optimizer (PRO) of RapidArc optimization takes care of finding the best energy, which now is a continuous quantity within the waveguide's lower and upper limits, and not one of the fixed energy steps. The resulting optimized energy is called EAP (Equivalent Accelerating Potential). It can only be accessed via the Calculation Notes (see Properties of a treatment field after dose calculation). The field itself will only have the label "TEE" instead of the photon energy (6X, 10X, 15X, ...).

First experiences with TEE

Some other prerequsites exist:

One should also know that TEE currently only works for RapidArc plans, since only the PRO of Eclipse 14 is capable of optimizing energy. It currently neither works for static Gantry IMRT plans nor for 3D-conformal plans. The continuous TEE energy cannot be set manually.

The TEE configuration in Eclipse Beam Configuration is easy. The Master Sets for the main Photon energies (6X, 10X, 15X) have to be measured and configured, as usual. From this FF-TEE Set, Eclipse computes some sort of linear combination of the photon spectra to produce the desired TEE energy.

A separate FFF-TEE Set is generated from the master energies 6FFF and 10FFF. In other words, there is no extra work for the physicist if he chooses to configure TEE in Eclipse. Also the MLC transmissions etc. are interpolated from the Master Transmissions.

From the above it becomes clear that TEE will never extrapolate, but always interpolate. If the lowest photon energy is 6X, the optimizer will never output 5.6X as the optimal energy.

One drawback of TEE exists when dosimetric plan QA is performed (it is more a nuisance than a real drawback). Since some ionization chamber correction factors need the photon energy as input quantity, it is quite cumbersome to look up the EAP in the calculation notes in order to get the corrections right.

Also machine QA is a little more complicated when using TEE. For instance, in order to make a water phantom cross-profile scan at 12.3 MV, one has to use a clinical plan (QA plan) with a high enough MU number, which has been produced by the optimizer of Eclipse. This plan has to be treated on the TrueBeam in QA mode (or Treatment mode), while the water phantom is scanning. Setting a TEE energy directly in Service Mode is not possible (yet).


TEE with the CE beamline is a promising way of getting better plan quality. The key technology is clearly the beamline. Up to now, the linac performed with the same stability for TEE beams as for beams which used one of the master energies.

We wonder what will come next. Maybe Dynamic TEE during a RapidArc treatment, where energy changes during beam-on?

1We are always talking about an accelerating potential producing a certain photon spectrum. 
2the one which gives the best plan according to the specified constraints.

back to Medical Physics home

Note, Oct 2013: If you have read this far and still believe that TEE, CE and EAP really exist, we have to disappoint you:
It is pure fiction. Please review your critical thinking skills (especially if you are working in a university setting).
Please do NOT ask Varian about TEE. They can't hear it anymore.
However, there have been rumors about TEE on the Siemens Artiste. If you belong to the exclusive circle of Artiste users, feel free to contact Siemens Healthcare about an upgrade solution ...

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