Couch Modelling in Eclipse 8.6

The Body Structure

Dose calculation usually ignores anything outside the Body Structure. Therefore, if the beam absorption (beam attenuation) of support structures like couchtops or immobilization devices (mask systems etc.) shall be taken into account, it is generally a good idea to include them inside the Body Structure:

If every structure which is in the beam is included, and a state-of-the-art dose calculation algorithm like AAA or VMC is used, one can be quite confident that the dosimetric impact of extracorporal material with the associated shift of isodoses, increased surface dose, absorption etc. will be handled properly.

Typical absorption values for hollow (sandwiched) carbon couchtops are 1-2%, depending on energy.

Immobilization devices which are made of solid carbon,

can have 350 HU or more, which is equivalent to some bones in the human body:

The higher the HU, the higher the absorption. Cutting out surfaces is suboptimal because it produces inhomogeneous absorption grids if a beam enters at oblique angle from the back side.

If a carbon fiber couchtop is thick (like the thick part of the Exact IGRT Couchtop from Varian), and the beam enters at a very oblique angle (e.g., 110°),

the fanline of a beam penetrating the couchtop can easily be over 20 cm long before it enters the patient.

Even if the top is made of sandwiched carbon, beam attenuation can become significant (8% or more), and should definitely be taken into account. This is where couch modelling comes in (since few people will have an Exact or ExactIGRT couchtop installed on their CT scanner).

The overlay installed on our CT scanner is very similar to the linac's Exact couchtop with flat panel. But it is not exactly the same. Up to now, we contoured the central part of the visible couchtop manually and included it in the Body Structure with Boolean operators. The resulting dose distributions take the absorption of the couchtop into account if the beam enters from the southern hemisphere:

The method has two drawbacks:

• The two couch rails of the Exact Couch - not present at the CT-scanner - cannot be drawn.
• The couch cannot be contoured to its full width of 53 cm.

Dose accuracy has been improved in Eclipse 8.6 with the introduction of Couch Modelling. Couch Modelling means that ready-made software-generated couchtops, which are dosimetrically and geometrically equivalent to their real-world counterparts can be placed below the CT-based 3D-model of the patient with three clicks of the mouse:

When New Couch Structures ... is selected from the Insert menu, the user gets the following choices:

Currently, only Varian couchtops are supported.

Eclipse automatically enlarges a standard 50 cm x 50 cm CT image to accommodate a wider couchtop. If a Body outline already exists, Eclipse places the couch below the patient. The couch structures, which are independent of the Body structure, can be moved around, deleted and recreated. That's it. Absorption is acounted for according to the HU numbers specified for the different couch parts. The values in the screenshot are the defaults.

The dosimetric improvement will be analyzed elsewhere.

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