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:

3D with manually drawn couchtop

The method has two drawbacks:

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:

3D with Exact couchtop

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

Possible Couch Selections

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.

3D with Exact couchtop

The dosimetric improvement will be analyzed elsewhere.