[vtkusers] How to position a vtkDIskSource?

[gemo]

Hello,
I need to display a circular region of interest over an image.

For this purpose I though to use a vtkDiskSource object where you can
set the inner and outer radius.

However there is no method to set the location of the disk centre (or at
least I could not find it :)), so how can I place the disk in an exact
location of the image?

Thanks a lot for your help,

Monica