SlicerRT isocenter configuration

[Mafer_Calix]

Hello. I need to know whether the isocenter for the slicerRT is fixed at 100 cm. Sure, in the software I can edit the isocenter position within the body and the SDD. But I need to know if the beam’s field defined in beam geometry forms at the SDD or at the isocenter position (eg. if I define a 100x100 mm beam geometry, SDD = 1000 mm and isocenter at the center of target volume, will the 100x100 mm area form at the surface of the volume target or the isocenter?)