We describe methods for the specification and modification of virtual resections in medical volume data. These techniques are focused on applications in therapy planning, but are also relevant for educational purposes. The basic idea of our approach is to convert resection lines specified by the user on an organ's surface into an initial mesh by means of a principal component analysis. This mesh can be flexibly deformed with an adjustable sphere of influence. We present applications in liver surgery planning. Keyword: 3d interaction, clipping, computer-assisted surgery