Interactive volume segmentation is an essential and important step in medical image processing. Conventional interactive methods typically demand significant amounts of time and do not lend to a natural interaction scheme with the 3D volume. In this paper we present a sketch-based interface for seeded region growing volume segmentation. In our approach, the user freely sketches regions of interest (ROI) directly over the 3D volume. Parts of the volume outside the ROIs are then automatically cut out in real-time. The user repeats this process as many times as necessary until he/she decides to specify the seed point 3D location directly at the ROI. To prevent unexpected segmentations, the region growing is restricted to the specified ROI. Our sketch-based system utilizes GPU programming to achieve real-time processing for both rendering and volumetric cutting independent from the size and shape of the sketched strokes.