Abstract: A computer-implemented arrangement (apparatus and method) for semi-automatically determining from CT image data the volume of a tumor. A primary application is to determining a tumor volume for patients with hepatoma and primary hepatic cholangiocarcinoma. CT image data of a scanned organ, stored on a magnetic tape, is read into a computer. This data includes for each of plural slices of predetermined thickness a plurality of pixels defining the slice and a CT number representing tissue density corresponding to each pixel. An image of the first slice is displayed on a monitor. A region of interest (ROI) approximating an outline of the organ is automatically generated, which outline is visually modified, if necessary, by an operator to closely correspond to the organ's outline.