This paper describes a computer program for volumetric treatment planning and image guidance during radiofrequency (RF) ablation of hepatic tumors. The procedure is performed by inserting an RF probe into the tumor under image guidance and generating heat to 'cook' a spherical region. If the tumor is too large to be ablated in a single burn, then multiple overlapping spherical burns are needed to encompass the entire target area. The computer program is designed to assist the physician in planning the sphere placement, as well as provide guidance in placing the probe using a magnetic tracking device. A pre-operative CT scan is routinely obtained before the procedure. On a slice by slice basis, the tumor, along with a 1 cm margin, is traced by the physician using the computer mouse. Once all of the images are traced, the program provides a three-dimensional rendering of the tumor. The minimum number of spheres necessary to cover the target lesion and the 1 cm margin are then computed by the program and displayed on the screen.