We present a fast and reliable algorithm that gives precise loca-tion of breast tumours for a partial mastectomy. Our algorithmis fully implemented in the Surface Evolver, which is a general-purpose simulator of physical experiments. By starting from theX-ray images that show a tumour one takes its 2D coordinates ineach view. These views are called CC (Craniocaudal) and MLO(Mediolateral Oblique). Together with some measurements of thepatient's breast, these coordinates are given as input to our simula-tor. From this point on the simulator reproduces all main steps oftaking mammography with a virtual transparent breast that matchesthe patient's. The virtual mammography procedure is graphicallydisplayed on the computer screen, so that users can track the vir-tual tumour inside the breast. As output we have the coordinates ofthe tumour position when the woman lies on the operating table forthe surgery. With these coordinates the surgeon can make a smallincision into the breast and reach the tumour for its removal. Aftera simple plastic correction the whole structure of the breast will bepreserved.