Having defined the quadrants, the tumor position is then randomly selected inside each quadrant - 3 spatial coordinates (x, y and z) are used to set the center of the tumor - and the excision volume is computed. Briefly, the line between the nearest point of the pectoral muscle and the tumor position sets the normal vector to the muscle, and a predefined cylinder (with a known radius, height and, consequently, volume) is aligned through this direction (Figure 6). Different tumor sizes (volumes) can then be modelled by varying the ratio between cylinder and breast volumes. Once the BCS protocol states that a breast tumor is eligible for BCS only if its removal do not require excision volumes higher than 20% of breast volume , cylinder volumes need to be limited to respect this threshold. In this study, excision volumes of 5%, 7.5% and 10% of the total breast volume were simulated, corresponding to three size categories: small, medium and large tumors, respectively (Figure 7a–c).