Breast conserving surgery is a common treatment option for early-stage breast cancer, and it relies on complete tumor excision such that no residual cancer is left in the resection cavity. However, the use of preoperative imaging to inform excision is compromised by intraoperative deformations that change the location, volume, and shape of the tumor compared to the imaging configuration. For intra-procedural guidance specifically, incision and retraction alter the tumor presentation and geometry. Being able to compensate for retraction deformations intraoperatively may increase the utility of image guidance technologies. In this work, a breast retraction phantom and deformation modeling approach are developed to explore the potential of modeling retraction for image guidance during BCS. Surface and subsurface beads were embedded in a realistic silicone breast phantom, and CT images were acquired in undeformed and retracted states. A reconstructive, sparse-data registration method was used to model retraction. Modeling accuracy was evaluated by comparing model-predicted and ground-truth bead displacements. The average surface bead registration error after retraction modeling in a region of interest was 0.5 ± 0.1 mm (maximum 0.5 mm). The average subsurface bead registration error in a region of interest was 1.2 ± 0.6 mm (maximum 2.6 mm). A biomechanical modeling method that includes retraction may improve the accuracy of image guidance for breast conserving surgery, but more work is needed to evaluate its utility.