Breast cancer remains one of the leading causes of morbidity and mortality among women worldwide, posing a persistent public health challenge. Although imaging modalities such as mammography, magnetic resonance imaging (MRI), and computed tomography (CT) are indispensable for diagnosis, they provide limited accuracy in defining tumor extension. Under- or overestimation of tumor margins can compromise surgical outcomes, increase recurrence risk, and delay treatment decisions. This chapter presents the development and application of patient-specific 3D printed breast models for the assessment of tumor extension. Clinical CT and MRI datasets were segmented to reconstruct anatomical features, including ducts, lobules, vascular structures, and pathological regions, before fabrication using resin-based additive manufacturing. The novelty of this approach lies in the integration of photoelastic and polarimetric optical analysis, whereby illumination under polarized light reveals stress distributions and highlights tumor-related discontinuities and atypical density within the models. The resulting patterns on the printed models demonstrated multiple benefits, as follows: Enhanced clinical interpretation—Improved three-dimensional visualization of tumor margins, supporting more confident preoperative planning. Patient–doctor communication—It is expected that tangible models translated radiological data into physical representations, facilitating informed discussions with patients. Educational impact—The models can be used as training tools for medical students and residents, integrating anatomy, biomechanics, and optical diagnostics. Preliminary evaluations confirmed that photoelastic 3D breast models improve margin visualization and offer a semi-quantitative framework for approximating tumor extension beyond imaging-based segmentation. These findings highlight the translational potential of combining additive manufacturing with experimental optics to advance personalized oncology. Future perspectives include the incorporation of soft biomaterials for more realistic tissue analogues, the extension of clinical validation through multi-center trials, and the establishment of hospital-based 3D printing laboratories to integrate this technology into routine surgical planning. (Martelli in Surgery 159:1485–1500, 2016; Witowski in Eur J Radiol 110:30–35, 2018).