 Introduction/Background Neoadjuvant chemotherapy (NAC) is often given before definitive surgical intervention for locally advanced breast cancer, which is defined as a tumor >5 cm with regional and/or metastatic lymph nodes, skin, or chest wall involvement. NAC is also indicated in T2 tumors (2-5 cm) in which lumpectomy might result in substantial cosmetic defect, triple-negative tumors 2 to 5 cm in size even if node-negative, and human epidermal growth factor receptor 2 (HER2)/neu-positive tumors 2 to 5 cm in size even if node-negative. The primary aims of this approach are to 1) reduce tumor burden, thereby permitting breast conservation rather than mastectomy; 2) promptly treat possible metastatic disease, whether or not it is detectable on preoperative staging; and 3) potentially tailor future chemotherapeutic decisions by monitoring in vivo tumor response [1,2]. Although the overall and disease-free survival for women receiving neoadjuvant versus adjuvant chemotherapy are not substantially different, women who do receive neoadjuvant therapy are less likely to undergo mastectomy and are more likely to be treated with breast conservation [1]. Imaging plays a vital role in managing patients undergoing NAC as treatment decisions rely heavily on accurate assessment of response to therapy. Beyond assessing the primary lesion, imaging is used to stage and monitor patients before, during, and after completion of initial therapy, including the axilla and potential distant metastatic sites. Accurate assessment of tumor burden is critical in determining the best management. Imaging plays an important role as clinical breast examination is challenging for primary tumors that are <2 cm in size, have an irregular shape or ill-defined margins, and show necrosis, fibrosis, or fragmentation with treatment [3]. Axillary imaging is increasingly used before, during, and after therapy to monitor response to treatment and help guide surgical management [4].
Monitoring Response to Neoadjuvant Systemic Therapy for Breast Cancer.  Introduction/Background Neoadjuvant chemotherapy (NAC) is often given before definitive surgical intervention for locally advanced breast cancer, which is defined as a tumor >5 cm with regional and/or metastatic lymph nodes, skin, or chest wall involvement. NAC is also indicated in T2 tumors (2-5 cm) in which lumpectomy might result in substantial cosmetic defect, triple-negative tumors 2 to 5 cm in size even if node-negative, and human epidermal growth factor receptor 2 (HER2)/neu-positive tumors 2 to 5 cm in size even if node-negative. The primary aims of this approach are to 1) reduce tumor burden, thereby permitting breast conservation rather than mastectomy; 2) promptly treat possible metastatic disease, whether or not it is detectable on preoperative staging; and 3) potentially tailor future chemotherapeutic decisions by monitoring in vivo tumor response [1,2]. Although the overall and disease-free survival for women receiving neoadjuvant versus adjuvant chemotherapy are not substantially different, women who do receive neoadjuvant therapy are less likely to undergo mastectomy and are more likely to be treated with breast conservation [1]. Imaging plays a vital role in managing patients undergoing NAC as treatment decisions rely heavily on accurate assessment of response to therapy. Beyond assessing the primary lesion, imaging is used to stage and monitor patients before, during, and after completion of initial therapy, including the axilla and potential distant metastatic sites. Accurate assessment of tumor burden is critical in determining the best management. Imaging plays an important role as clinical breast examination is challenging for primary tumors that are <2 cm in size, have an irregular shape or ill-defined margins, and show necrosis, fibrosis, or fragmentation with treatment [3]. Axillary imaging is increasingly used before, during, and after therapy to monitor response to treatment and help guide surgical management [4].