Neoadjuvant chemotherapy (NACT) is often used to treat patients with locally advanced breast cancer, large tumor but operable breast cancer, or proven lymph node metastasis. The advantage of NACT is that it can downstage tumor size and increase the rate of breast-conserving surgery as well as providing information on the drug response through assessment of the changes of tumor size . The pathological complete response has consistently been associated with good long-term outcomes, but is achieved in only 10%-20% of cases. Patients who achieve a pathological complete response have a better prognosis than those who do not . Higher rates of pathological complete response can be achieved when selecting for certain breast cancer subtypes and treatment regimens .

Breast cancer is a heterogeneous disease that can be divided into different subtypes by immunohistochemical marker expression or gene expression array data. A new surrogate intrinsic subtype was proposed at the St Gallen meeting to separate luminal A, luminal B (HER2-/HER2+), HER2-enriched, and triple-negative disease . Similarly, tumors may have different prognosis based on their molecular subtypes. The luminal A subtype has a better prognosis than the other subtypes, and the triple-negative subgroup has the worst prognosis . On the other hand, the triple-negative subtype is more sensitive to chemotherapy than luminal A and B breast cancer .

Current techniques available for monitoring response to NACT are positron emission tomography (PET) , sonography, mammography, magnetic resonance imaging (MRI) , and shear wave elastography (SWE) . Conventional sonography and mammography have poor reliability in evaluating the size of residual tumor after chemotherapy . SWE is a recently developed low-cost imaging technique for measuring tissue stiffness in a noninvasive and quantitative manner with high reproducibility . Tissue stiffness has been demonstrated to be significantly correlated with tumor growth as cancer development and progression require extensive reorganization of the extracellular matrix (ECM) . Increased deposition of collagen and other ECM molecules enhances the stiffness of tumoral stroma . Changes in tumor stiffness were significantly greater in patients who had a good response to NACT compared to those resistant to NACT. Breast cancer pre- and post-treatment stiffness obtained from SWE was significantly correlated with the presence of residual cancer . A study in showed that the SWE stiffness measured after 3 cycles of NACT and changes in stiffness from baseline were strongly associated with pCR after 6 cycles. The combination of the post-treatment SWE and grey scale ultrasound has also been shown to be promising for end-of-treatment identification of residual disease and thus response to NACT, with similar accuracies found in assessment by MRI .

Several histopathological classifications are available to categorize the tumor response to NACT. The Miller-Payne grading (MPG) and Residual Disease in Breast and Nodes (RDBN) are systems to assess the pathological response of NACT. MPG provides a five-step scale based on tumor cellularity in the excision/mastectomy specimen compared with the pretreatment core biopsy as follows: grade 1, no reduction in overall cellularity; grade 2, minor (\<30%) loss of cellularity; grade 3, estimated 30%-90% reduction in tumor cells; grade 4, \>90% loss of tumor cells; and grade 5, no invasive carcinoma (IC); ductal carcinoma in situ may be present.\[30\] RDBN uses the following formula: level 1, pathological complete response in breast and nodes, without or with carcinoma in situ, and levels 2-4, residual disease in three different amounts, calculated as 0.2 (residual breast tumor size in cm) + the index for the involved nodes (0 for no positive nodes, 1 for 1-4 nodes, 2 for 5-7 nodes, 3 for ≥8 nodes) + the Scarff-Bloom-Richardson grade (1, 2, or 3), which takes into account tumor size, lymph node stage, and histological grade to determine response.