PROJECT SUMMARY Breast cancer is now the most prevalent cancer, with 2.3 million women diagnosed and 685,000 deaths globally. Although, the death rate has declined by about 1% per year over the last decade due in large part to earlier detection through screening, the current screening standard, digital mammography, lacks sensitivity and is challenged in radiographically dense breasts. Sensitivity in dense breasts is improved by magnetic resonance imaging (MRI), but required contrast enhancement and substantial additional cost are drawbacks. A non-contrast, low-cost alternative, ultrasound strain elastography (SE) has been shown to achieve high sensitivity and specificity for breast cancer diagnosis. However, while SE diagnoses suspicious breast massed by interrogating tissue stiffness, the underlying cause of stiffening is not discerned. Among the many possible causes, stromal collagen organization is newly a focus of intense interest due to its recently proven association with breast cancer stage, prognosis, treatment response, and other clinical features. Therefore, delineating stromal collagen organization in vivo would not only improve breast cancer diagnosis and treatment management but also help to elucidate the complex and poorly understood pathophysiology of breast cancer development, progression, and aggressiveness. One approach to noninvasively evaluating stromal collagen organization in the breast is ultrasound shear wave elastography (SWE), but shear wave propagation is inhibited in 63% of malignant breast masses, leading to low-quality measurements. The lack of a robust ultrasound approach to interrogating stromal collagen organization represents a major gap in exploiting a rapidly emerging and highly promising biomarker for breast cancer. To fill this gap, our group has developed Viscoelastic Response (VisR) ultrasound, an on-axis approach to interrogating collagen fiber organization via stiffness anisotropy in breast masses and surrounding stroma without observing shear wave propagation. Our preliminary in vivo data, acquired in 30 women with BIRADS-4 or -5 masses, demonstrate that VisR-derived ratio of mass-to-surrounding tissue stiffness anisotropy (RMSA) differentiated biopsy-confirmed malignant (n=9) from benign (n=21) breast masses with a sensitivity of 95% and a specificity of 89%. Further, in our pilot feasibility study of four women monitored serially while undergoing neoadjuvant chemotherapy (NAC), VisR RMSA trended from malignant to benign indication with response to treatment. The success of our investigations motivates further advancement of VisR RMSA measurement technology, its extension to revealing the relationship between VisR RMSA outcomes and stromal collagen organization, and broader application to cancer detection and treatment monitoring. We hypothesize: Advanced VisR RMSA correlates to stromal collagen fiber organization, which is relevant to differentiating malignant from benign breast masses and predicting p...