## 2 Materials and Methods
### BI-RADS Categories  
In the fifth edition of BI-RADS Ultrasound [37], the descriptive terms for characterizing the main sonographic attributes of a mass are standardized and classified into five categories: shape, orientation, margin, echo pattern, and posterior acoustic features. Shape: In the BI-RADS framework, the morphological characterization of a lesion is crucial for assessing breast cancer. The classification of lesion shape is delineated into three specific categories: oval, which describes lesions with an elliptical form and 2-3 curvatures; round, indicating a circular configuration; and irregular, which indicates shapes that do not conform to either oval or round (Figure 2). Typically, benign tumors, which are non-proliferative and lack metastatic potential, exhibit smooth, well-defined shapes, often manifesting as oval or round. Conversely, malignant tumors display irregular shapes, a reflection of their invasive growth and metastatic properties. Orientation: Orientation pertains to the alignment of the tumor's long axis. When this long axis runs parallel to the skin line, the orientation is termed parallel, or \"wider  
Figure 1: The conceptual architecture of the breast ultrasound CAD framework. than tall\". Conversely, if the long axis is perpendicular to the skin line, the orientation is described as not parallel, or \"taller than wide\" (Figure 3). The \"taller than wide\" orientation is particularly concerning, as it suggests malignancy due to the tumor's reduced compressibility and its propensity to infiltrate across tissue planes. Margin: The margin category refers to the delineation between a lesion and the surrounding tissue. When this boundary is distinct and sharply defined, it is labeled circumscribed. On the other hand, if the boundary is unclear, it falls under the not circumscribed category. Not-circumscribed margins are further divided into specific subcategories: indistinct, where the boundary is blurred and poorly defined; angular, where edges are sharp and form acute angles; microlobulated, featuring short, radiating protrusions along the lesion's boundary; and spiculated, characterized by needle-like projections (Figure 4). The presence of one or more of these subcategories results in a not-circumscribed classification. Benign tumors, typically enclosed by a fibrous capsule, exhibit circumscribed margins due to their clear separation from surrounding tissues. In contrast, malignant tumors, which lack such encapsulation, frequently present with not circumscribed margins, reflecting their invasive nature. Echo pattern: The internal echo pattern of a mass encompasses several distinct classifications: anechoic, hyperechoic, complex cystic and solid, hypoechoic, isoechoic (equal), and heterogeneous (Figure 5). Echogenicity is determined in relation to subcutaneous fat. An anechoic mass, devoid of internal echoes, presents as entirely black on ultrasound. Hyperechoic masses exhibit increased echogenicity, isoechoic masses show echogenicity comparable to subcutaneous fat, and hypoechoic masses display reduced echogenicity. Complex cystic and solid masses contain both anechoic cystic regions and echogenic solid elements, such as intracystic papillomas. A heterogeneous echo pattern is characterized by a combination of varying echogenicities within a single solid mass, reflecting a diverse internal texture. Figure 4: Examples of mass margin categories in ultrasound images. Figure 3: Example of mass orientation: parallel and not parallel. Posterior acoustic features: The posterior acoustic feature of a lesion is categorized into several distinct patterns: enhancement, which refers to an increase in echogenicity behind the tumor; shadowing, characterized by a reduction in echogenicity (excluding edge artifacts such as border shadow); absence of posterior acoustic features, where neither enhancement nor shadowing is observed; and a combined pattern, where there is a presence of both attenuation and either enhancement or shadowing at the posterior aspect (Figure 6).