A smooth round or oval mass with homogeneous internal echoes is the typical US appearance of a fibroadenoma. There is a peripheral pseudocapsule adjacent to packed breast parenchymal tissue, allowing the surgeon to easily resect the mass. The echogenicity of fibroadenoma tends to be determined by its histologic composition. Microscopically, fibroadenoma arises from a terminal ductolobular unit (TDLU) and a variable amount of stroma and epithelial elements. Generally, a higher degree of epithelial elements is associated with a hypoechoic appearance, whereas a higher stromal composition yields a more echogenic appearance [4]. Moreover, the acellular stroma is more isoechoic and can even be mildly hyperechoic if the stroma is affected by sclerosis, hyalinization, myxoid change, or calcification [4]. In summary, a more hyperechoic appearance tends to be found in lesions with a higher stromal composition relative to the epithelial composition and lesser cellular stroma combined with stromal changes (Fig. 1).

Phyllodes tumor is a rare fibroepithelial lesion of the breast with a proclivity for rapid growth. It is made up of epithelial and stromal components, with cleft-like cystic spaces acting as a diagnostic clue. This entity has a distinct leaf-like architecture with abundant cellular stroma, which distinguishes it from fibroadenoma and other stromal sarcomas [5]. Phyllodes tumors are classified as benign, borderline, or malignant based on their histopathologic features, such as the degree of stromal cellularity and atypia, mitotic count, and stromal overgrowth.

A benign phyllodes tumor resembles cellular fibroadenoma, while a malignant phyllodes tumor can be mistaken for primary breast sarcoma or spindle cell carcinoma. Distinguishing benign phyllodes tumor from fibroadenoma is difficult on core needle biopsy (CNB). Therefore, fibroepithelial lesions diagnosed using CNB are currently treated with either vacuum-assisted or open excision. Surgical excision is usually preferred because it enables negative margins in the event that the final diagnosis is a phyllodes tumor.

On US, phyllodes tumor presents as an inhomogeneous, solid-appearing mass with cleft-like cystic spaces and posterior acoustic enhancement. The hyperechogenicity of the lesion may be due to the heterogeneous distribution of stromal elements intermingled with collagen lay-downs (Fig. 2) [6].

Adenosis of the breast is characterized by hyperplasia of all TDLU constituents (epithelial cells, myoepithelial cells, and connective tissue). Clinically, it may manifest as a slowly growing breast swelling. Pathologically, it is classified into three groups: sclerosing adenosis, blunt duct adenosis, and microglandular adenosis [7].

Adenosis has nonspecific US findings, with variable echogenicity, and hyperechogenicity of an adenosis tumor could be attributed to an increased extent of the interfaces between prominent glandular tissue and fibrosis (Fig. 3).

Myofibroblastoma is a rare benign spindle cell tumor that has myofibroblastic characteristics. These tumors have uniform spindle cells in short fascicular clusters and interspersed bands of hyalinized collagen. The spindle cells express vimentin, CD34, and desmin, but not S-100 protein nor cytokeratins [8].

The mammographic appearance of the tumor can be well-circumscribed or ill-defined, and it can be round, oval, or lobular in shape. The most common US features are a homogeneously hypoechoic well-circumscribed solid mass that may resemble a fibroadenoma [8]. The fat component and multiple internal tissue interfaces among fibrous tissue, fat, and cellular components in the tumor may be the cause of hyperechogenicity (Fig. 4) [9].

Because of the wide range of histological findings in invasive ductal carcinoma (IDC), mammographic and US findings are also diverse. The most common US finding is an irregular hypoechoic mass with a non-circumscribed margin. The internal echo of usual breast cancer is hypoechoic in the majority of cases, but hyperechoic masses can occur in rare cases. Malignant masses can be composed of tumor cells mixed with other tissue components with varying echogenicity, such as adipose tissue (Fig. 5), fluids (secretions, intercellular fluid, or blood), hemosiderin, and/or microcalcifications (Fig. 6) [10]. The complex intermixture of various elements is thought to play a role in hyperechogenicity on US. Another possible explanation is tumor cellular heterogeneity, such as a cribriform pattern, tubular structure, solid nests, and scirrhous pattern.

Invasive lobular carcinoma (ILC) is the second most common type of breast cancer, accounting for approximately 7%-10% of all cases. Microscopically, ILC is made up of malignant cells that grow in the mammary stroma and adipose tissue in a single-file fashion around normal breast ducts. On mammography, the most common finding is asymmetric density. However, ILC presents with nonspecific findings more often than other breast carcinomas, and it frequently exhibits multifocal and multicentrality, as well as no abnormality at all. The US findings range from no detectable lesion to a mass similar to IDC. US detectability depends on whether the tumor forms a mass and the relative proportion of the lesion that is diffusely infiltrating [12]. The most common US finding is a heterogeneous mass with irregular or indistinct margins and posterior acoustic shadowing [12]. According to Watermann et al. [13], hyperechoic and isoechoic patterns are more common in ILCs than in tumors of other histologic differentiation. These specific US findings could be explained in two ways by the distinct histologic growth patterns of ILC. First, ILC involves the increasing infiltration of the surrounding parenchyma as rows of single cells and in concentric rings around normal ducts, resulting in an increase in reflective surfaces and increased echogenicity with posterior acoustic shadowing [12,13]. In addition, the alternating repetition of tumor cells and intercellular fibrotic stroma could contribute to hyperechogenicity, as in the case presented herein (Fig. 7). Another plausible explanation is that the tumor can infiltrate adjacent fat tissue, increasing the echogenicity of the mass.

Mucinous carcinoma is an extremely rare type of IDC, accounting for <2% of all breast cancers. In this type of cancer, the tumor is made up of abnormal cells that "float" in pools of mucin. Pathologically, mucinous carcinoma is classified into pure mucinous carcinoma and mixed mucinous carcinoma based on the mucinous component.

On mammography, mucin appears with increased density with a well-circumscribed or microlobulated mass, but the margin can be obscured at times. On US, it has a heterogeneous echo with a mixed internal solid and cystic area and posterior acoustic enhancement. Moreover, extracellular viscous mucin and microcalcifications may explain the hyperechogenicity (Fig. 8) [14]. When the suspicious mass has increased echogenicity with a relatively circumscribed margin on US and high signal intensity on T2-weighted magnetic resonance imaging, mucinous carcinoma should be considered (Fig. 8D).

Primary breast lymphoma (PBL) is a rare hematologic malignancy, accounting for only 0.05%-0.53% of all breast malignancies [15]. PBL is mostly non-Hodgkin lymphoma (specifically, high-grade B-cell lymphoma). Most cases of PBL were shown to be oval-shaped and high-density masses on mammography. A hypoechoic, homogeneous or heterogeneous well-defined mass is the most common US finding in malignant lymphoma of the breast. According to Yang et al. [16], a hypervascular irregular mass with indistinct margins or an echogenic boundary is an even rarer subset of PBL and has a poorer prognosis than the B-cell type. T-cell lymphoma involving the breast is considered an aggressive disease. T-cell lymphoma shows diffuse infiltration of the normal architecture by medium-sized to large cells with pleomorphic nuclei (Fig. 9). Invasion of the breast of T-cell lymphoma usually presents as a bilateral involvement, mimicking mastitis. The hyperechogenicity of the tumor may be explained by tumor cells infiltrating the breast parenchyma and fat (Fig. 9).

In locally advanced breast cancer and inflammatory breast cancer, neoadjuvant chemotherapy (NAC) is the treatment of choice before surgical treatment. A pathologic complete response has been considered as the most important predictor of NAC outcomes and has emerged as a surrogate prognostic marker in many clinical trials. During NAC, the change in tumor echogenicity can predict a pathological response in monitoring. As a result of NAC, the tumor becomes composed of tumor cells with fibrosis, collagenization, and microcalcifications in the stroma. These multiple reflective surfaces can cause hyperechogenicity on breast US (Fig. 10) [17]. Therefore, an increase in echogenicity during NAC may indicate a more effective tumor response. Dobruch et al. suggested that persistent tumor hypoechogenicity after three courses of NAC predicts a poor response to treatment [18].

Metastasis to the breast from extramammary neoplasms is infrequent. A correct diagnosis of breast metastasis is critical because the treatment and outcomes of primary and secondary breast malignancies differ [19].

The most common mammographic appearances of metastatic breast cancer are one or more well-circumscribed masses without spiculation, calcifications, or other signs of a surrounding desmoplastic reaction that characterize primary breast cancer. The characteristic US findings are a hypoechoic, circumscribed mass with a round or oval shape. However, various US findings have been reported, including circumscribed or not circumscribed hyperechoic or hypoechoic solid masses with acoustic shadowing or enhancement [19]. In metastatic breast lesions, tumor cells infiltrating adipose tissue, hemorrhage with hemosiderin deposition (melanoma), papillary fronds of the tumor surrounded by extracellular fluid (papillary serous carcinoma), and necrosis can exhibit hyperechogenicity on US (Fig. 11) [20].