The presence of a lump in the breast is one of the most common features of breast carcinoma.^2,3 Benign masses usually have an oval shape and circumscribed margin with an abrupt interface with the surrounding tissue. In contrast, malignant masses often have an irregular shape with indistinct or spiculated margins caused by infiltrating nearby normal tissue.^2,3

Benign breast lesions are a heterogeneous group of conditions that can be categorized into three different etiologies: inflammatory, proliferative, and benign breast tumors.^2,4 Inflammatory lesions include abscess, hematoma, granulomatous mastitis, and lymphocytic mastopathy.^2,3 Proliferative conditions encompass fat necrosis, radial scar, and sclerosing adenosis.^2,3 Benign tumors mimicking malignancy include pseudoangiomatous stromal hyperplasia, desmoid fibromatosis, and granular cell tumors.^2,3

Mastitis and Abscess

A breast abscess typically develops as a complication of mastitis, and Staphylococcus aureus and Streptococcus are the most common pathogens involved.^2,5 The patient usually presents with a tender palpable breast mass on clinical exam.² It can occur in puerperal women who are breastfeeding or in nonpuerperal women with risk factors such as smoking, diabetes, or a weakened immune system.² On mammography, breast abscess often appears as architectural distortion, asymmetry, or a dense mass..^2,5,6 Focal or diffuse skin thickening may also be present.^2,5 On ultrasound, it corresponds to a heterogeneous fluid collection containing debris or septations, with indistinct margins and posterior acoustic enhancement.^2,5,6 Typically, the abscess has a thick echogenic rim and increased vascularity.² Associated mastitis presents as increased echogenicity of adjacent tissues and thickening of the skin.⁵ The main differential diagnosis for breast mastitis is inflammatory carcinoma. The skin thickening in inflammatory carcinoma is diffuse, while it is localized to the affected area in mastitis and abscess.² If symptoms persist after appropriate treatment, a biopsy should be considered to exclude malignancy.⁵ (Figures 1-2)

Hematoma

A hematoma is typically associated with trauma.⁵ On mammography, it appears as focal asymmetry with circumcised or ill-defined borders, and its density can vary.^2,5,6 In the acute stage, surrounding hemorrhage may obscure the hematoma.⁵ Over time, it becomes more organized and homogeneous and may result in slight architectural distortion or fat necrosis.⁵ On ultrasound, it presents as a mass with mixed echotexture that gradually becomes hypoechoic.^2,5 It may contain debris, solid components, or fluid-fluid levels.⁵ As the lesion is followed, the size should decrease, and resolution should occur. If there is any doubt or an unclear clinical history, short-term follow-up is recommended to rule out a small underlying malignancy that may have bled.² (Figures 3-4)

Granulomatous Mastitis

Granulomatous Mastitis (GM) is an inflammatory condition of unknown cause, typically affecting younger women within months and years after pregnancy.⁵ Histologically, it is characterized by non-caseating granulomatous inflammation of the lobules without a clear infectious or inflammatory origin.² GM typically presents as a firm mass with skin erythema, often accompanied by pain or nipple retraction.⁵ In some cases, draining sinus tracts or nipple discharge may also be present.² On mammography, it appears as focal asymmetry, ill-defined breast mass, skin thickening, or negative mammograms.² On ultrasound, findings include multiple hypoechoic masses or parenchymal distortion with acoustic shadowing.^2,5 MRI can identify fistulous tracts to the skin.⁵ GM diagnosis requires a biopsy to exclude inflammatory and infectious diseases .^2,5 (Figures 5-6)

Lymphocytic Mastopathy

Lymphocytic Mastopathy (LM) is an inflammatory breast condition characterized by lymphocytic infiltration in the periductal and perilobular spaces, often associated with fibrotic changes.^5,6 This condition is commonly associated with diabetes but may also occur in autoimmune disorders.² Diabetic mastopathy is a type of LM associated with diabetes, most commonly seen in younger premenopausal women with type 1 diabetes.² Mammography displays a regional asymmetric density with ill-defined margins.⁵ On ultrasound, it appears as a poorly defined hypoechoic mass with posterior acoustic shadowing and increased vascularity.^2,5 Biopsy is sufficient to diagnose LM with the appropriate clinical history and imaging features.^2,5,6 (Figures 7-10)

Fat Necrosis

Fat Necrosis (FN) is a proliferative condition commonly seen after previous surgery or trauma.⁵ Typical benign findings include dystrophic calcifications and round opacity with a radiolucent center.^2,6 However, atypical appearances can sometimes resemble malignancy, including architectural distortion, asymmetries, spiculated masses, or pleomorphic calcifications.² On ultrasound, FN usually presents as a solid mass, an anechoic, or complex cystic lesion.⁵ When mimicking malignancy, it appears as a hypoechoic, irregular mass with posterior acoustic shadowing.² Typical benign imaging features do not require further evaluation.² In the appropriate clinical context, FN with atypical imaging findings can be managed with short-term follow-up.⁵ If growth is observed, a biopsy should be performed.² (Figures 11-12)

Radial Scar

Radial Scar (RS) is a proliferative lesion histologically characterized by a central fibroelastotic core, surrounded by compressed glandular structures at the periphery.⁷ It can contain a malignant component, which is usually located eccentrically.⁷ On mammography, RS appears as an area of architectural distortion resembling malignancy.⁵ However, there are imaging features that may help differentiate it from carcinoma, such as varying appearances in different projections, the absence of a central mass, the presence of long, thin radiating spicules originating from a radiolucent center that creates a “black star appearance”, and the absence of a palpable mass or skin changes.^5–7 On ultrasound, RS appears as a hypoechoic mass, with or without posterior acoustic shadowing.^5,7 (Figure 13)

Sclerosing Adenosis 

Sclerosing Adenosis (SA) is a proliferative lesion, resulting from lobular hyperplasia.^2,5 SA is common in perimenopausal women and often occurs with other proliferative lesions. On mammography, SA presents as amorphous or pleomorphic calcifications.² It can also appear as a mass or as an architectural distortion.² On ultrasound, SA may appear as a region of posterior shadowing, with or without an associated mass.³ A biopsy of the mass or calcifications is recommended.^2,3 (Figure 14)

Granular Cell Tumor

Granular Cell Tumor (GCT) is a rare benign tumor of neural origin (4-6% of cases in the breast).² It typically affects premenopausal women and occurs in the upper-inner quadrants.² The sonographic, mammographic, and clinical features resemble breast cancer.⁵ The lesion typically presents as a painless, and firm mass, often adhered to the pectoral fascia or chest wall.² GCT can appear as a dense opacity with spiculated margins on mammography. On ultrasound, it presents as a hypoechoic mass with irregular margins and can have a hyperechoic halo.^2,3 The diagnosis is usually performed by biopsy.² (Figures 15-16)

Desmoid Tumor

Desmoid Tumor (DT) is a rare benign infiltrating mass, composed of fibroblasts and collagen.⁵ They account for less than 0.2% of breast tumors.⁵ The development of these tumors is associated with trauma, surgery, and Gardner syndrome.²  It has been reported to occur in women with breast implants.⁵ DT presents as a solitary, firm, and painless mass, sometimes adherent to the skin or pectoral fascia.⁵ On mammography, DT appears as high-density opacity, with spiculated or indistinct margins.^2,5 Ultrasound reveals an irregular hypoechoic solid mass with posterior acoustic shadowing.² Since a DT is difficult to distinguish from breast cancer, its diagnosis requires a biopsy.² (Figures 17-18)

Pseudoangiogiomatous stromal hyperplasia

Pseudoangiomatous stromal hyperplasia (PASH) is a benign entity, representing a wide spectrum of imaging features.^2,5 The etiology of this condition is unknown, and it is composed of stromal and epithelial proliferation, forming pseudovascular spaces.³ Clinically, it presents as a painless, well-defined, palpable mass in premenopausal women or postmenopausal women undergoing exogenous hormone therapy.⁵ On mammography, PASH typically appears as a round or oval noncalcified mass with well-defined margins, that can reach up to 11 cm in size.^2,5 On ultrasound, its appearance is variable, ranging from an oval, well-circumscribed hypoechoic mass to an irregular mass with mixed echogenicity.² Histologically, PASH should be differentiated from low-grade angiosarcoma.² After histological diagnosis and in the absence of clinical suspicion, these lesions can be monitored with imaging. ^2,3 (Figures 19-20)