Mastitis that occurs during lactation is termed puerperal mastitis. This is the most common form of mastitis and usually occurs from infection with Staphylococcus aureus or Streptococcus bacteria. This may lead to reduced milk production and is a common reason for mothers to stop breast feeding (6). Infection often arises from disruption to the skin surface of the nipple areolar complex and milk stasis is a risk factor for infection (7). Most patients will respond to antibiotic treatment, but a small percentage may develop abscess collections, that may complicate treatment and require further drainage. Ultrasound is useful for detecting these collections (Fig. 1B). Although rare, one should always keep in mind the possibility of cancer in puerperal mastitis that does not respond to antibiotic therapy, in which case histological sampling should be conducted (138).

Galactoceles are the most common lesions in lactating women. Milk is retained and stagnant when breast-feeding is stopped, causing ductal dilatation and development of cysts or galactoceles. Infection is a relative common complication of galactoceles (8). Galactoceles will appear similar to abscesses but follow a ductal distribution (3). Diagnosis is made based on an appropriate clinical picture (i.e., history of recent lactation) and aspiration of mixed milky and purulent material from the cyst (8) (Fig. 3).

Non-puerperal subareolar mastitis and breast abscess, also known as Zuska's disease is an uncommon and recurring disease strongly linked to cigarette smoking (29) (Fig. 4). It frequently affects middle-age females in their 40s but may affect a wide range of ages (mid-teens-80s) and may even be observed in males (2) (Fig. 5). This is caused by squamous metaplasia and proliferation of the epithelium in the lactiferious ducts, causing obstruction and secretory stasis leading to abscess formation. They present as recurring abscesses, often with fistulous tracks extending to the skin near the periareolar region (1210). Initial infections are usually caused by Staphylococcus bacteria. Subsequent recurring episodes of infection may be caused by mixed flora of bacteria, including anaerobes, and can be more complicated to treat with higher rates of recurrence. Radiological features of subareolar mastitis and inflammatory breast carcinoma may be similar (2). If clinically suspected, biopsy should be conducted to exclude presence of malignancy.

Peripheral breast abscesses usually occur in older women. In some cases, it may occur in patients with pre-existing conditions such as diabetes and rheumatoid arthritis. Staphylococcus aureus is the most common pathogen. These abscesses typically respond well to antibiotic treatment and drainage, and they tend not to recur (10).

Peripheral breast abscesses may also occur in patients with pre-existing cystic breast conditions, with superimposed infection of breast cyst (1). Patients may present with a painful palpable lump in the breast. Typical features of a infected breast cysts include thickening of the cyst wall, increased peripheral vascularity and presence of echogenic debris within the cyst (13) (Fig. 6).

Infections following invasive procedures such as breast surgery or biopsy are a relatively common cause of breast infection. Diagnosis is usually straightforward with the appropriate clinical history (1) (Fig. 7).

Uncommon microorganisms such as mycobacterium tuberculosis, salmonella and fungi may also infect the breast. Tuberculosis of the breast is a rare extra-pulmonary manifestation of tuberculosis infection. It most commonly presents as a unilateral solitary hard breast lump, that may turn into an abscess, form fistulas or ulcerate into the skin. Less commonly, tuberculosis of the breast can present as a multifocal disseminated disease with multiple communicating abscesses and discharging sinuses (Fig. 8). A rare sclerosing type, of which excessive fibrosis is a dominant feature, has also been described, in which the whole breast is hard with nipple retraction. Diagnosis is usually conformed on tissue sampling and detection of acid-fast bacilli on Ziehl-Neelsen stain or by culture (91112).

Skin lesions such as epidermal inclusion cysts or sebaceous cysts can be infected. Radiological investigations reveal dermal lesion, often well circumscribed with mixed echogenicity, reflecting varying internal contents (Fig. 9). There may be presence of a tract extending to the skin surface (13). Inflammatory changes may extend into the surrounding subcutaneous tissue or even into the breast parenchyma (1).

This is a rare form of mastitis that usually occurs in young women within a few years following childbirth (13). Its clinical and radiological presentation mimics breast malignancy (7). It may present with a palpable lump, or a more diffuse involvement of the breast (Fig. 10). This is usually accompanied with lymphadenopathy. In some cases, aseptic abscesses can develop, that may form sinus tracts with the overlying skin. Diagnosis is usually confirmed on histology, which demonstrates well-formed non-caseating granulomata with epithelioid macrophages and Langhans-type multinucleated giant cells (139).

Xanthogranulomas are usually well-circumscribed cutaneous nodules or indurated plaques that occur elsewhere in the body, with predilection for the periorbital area. Extra-cutaneous involvement is rare; involvement of the breast is even more uncommon. Imaging findings in the breast are variable (Fig. 11). These lesions are characterized on histology by the presence of multi-nucleated giant cells, lipid-laden macrophages (also called xanthoma cells or foamy histiocytes) and cholesterol crystals (1214).

Plasma cell mastitis, also known as duct ectasia or periductal mastitis, is a benign inflammatory disease of the breast that may present with nipple discharge and nipple retraction, which mimics malignancy. On ultrasound, dilated ducts are noted with echogenic material within (debris) and surrounding increased vascularity (Fig. 12A). On mammography, inspissated calcified intra-ductal content will show up as large rod-like branching calcifications radiating from the nipple (39) (Fig. 12B).

Diabetic mastopathy is another condition that is frequently mistaken for carcinoma due to its often indistinguishable radiological presentation (39). It usually occurs in patients with longstanding insulin dependent diabetes. Patients usually present with large, painless, hard breast masses. Multifocal or bilateral breast involvement is not uncommon (7). On mammogram, ill-defined masses or asymmetrical densities can be seen. On ultrasound, diabetic mastopathy often manifest as ill-defined hypoechoic lesions with significant posterior acoustic shadowing (12) (Fig. 13). Diagnosis is usually confirmed on histology (3).

Inflammatory breast cancer is an invasive aggressive disease with high rate of metastasis at diagnosis (1). Clinical symptoms include palpable tumor, skin erythema, edema and warmth, breast enlargement and nipple retraction. Some patients may present with breast pain. Mammographic findings include skin thickening, increased breast density and trabeculation (15). On ultrasound, subareolar mastitis tends to show mixed solid-cystic lesions or collections. However, inflammatory breast cancers are more likely to show solid mass lesions. On MR imaging, both inflammatory breast cancers and benign mastitis may exhibit similar morphological features such as skin thickening, edema and presence of mass lesions or non-mass like enhancement. Their differences, however, lie in their enhancement characteristics. Masses in inflammatory breast cancer tend to display greater initial enhancement and more frequent subsequent washout. Lesions in benign mastitis tend to show a more persistent or plateaued enhancement pattern. Lesions in benign mastitis also tend to occur in a subareolar location, whereas malignant lesions tend to occur either centrally or dorsally within the breast tissue (2). Any non-lactating patient with mastitis that does not respond to antibiotic therapy should undergo biopsy to exclude presence of malignancy (315) (Fig. 14).