Paired ectodermal primitive mammary streaks (milk streaks) form along the ventral surface of the embryo during the fifth week of gestation. These extend from the axilla to the inguinal region. The mammary ridges arise in the thorax from these streaks and there is progressive differentiation into breast parenchyma during fetal development. The other portions of the mammary streaks normally regress; incomplete involution of embryonic breast tissue can result in accessory or ectopic breast tissue anywhere along the primitive mammary streaks.

The prepubertal breast is a rudimentary organ that consists of simple branched ducts surrounded by a connective tissue stroma. With the onset of puberty, the ducts elongate, divide, and form terminal duct lobular units. Lobular differentiation begins in the peripheral regions of the breast and progresses centrally. At the end of pubertal development, the breast consists of a ductal system lined by epithelial cells and ending in terminal duct lobular units. Normal breast development is classified into a 5-stage Tanner grading system.¹

Tanner stage 1 is prepubertal. Sonography of the prepubertal breast may show thin ill-defined retroareolar tissue that is slightly hypoechoic or hyperechoic with respect to adjacent subcutaneous fat (Figure 9-1).

At the beginning of puberty, the breast bud develops; this is Tanner stage 2. Initially, there is a subareolar lump that is appreciable by palpation only. Enlargement of the breast bud produces a single mound of tissue that includes breast tissue and the nipple. The breast bud is demonstrated sonographically as a retroareolar nodule that is hypoechoic relative to adjacent fat and contains low-level echoes. Often, there is a central star-shaped or linear hypoechoic area that correlates histologically with simple branched ducts.

During Tanner stage 3, enlargement and elevation of the entire breast occur. As with stage 2, the breast tissue and nipple form a single mound of tissue. Development of glandular tissue occurs during this stage. The glandular tissue is relatively hyperechoic on sonography. A central retroareolar spider-shaped hypoechoic region is often visible, representing the ductal system.

Tanner stage 4 is transient and does not occur in all individuals. A secondary mound develops, with the nipple and areola projecting anterior to the breast tissue. Sonography shows a prominent hypoechoic nodule in the retroareolar region, surrounded by hyperechoic periareolar fibroglandular tissue.

Tanner stage 5 indicates development of a mature breast contour, with regression of the areola to form a smooth contour with the remainder of the breast tissue. The hypoechoic central nodule visible in the earlier Tanner stages is no longer visible with sonography. The breast predominantly consists of hyperechoic glandular tissue.²

Newborns sometimes have mild enlargement of the breast buds as a result of hormonal stimulation; this is termed transient gynecomastia of the newborn. This occurs in males and females. Some degree of ductal development can also occur, leading to nipple secretion (termed witch’s milk). Sonographic examination of these infants shows prominent subareolar ducts, which can be symmetric or asymmetric (Figure 9-2). Spontaneous resolution occurs over the next weeks to months, and sonography shows regression of the ducts to age-appropriate morphology. Resolution may take longer in breastfed infants because of estrogenic substances in human milk.

The initial development of the breast bud is termed thelarche. The typical age range of thelarche is 8 to 13 years, with a mean of 9.8 years. The ductal system of the breast bud proliferates and enlarges, resulting in a lump deep to the nipple; that is, Tanner stage 2. Occasionally, this process occurs asymmetrically, and sonography may be requested to determine if there is a mass or cyst (Figure 9-3). The appearance at this stage is identical to that demonstrated in males with gynecomastia: a hypoechoic subareolar “nodule.” The breast tissue appears hypoechoic because the ducts are predominantly end-on to the transducer. Manual rotation of the breast tissue while angling the transducer helps confirm the solid nature of the structure, which now appears moderately echogenic and contains thin hypoechoic ducts. It is essential that this nodule not be removed surgically, as this would lead to iatrogenic amastia. Spontaneous resolution of breast bud asymmetry occurs in nearly all affected children.

Premature Breast Development

Premature thelarche indicates abnormally early breast development. Because thelarche tends to occur earlier in girls of black African descent than in others, the most commonly utilized clinical definition of premature thelarche is the onset of breast development prior to 7 years of age in blacks and 8 years in whites. Premature thelarche can be unilateral or bilateral, and may occur as an isolated abnormality or in association with central precocious puberty.^3–5 The most common cause of premature thelarche appears to be subtle overfunction of the pituitary–ovarian axis. Menarche in these patients occurs at the usual age. Isolated premature thelarche generally occurs between 1 and 3 years of age and is nonprogressive.

Sonography is useful for selected children with premature breast development to document that the palpable “lump” represents normal breast tissue (Figure 9-4). In addition, sonographic evaluation of the uterus and ovaries can aid in the differentiation between isolated premature thelarche and central precocious puberty. A radiographic bone age assessment is usually indicated for patients with clinical findings suspicious for precocious puberty.

Accessory Breast Tissue

Accessory breast tissue refers to normal breast tissue in an ectopic location. Because the ectopic breast tissue typically occurs in conjunction with a normal ipsilateral breast, this represents polymastia, or multiple breasts. Accessory breast tissue can occur anywhere along the courses of the embryonic primitive mammary streaks, from the axillae to the inguinal regions. The most common location is the axilla; the vulva is the next most common site. The accessory breast tissue may occur as a very small asymptomatic focus of parenchyma to a more developed structure with a nipple and areola. The most common clinical manifestation of supernumerary breast tissue is the isolated presence of a supernumerary (accessory) nipple; this is termed polythelia.⁶

The clinical presentation of accessory parenchymal breast tissue often occurs during thelarche, when hormonal stimulation causes the tissue to enlarge. Discomfort and swelling may occur; the symptoms are exacerbated by pregnancy, during which milk secretion may be observed. When ectopic parenchymal tissue is present without a nipple or areola, the clinical diagnosis is more difficult. The anomaly is sometimes mistaken for a lipoma, lymphatic malformation, or adenopathy.⁷

The typical sonographic appearance of accessory breast tissue is that of moderately echogenic tissue that has a similar character to the normal breast. The tissue may contain varying proportions of glandular, fatty, and fibrous tissue. If the accessory breast lacks a pathway for drainage through an accessory nipple, secretory response during hormonal stimulation (particularly during pregnancy) can lead to sonographically detectable duct ectasia or a galactocele.⁸

Accessory breast tissue is demonstrated with MR imaging as a poorly demarcated mass that contains tissue of mixed single intensity on T1-weighted images. Fat-suppressed T2-weighted images show moderately high signal intensity within the parenchyma and interspersed hypointense areas of fat. The amount of fatty tissue within the lesion varies between patients. As with normal breast tissue, there is a moderate degree of contrast enhancement of accessory breast tissue.⁹

Amastia

Amastia refers to developmental absence of the breast. Individuals with this rare anomaly often have additional manifestations of Poland syndrome (unilateral pectoralis muscle aplasia, undergrowth/hypoplasia of ipsilateral chest wall and upper-extremity structures). Athelia (absence of the nipple) accompanies amastia in many patients. There are rare instances of bilateral amastia associated with ectodermal dysplasia.¹⁰

Gynecomastia

Gynecomastia refers to excessive development of the breast in a male. In children, the peak frequencies are in the neonatal period and during adolescence. Neonatal gynecomastia in infants of both genders is a result of the effects of maternal estrogens. This transient gynecomastia of the newborn is a self-limited abnormality that usually resolves by about 12 months of age. Breast tissue enlargement in otherwise normal adolescent boys is termed physiological pubertal gynecomastia. This most often occurs approximately 1 year after the onset of puberty; the peak age frequency is 13 to 14 years. Some degree of gynecomastia occurs in 60% to 75% of healthy boys. Spontaneous resolution occurs within 1 to 2 years. Physiological pubertal gynecomastia is caused by an increase in free testosterone, which is metabolized to estradiol by the enzyme aromatase. Excessive body fat facilitates this conversion.¹¹

Although gynecomastia is physiological in the great majority of affected boys, various pathological conditions can cause mammary tissue enlargement, including Klinefelter syndrome, testicular insufficiency, and androgen receptor defects. Other rare causes of gynecomastia include Sertoli or Leydig cell tumors of the testis, feminizing adrenal cortical tumors, gonadotropin-secreting tumors (hepatoblastoma, fibrolamellar carcinoma, chorio-carcinoma), prolactinoma, liver disease, and neurofibromatosis type 1. Drugs that can lead to gynecomastia include digitalis, isoniazid, tricyclic antidepressants, cimetidine, and marijuana. Athletes or bodybuilders who consume nutritional supplements (e.g., dehydroepiandrosterone [DHEA]) or anabolic steroids can develop gynecomastia.^5,12

When the clinical diagnosis is in doubt, sonography serves to confirm that the palpable mass in a boy with suspected gynecomastia is breast tissue (Figure 9-5). These sonographic findings are identical to those of a girl with premature thelarche. The breast tissue is usually hypoechoic relative to adjacent subcutaneous fat, and is located in the subareolar region (Figure 9-6). The echogenicity increases when transducer angulation provides interrogation at an oblique or perpendicular angle to the majority of the ducts.^2,13