Teaching Case

An ultrasound (US) examination of the left axilla was requested to evaluate a sudden appearing nodular axillary tumefaction in a 35-year-old woman in her first day of the puerperium. No other inflammatory signs were apparent and the patient denied any accompanying symptoms such as fever or local pain.

The US examination of the left axilla (performed with a high frequency linear array transducer) showed multiple tubular anechoic structures admixed with echogenic vascularized tissue in between, globally measuring 38x21x11mm (Fig. 1 and Fig. 2). A smaller amount of tissue (12x5x3mm) with similar echotexture was found at the contralateral axilla. These findings were considered compatible with ectopic axillary breast tissue with prominent lactating ducts and a careful clinical observation was performed subsequently revealing several supernumerary nipples along the left and right milk lines that were previously not noticed by the patient (Fig. 3, 4 and 5).

Background
Polymastia, supernumerary breasts, accessory breasts and ectopic breast tissue are all synonyms designating the presence of more than two breasts in humans [1]. This variant can occur at up to 6% of the population, more frequently in the axilla, although it may appear in any location along the milk line [2] extending from the axilla to the external genitalia [3] or even in locations outside the mammary ridge, such as the face, posterior neck, middle back, hip, shoulder, upper extremities and posterior and lateral thigh [2]. A classification system (comprising 8 classes) for supernumerary breast tissue created by Kajava in 1915 is still applied today [1] and it is based on which elements are present in the accessory breast tissue (nipple, areola, glandular tissue, hair) and their possible combinations. Our case is a class 2, consisting of glandular tissue with a nipple and without an areola.

Clinical Perspective
Ectopic breast tissue is under the same hormonal influences and risk of disease as normally located breast tissue [4]. Lactational changes can occur in ectopic breast tissue [4], as in our case, and this variant should not be misdiagnosed as pathological. Apart from the physiological and pathological (benign and malignant) conditions that can occur in ectopic breast tissue, this entity is sometimes associated with congenital anomalies of the urogenital system [2].

Imaging Perspective
The differential diagnosis of sudden appearing axillary tumefaction in a lactating woman should include the hypothesis of ectopic axillary breast tissue.
On US, mammography and magnetic resonance (MR), accessory breast tissue has the same imaging appearance of eutopic breast tissue [4].
US is the most appropriate imaging modality to evaluate breast disorders in pregnant and lactating women [5]. The US features of the lactating breast include a prominent ductal system (due to milk secretion) and an increased vascularity on color Doppler [5], both observed in our case of ectopic breast tissue.
Overall, mammography has a decreased sensitivity in the lactating breast due to its increased density, nevertheless it may be helpful if malignancy is suspected. MR is controversial and difficult to interpret because of the pattern of enhancement of the lactating parenchyma and its diffuse high signal on T2-weighted sequences. [5]

Outcome
Most patients with accessory breast tissue are asymptomatic. If required, surgical treatment is the option for cosmetic and physical discomfort reasons. Management of accessory breast tissue cancer should follow the recommendations for pectoral breast cancer of parallel TNM classification [2].