Ectopic breast tissue is a remnant of the mammary ridge during embryogenesis without regression [1, 3, 5–9], and EBC originating from ectopic breast tissue is rare, accounting for only 0.3–0.6% of all breast cancers [1]. It is very rare for EBC and pectoral breast cancer to occur simultaneously or metachronously as in this case. Only 5 cases [10, 14–16] are found in the literature, and there are no reports of postoperative cases of bilateral breast cancer.
Diagnosis is often delayed because it is rarely considered clinically and because it occurs in areas that are not examined by usual breast cancer screening [2–4]. Since the majority of EBC cases occur in the axilla [2, 6, 10], it is crucial for patients and doctors to recognize axillary masses at an early stage. Since axillary ectopic breast tissue is usually located just beneath the skin and outside the site where axillary lymph nodes are present, EBC should be considered first if a mass is found at that site [4, 9].
The ultrasound findings of EBC are similar to those of pectoral breast cancer, most of which involve irregularly shaped, hypoechoic, indistinct-border or spiculated masses [6, 7, 9]. However, they may vary based on histological structure and type. It has been reported that internal echoes are hyperechoic in medullary carcinoma and mucinous carcinoma [2, 11]. Therefore, ultrasound-based morphology is thought to depend on the tissue architecture of breast cancer cells. The presence of hyperechoic areas surrounding the mass, which indicate mammary glands, is a suspicious feature of axillary EBC [4, 9]. Fibroadenoma and phyllodes tumours also originate from ectopic breast tissue [12, 13] and they also should be included in the differential diagnoses in addition to lymph node metastasis.
For this patient, the possibility of axillary lymph node recurrence was considered low because 28 years had passed since surgery for left breast cancer, and there was no lymph node metastasis. However, the mass was oval in shape, the boundaries were clear, and the internal echo was relatively homogeneous; therefore, lymph node metastasis replaced by breast cancer cells was first suspected [17]. Since there was no palpable induration suggestive of ectopic breast tissue around the mass, EBC was not actively considered. This was later confirmed by histopathological examination, in which only minimal breast tissue was observed around the mass. As a result, it was thought that no hyperechoic mammary gland tissue was detected via ultrasound. Therefore, as indicated earlier, attention should be given to the site of the tumour, namely, just beneath the skin and outside the axilla.
An excisional biopsy showed no lymphoid tissue, and the status of the surrounding breast tissue led to the diagnosis of EBC. The tumour appeared to be an expansive growth pattern of ductal carcinoma in situ, but it was histologically identified as invasive ductal carcinoma because most of the lesion was missing the myoepithelium. On ultrasound, the tumour had an expansile morphology, solid and homogenous tumour cells, well-defined boundaries, and an overall oval shape, so it resembled a metastatic lymph node replaced by breast cancer cells[4, 17].
Mastectomy was previously performed for EBC. However, there is currently no evidence favouring mastectomy over wide excision, which includes the removal of ectopic breast tissue; wide excision is usually chosen currently [2, 3, 7, 18]. In this patient, mastectomy and axillary clearance had already been performed, and only wide excision was performed. Postoperative therapy was administered in accordance with the criteria for pectoral breast cancer [4, 7, 19]. The risk of recurrence was low because of the tumour findings of diameter of 6 mm, grade 1, ER-positive, PR-positive, HER2-negative, and Ki67 of 14%, hormone therapy alone was administered.