Accessory breast tissue originates from embryonic mammary ridges extending from the axillary to inguinal regions, resulting from incomplete regression during embryogenesis. Malignant transformation occurs in the epithelial tissue of accessory breasts is classified as accessory breast carcinoma. The overall incidence of accessory breast carcinoma is about 0.3%-0.6% (1), with males accounting for only 2.4%-5.3% of these cases (2). Due to the scarcity of documented cases, the management of male accessory breast carcinoma remains nonstandardized, complicating the formulation of evidence-based diagnostic and therapeutic protocols. Here we present a case of male axillary accessory breast carcinoma. Given its exceptional rarity, we herein describe the diagnostic challenges, therapeutic approach, and clinicopathological characteristics to augment the limited clinical experience with this entity.

A 72-year-old male presented to our clinic with a right axillary mass that had been present for three years. The patient first noticed the mass approximately three years ago, when it measured approximately 0.8×0.5 cm. At that time, the mass was painless and not associated with any redness, swelling, ulceration, or other discomfort. Physical examination at initial discovery showed no changes in the skin or contour of either breast, no nipple discharge or bleeding, and no swelling or pain in the right upper limb or neck region.

Over the following three years, although the patient reported no particular discomfort, he noticed gradual enlargement of the mass. During the most recent 1–2 years, the mass grew to approximately 1.5×0.8 cm, became raised above the skin surface, and developed tenderness when pressed, though it remained without spontaneous pain, local redness, congestion, or ulceration. Repeated examinations confirmed no changes in breast skin or contour, no nipple discharge or bleeding, and no swelling or pain in the right upper limb or neck region.

On current physical examination, we identified a well-circumscribed, smooth-surfaced, mobile mass (2.5×1×1 cm) elevated above the skin in the right axilla. The lesion showed no ulceration, vesicle formation, erythema, edema, or peau d’orange sign. No vascular bruits were auscultated. Examination of both breasts revealed no abnormalities. Ultrasonography demonstrated a solid hypoechoic mass (29.3 mm × 13.7 mm × 16.2 mm) located 9 mm beneath the epidermal surface in the right axilla. The lesion exhibited well-defined margins, regular morphology, and heterogeneous internal echogenicity. Color Doppler flow imaging (CDFI) revealed moderately abundant intralesional vascularity (Figures 1A, B). Preoperative abdominal ultrasonography and chest radiography revealed no detectable abnormalities.

The patient’s medical history was notable for long-term tobacco use (15 years, averaging 10 cigarettes per day) and regular alcohol consumption (10 years, averaging 100 mL per day). No other significant medical history or family history of malignancy was reported.

The patient underwent transaxillary subcutaneous mass excision under local anesthesia. The mass was completely excised. Intraoperative findings revealed a 2.5 cm × 1.0 cm × 1.0 cm cystic-solid mass with a tan-brown surface and a well-circumscribed capsule. Sectioning of the mass demonstrated a cystic component containing colorless, transparent gelatinous fluid, while the solid component exhibited moderate consistency.

Histopathological examination revealed a tumor involved the subcutaneous tissue, without a definitive capsule or lymphoid stroma, with infiltrative growth patterns adjacent to cutaneous adnexal eccrine glands (Figure 2A). Neoplastic cells were arranged in tubular and solid sheet patterns, demonstrating moderate cytological atypia characterized by eosinophilic cytoplasm, conspicuous nucleoli, and absence of apocrine snouts. The mitotic count was approximately 10 per 2 mm² (Figure 2B). Morphological features were consistent with mammary-origin invasive ductal carcinoma. The resection margins were negative and without lymphovascular invasion.

Immunohistochemistry: CK-Pan (+), CK7 (+), ER (strong nuclear positivity, 90%; Figure 2C), PR (strong nuclear positivity, 90%; Figure 2D), AR (moderate-to-strong nuclear positivity, 90%), E-cad (+; Figure 2E), P120 (membranous +; Figure 2F), GATA-3 (+; Figure 2G), TRPS1 (+; Figure 2H). CK5/6 (−), HER2 (0), P53 (−), CD117 (−), P63 (−), neuroendocrine markers CD56 (−), Syn (−), CgA (−), INSM1 (−). Ki-67 (10% +; Figure 2I).

Diagnosis: Invasive ductal carcinoma, not otherwise specified, grade II, right axilla.

Subsequent Therapeutic Interventions.

Based on the histopathological findings, the multidisciplinary tumor board recommended guideline-directed care comprising: I, systemic staging via breast and axillary Magnetic Resonance Imaging (MRI), thoracic/abdominal Computed Tomography (CT) scan; II, additional comprehensive axillary lymph node dissection; III, adjuvant chemotherapy combined with endocrine therapy based on hormone receptor status. The patient declined the recommended interventions. Sutures were removed on postoperative day 10, with the incision achieving Grade I/A healing. At the 11-month follow-up, the right axillary surgical site exhibited optimal wound healing without evidence of palpable lymphadenopathy.

The common risk factors for male breast cancer include: ① age, specifically males aged 60 years and above; ② endocrine factors such as elevated estrogen levels and testicular diseases; ③ obesity and physical inactivity; ④ genetic predisposition in first-degree relatives; and ⑤ radiation exposure.In the current follow-up, the patient was found to have adverse lifestyle habits characterized by a long-term history of tobacco and alcohol use (smoking 10 cigarettes per day for 15 years, and consuming 100 ml of alcohol per day for 10 years). Long-term tobacco and alcohol use is a high-risk factor for tumors. Previous studies have demonstrated that smoking and alcohol consumption increase the risk of breast cancer in females (3, 4); however, there is a lack of research on the correlation with male accessory breast cancer. The report of this case serves as a reminder in this regard.

Accessory breast tissue may occur at diverse anatomical sites, most frequently in the axillary region (approximately 70% of cases), followed by the chest wall, abdominal wall, and inguinal regions (1, 5). The incidence of accessory breast tissue in males is exceedingly low, and clinicians may not readily consider accessory breast pathologies during initial evaluation. This case underscores the necessity of including accessory breast carcinoma in the differential diagnosis when evaluating superficial masses in the axilla, chest wall, abdominal wall, or inguinal regions, even when clinical suspicion is low.

The predominant histological subtype of accessory breast carcinoma is invasive ductal carcinoma, accounting for 70%–90% of reported cases (6, 7). This malignancy is clinically insidious, posing significant diagnostic challenges in early stages. Initial clinical manifestations typically involve painless masses in the axillary or inguinal regions, while ancillary symptoms such as localized swelling, mastalgia, or nipple discharge are less frequently reported (8, 9). The clinical presentation in our reported case aligns with this phenotypic pattern.

The following entities constitute critical differential diagnoses requiring comprehensive evaluation (10–15): ① Accessory breast tissue-related pathologies, (e.g., fibroadenomas, including juvenile gynecomastia in adolescent males); ② Sebaceous cysts; ③ Lipomas; ④ Benign or malignant neoplasms arising from cutaneous adnexal structures; ⑤ Lymphadenopathy, encompassing benign entities (e.g., lymphadenitis, reactive lymphoid hyperplasia, tuberculous lymphadenitis) and malignant conditions (e.g., lymphoma, metastatic carcinoma); ⑥ Hidradenitis suppurativa; ⑦ Castleman disease. (Table 1).

For axillary masses with postoperative pathological confirmation of breast carcinoma, the diagnostic workup must first exclude the following possibilities: ① Secondary metastatic carcinoma, particularly axillary lymph node metastases from primary adenocarcinomas of the breast, gastrointestinal tract, prostate, or lung that may mimic subcutaneous axillary masses; ② For pathological diagnosis, the pathological type should support that the heteromorphic cells originate from the mammary glands. Since the mammary glands, sebaceous glands, and sweat glands all originate from the same stem cells, during the diagnostic process, immunohistochemical auxiliary diagnosis should be employed to ensure the accuracy of the diagnosis.

The histological features of this case necessitate primary differentiation from apocrine carcinoma, a rare malignancy predominantly localized to the axillary region. Apocrine carcinoma typically manifests as solitary or multiple slow-growing nodules or plaques with erythematous to violaceous surfaces, occasionally demonstrating ulceration. Histopathologically, it is characterized by glandular, ductal, papillary, or solid growth patterns predominantly involving the deep dermis and subcutaneous adipose tissue. Tumor cells exhibit abundant eosinophilic cytoplasm with definitive apocrine secretion. The nuclei are round to oval, vesicular, and exhibit a single prominent nucleolus (16, 17).

In contrast, the present case demonstrated tubular and solid growth architectures without papillary or ductopapillary structures. Neoplastic cells displayed moderate eosinophilic cytoplasm devoid of apocrine differentiation, a critical morphological distinction. Immunohistochemically, both entities share overlapping profiles (AE1/AE3, ER/PR/AR, GATA3), limiting their discriminative utility. However, TRPS1—a highly sensitive and specific biomarker for mammary-origin malignancies (18)—exhibited strong positivity in this case, further supporting the diagnosis of accessory breast carcinoma. In summary, histomorphology characteristics, particularly the presence or absence of apocrine differentiation, constitute pivotal diagnostic discriminators between these entities.

The current case was diagnosed as invasive ductal carcinoma. To better understand the pathological features of invasive breast cancer, we also reviewed and compared it with non-invasive carcinoma. The fundamental pathological distinction between invasive and non-invasive carcinoma lies in the integrity of the basement membrane. In non-invasive ductal carcinoma, tumor cells are confined within the ducts without breaching the basement membrane, while in invasive carcinoma, tumor cells penetrate the basement membrane and infiltrate into the stroma or surrounding tissues. Non-invasive carcinomas include ductal carcinoma in situ (DCIS), where tumor cells remain within the ductal basement membrane, and lobular carcinoma in situ (LCIS), where tumor cells do not breach the basement membrane of terminal ducts or acini. Currently, DCIS is primarily graded based on nuclear features (19) (Table 2). LCIS represents an important special lesion that requires differentiation from DCIS. LCIS cells demonstrate poor adhesion and distend ≥50% of acini in the terminal duct-lobular unit, and can be classified as classic, florid, or pleomorphic types. When involving <50% of acini, it is diagnosed as atypical lobular hyperplasia (ALH). Immunohistochemically, LCIS typically shows loss of E-cadherin membranous staining or other abnormal expression patterns, strong diffuse cytoplasmic staining of p120 and β-catenin, and loss of β-catenin membranous staining. The differential diagnosis between LCIS and DCIS should integrate both histomorphology and immunohistochemical markers. Classic and florid LCIS usually show diffuse and homogeneous ER positivity with low Ki-67 index, while the pleomorphic subtype is often ER-negative but may express androgen receptor (AR). HER2 overexpression and gene amplification can also occur, with approximately 10% of pleomorphic LCIS cases being triple-negative (20–22).

Given the limited clinical experience among practitioners, initial diagnostic evaluations are typically restricted to conventional imaging modalities such as Doppler ultrasonography. Advanced modalities including CT, MRI, and positron emission tomography - computed tomography (PET-CT), while diagnostically valuable, are reserved for selective use in clinical practice due to cost and accessibility constraints. For cases with high clinical suspicion of accessory breast carcinoma, image-guided core needle biopsy may be considered (23, 24).

In cases where malignancy demonstrates low clinical suspicion, local excision is usually performed during the index procedure, leading to a definitive diagnosis of accessory breast carcinoma established only after comprehensive postoperative histopathological evaluation. Due to limited evidence, the prognostic implications of this conservative surgical approach remain inconclusive.

Currently, no established therapeutic consensus exists for male accessory breast carcinoma, and clinical management primarily aligns with protocols for male breast cancer (25, 26). Treatment strategies should be tailored according to tumor stage and molecular subtype. Notably, limited evidence supports the routine use of neoadjuvant therapy in male breast cancer, with controversial indications largely extrapolated from female breast cancer guidelines (25). Consequently, neoadjuvant therapy is not recommended as first-line intervention for male accessory breast carcinoma. Surgical resection, including wide local excision with lymph node dissection, remains the cornerstone of definitive management. The performance of a mastectomy depends on anatomical proximity. Modified radical mastectomy should be considered when the lesion is contiguous with native breast tissue. Conversely, preservation of native breast parenchyma is advised if the accessory breast constitutes a separate anatomical entity, as prophylactic mastectomy offers no oncologic benefit. Postoperative adjuvant therapies—including chemotherapy, radiotherapy, endocrine therapy, and targeted therapies—should be individualized based on clinicopathological risk stratification (19, 20). Over 90% of male breast cancers are hormone receptor-positive. Current international guidelines recommend a 5-year tamoxifen regimen, with extended adjuvant therapy (additional 5 years) reserved for high-risk patients demonstrating adequate tolerance (26).

Therefore, if the patient agrees to further treatment, given that this case is ER/PR-positive, HER2-negative, and has a Ki-67 ≤20%—consistent with Luminal A breast cancer—the tumor is likely highly sensitive to endocrine therapy but generally less responsive to chemotherapy. We recommend completing 5 years of tamoxifen therapy. Although some studies have explored neoadjuvant chemotherapy (NAC) in male breast cancer (27), the evidence remains limited, and clinical practice largely extrapolates from female breast cancer data, which remains controversial. Notably, studies report that pathologic complete response (pCR) rates in men are consistently lower than in women, particularly in hormone receptor-positive subtypes (28). Currently, there is insufficient high-quality evidence to guide chemotherapy decisions for male breast cancer patients. Based on available clinical data, TC (docetaxel + cyclophosphamide) or AC (doxorubicin + cyclophosphamide) regimens may be considered for this patient if chemotherapy is pursued.

Male accessory breast carcinoma is extremely rare, exhibits relatively high malignancy, and has a low 5-year survival rate. Preoperative diagnosis remains one of the most significant challenges. Clinically, when encountering masses in the axillary or inguinal regions, the possibility of accessory breast carcinoma should not be easily excluded regardless of the patient’s sex. Preoperative evaluations should include necessary examinations, and core needle biopsy is a practical diagnostic approach for suspicious lesions. Surgical intervention primarily involves local excision combined with axillary lymph node dissection. Postoperative adjuvant therapies, including chemotherapy, radiotherapy, endocrine therapy, and targeted therapy, should be administered based on individualized assessments.