Mucoepidermoid carcinoma: An unusual presentation in breast. A case report

A 76-year-old woman was referred to our institution for assessment of a BI-RADS 5 abnormality in her mammogram. The patient related a painless palpable mass in the right breast a week ago. She has no personal or familiar relevant history.

A mammogram revealed a high-density spiculated mass with associated microcalcifications in the upper outer quadrant (Figures 1a and 1b), corresponding with the palpable abnormality. Ultrasound showed a hypoechogenic irregular solid mass, with spiculated margins and hyperechogenic halo, associated with microcalcifications and posterior shadowing, measuring 20 mm. At Doppler colour, the lesion was moderately vascularised (Figure 2). No axillary lymphadenopathies were identified. The lesion was classified as BI-RADS 5, and a Core biopsy was recommended.

A contrast-enhanced mammogram (CEM) was performed for local staging. The lesion showed an early, intense contrast enhancement measuring 24 x 18 mm. No new lesions were detected (Figures 3a and 3b). No further staging studies were performed. However, no other suspicious or secondary lesions were observed on CEM.

The histopathological report concluded a low-grade infiltrating mucoepidermoid carcinoma (MEC). Immunohistochemistry test reported oestrogen and progesterone hormone receptor-negative, human epidermal growth factor receptor 2 negative, and a Ki67 score of 10%. Additionally, immunoreactivity was positive for CK7 of the mucosecretory cells, CK5/6 and p63 of the squamous cells, and GATA 3 in a diffuse and intense form. The sentinel lymph node was negative.

MEC is a malignant neoplasm of epithelial origin in the salivary glands and is less frequently described in the thyroid, oesophagus, and nasal cavity, among others [1].

In the breast, its presentation is extremely unusual; the estimated incidence is 0.2–0.3% of all breast cancers [1,2]. So far, there are 44 cases published in the English literature between 1979 and 2022 [2] in women between 29 and 80 years of age [3].

Imaging appearance is variable and indistinguishable from other breast cancers by all imaging techniques, so histopathology and immunohistochemistry are essential for diagnosis [4].

Macroscopically, it appears as a solid or cystic-circumscribed mass [3] (Figure 4). Histopathologically, microscopy mucinous, intermediate (transitional), and squamous cells are observed (Figure 5). The absence of true keratinisation is essential to confirm the diagnosis [3]. Mucinous cells are positive for CK7 and squamous cells for CK5/6 and p63 and negative for oestrogen, progesterone, and c-erbB-2 receptors (triple negative) [5]. Neoplastic cells are also immunoreactive for GATA3 and mammoglobin [3,6]. Histologic grade is the most important prognostic factor. They can be graded according to the parameters used for salivary gland or mammary tumours with similar results [5].

Treatment is not standardised; as they are triple negative, they do not respond to hormonal therapy [4]. Usually, conservative surgery is performed. The management of the axilla will depend on the preoperative staging [4]. Chemoradiotherapy is considered in lesions larger than 3 cm or with metastasis. The prognosis depends on the histological grade, and despite being triple negative neoplasms, they have a sluggish behaviour [2,4]. Low-grade neoplasms have a better prognosis than their counterpart, with a low risk of metastasis and recurrence [4].

Our case is the only one with contrast-enhanced digital mammography correlation, a method that is gaining more and more ground daily. The case of our patient corresponds to low-grade MEC, which is reported even less frequently than high-grade MEC, and despite being classified as a triple-negative cancer, it has a better prognosis and response to treatment, with a low probability of metastasis and recurrence. For this reason, conservative management was decided upon in addition to the patient's longevity, without the need for adjuvant therapy.

All patient data have been completely anonymised throughout the manuscript and related files.