Definition

Mucocele­-like lesions (MLL) of the breast were first described by Rosen in 1986 as rare benign lesions, defined as mucin filled cysts and extravasated mucin in the adjacent stroma without (or with minimal) inflammatory reaction. The similarity in pathologic appearance to mucocele lesions elsewhere in the body inspired the name given to the breast lesion by Rosen.

About four years later, studies reported that the MLL of the breast may be associated with malignancy, showing an association with columnar cell lesions (CCLs), atypical ductal hyperplasia (ADH), and even with cribriform and micropapillary ductal carcinoma in situ (DCIS) and invasive carcinoma, usually of the mucinous type. Other studies have suggested that MLL and mucinous carcinoma may represent the two ends of the pathological spectrum of the mucinous lesions of the breast.

As the literature presents controversies and a lack of consensus on multiple issues, the terminology used in the literature is quite confusing. The terms “mucocele-like tumors” and “mucocele-like lesions” are synonymous.

Etiopathogeny and Risk Factors

Despite the fact that mucocele-like breast tumors are uncommon, there are some reports of an increasing frequency of this diagnosis at percutaneous imaging-guided large-core biopsy.

However, the pathogenesis of this lesion is still not entirely clear, what makes it difficult to understand what factors may influence the association of this lesion with atypia and carcinomas. In addition, the risk factors that predispose the MLL and the radiological presentations that suggest such injury are still unknown.

Some studies propose that excessive mucinous secretions or ductal obstruction may be contributing factors. Other studies point out that distention of the cysts, associated with incidental trauma to the breast may result in rupture and subsequent extravasation of the cyst contents.

Radiological Presentation

Mucocele-like lesion can be an asymptomatic lesion detected on a screening mammogram or it may present as a palpable mass.

On mammography, the most common form of presentation are microcalcifications and, less commonly, nodules.

On ultrasonography, the most common findings are a unique or a cluster of apparently complex cysts or hypoechogenic tubular structures with no flow visualized on color Doppler evaluation that may be associated with calcifications and / or mural nodules.

Surgery x Follow Up

The management after the identification of mucocele-­like lesions diagnosed on core biopsy percutaneous biopsy is not well established. The literature is still limited  on determining if excisional biopsy is necessary, particularly in the absence of MLL associated with atypical ductal hyperplasia or masses. This lack of consensus occurs mainly due to the wide spectrum rates, which range from 0% to 43%, between studies that have shown an upgrade to atypia or malignancy on excision after a MLLs diagnosed on core biopsy.

Despite the well-known reliability of core-needle biopsy (CNB) as a diagnostic method for breast lesions, controversy remains on whether CNB of an MLL is sufficient to diagnose benign pathology and thus, to avoid surgical excision. Due to the limited material available on CNB specimen, paucicellular mucinous carcinoma cannot be excluded, particularly when the mass is palpable or radiologically evident, or when the sample exhibits ADH. On the other hand, for pure MLL on CNB, some recent studies suggests that surgical excision may not be required.

The decision of how to manage such lesion also varies according to the type of procedure performed and age. Differentiating a mucocele-like tumor from a mucinous carcinoma can be difficult with fine-needle aspiration or large-core needle biopsy, although the rate of false-negative biopsy findings may be reduced with the use of vacuum-assisted devices.  

An information that can helps in differentiating those lesions is that mucinous carcinoma is uncommon in women younger than 40 years, so if a biopsy in a young woman reveals extravasation of mucin into the adjacent stroma, a mucocele-like tumor should be considered as a possible diagnosis.

As we have shown there are limited studies regarding the clinical and radiologic features that can help to predict malignancy or the image-pathologic correlation of pure MLLs diagnosed on CNB to determine which lesions truly require or do not require excision. There is also no concordance among pathologists in the recommendation that follows a benign diagnosis of mucocele-like tumor at percutaneous imaging-guided large-core biopsy. Because of the wide spectrum of presentations of mucocele-like lesions and the lack of information of how to manage each of them, it turns to be an interesting topic to discuss.