Teaching Case

A 51-year-old woman underwent her first screening mammography without any clinical evidence or family history of breast cancer. She was referred to our hospital because of a positive screening mammography.

Mammography showed an irregular and ill defined nodule with gross calcifications, located in the lower outer quadrant of the left breast.

The patient underwent ultrasound (US) examination, which demonstrated the presence of a solid, hypoechoic and ill defined mass of 4 cm with posterior acoustic shadowing. Pathologic homolateral axillary lymph nodes were found.

A tru-cut biopsy (14G needle, 4 specimens from the mass and 3 from axillary lymph nodes) guided by US was performed. Histology revealed a mucinous carcinoma and pathologic homolateral axillary lymph nodes.

We performed a MR to evaluate the local extension. The MR showed a lobulated and ill defined mass. It had a hypointense signal on T1 weight images and hyperintense signal on T2. The mass showed progressive enhancement pattern with a curve type I.

Mucinous carcinoma (MC) of the breast is a rare histological type of invasive adenocarcinoma, which is characterised by a large amount of mucin production. [1] MC of the breast is defined as having a mucinous component of 50% or more. [2]

The incidence of MC was reported to be 1-6% of all primary breast cancers. The median age at diagnosis is older than 55-60 years. [1]

At histologic examination, MC consists of well-differentiated carcinoma cells arranged in small clusters that are surrounded by a pool of mucus. It can be histologically categorised into pure and mixed. It is important to apply strict histologic criteria to differentiate between pure and mixed mucinous carcinoma because pure tumours metastasise to axillary lymph nodes less frequently than mixed tumours and can be managed with less invasive surgery than mixed mucinous tumours. Axillary lymph node metastases have been reported in 14-15% cases of pure tumours meanwhile in mixed tumours they were 46% [3].
MC commonly manifests as a well-circumscribed round or lobular mass that sometimes has a microlobulated appearance at mammography and US. A well-circumscribed margin, homogeneity, and an isoechoic appearance are features associated with pure mucinous carcinoma; whereas an indistinct margin, heterogeneity, and a hypoechoic appearance may be more commonly involved with the mixed type of mucinous carcinoma. [1, 2]

MR features of pure MC of the breast include lobulated shape mass, very high signal intensity on T2-weighted images (owing to the abundant accumulation of extracellular mucus rich in free water). Although T2-weighted signal hyperintensity lesion is a strong indicator of benign histology as myxoid fibroadenoma and some papillomas, a MC may also show this feature. [4]

MC shows gradual enhancement on dynamic MR images, pattern type I, with a continuous increase in signal intensity on each successive contrast enhanced image. [1, 5]
On T1-weighted images they varied from low intensity to high intensity.

In summary, MC can appear with relatively benign findings. We have to add MC to the differential diagnosis when we find a new well/partially circumscribed round/lobular mass or if a mass shows a larger size than in previous mammography.
We have to think about a MC if we see a hyperintense mass on T2, although myxoid fibroadenoma and some papillomas show this feature.
MC show gradual enhancement on dynamic MR images, this pattern is usually associated with a benign finding; only 9% of lesions with this type of curve are malignant.