Mammography shows normal fibroglandular tissue and was therefore classified as false negative according to standard definitions in the BI-RADS Atlas. As many as 21% of mucinous carcinomas might at any age and tumor size might not be detected mammographically.

The patient underwent breast-conserving surgery and sentinel node dissection. 

Pathological examination confirms the diagnosis of a hypercellular type B or mixed mucinous breast cancer of 13 mm, grade 2. The positivity of estrogen receptor (ER) and progesterone receptor (PR) is 100% and 90%, respectively. Ki67 was 25%. HER2 (human epidermal growth factor receptor 2) is equivocal on IHC (immunohistochemistry) and negative on FISH test (fluorescence in situ hybridization). The sentinel node is negative.

Mucinous carcinoma of the breast is a rare tumor with a favorable prognosis due to the low incidence of lymph node metastases. The prevalence is only 1% in women younger than 35 years.