A Diagnostic Revelation: Case of a Mucinous Carcinoma of the Breast

Mucinous or colloid cancers are a rare subtype of invasive ductal carcinoma, making up only 2-3% of infiltrating carcinomas. Prevalence of pure mucinous breast cancer(PMBC) among infiltrating duct carcinomas in less than 60 year olds is 2-7%, and in less than 35 year olds, it is 1%. Mucinous carcinoma of the breast is divided into 2 subtypes, the pure type and mixed type. PMBC is characterized by a lower incidence of nodal involvement, favourable histological grade, and higher ER/PR expression. Axillary metastases are rare, though found in 12-14%. It has a better prognosis than infiltrative ductal cancer with 10-year survival being more than 90%. Here is the case of a 70-year-old female who presented with lump in the left breast since 3 years. On examination, we detected a left breast lump occupying the whole breast except lower outer quadrant, measuring 10 × 8 cm with overlying skin stretched with puckering and engorged veins seen, nipple displaced laterally and higher by 1 cm, firm to hard in consistency, and mobile with breast tissue. Sonomammography, mammography, FNAC and biopsy were suggestive of benign phyllodes tumour. Patient was hence posted for simple mastectomy on the left side with removal of attached lymph nodes (near axillary tail). Histopathological examination revealed pure mucinous breast carcinoma with nine lymph nodes, free from tumour and showing reactive hyperplasia. Immunohistochemistry studies demonstrated ER + , PR + , HER-2-NEU-. The patient was started on hormonal therapy. Therefore, mucinous carcinoma of the breast is a rare entity with imaging features sometimes mimicking a benign tumour such as Phyllodes tumour, hence making it important to include it as a differential diagnosis in our daily practice. It is especially important in the subtyping of carcinoma of the breast since it carries a favourable risk profile with less chances of lymph node involvement, higher hormone receptor positivity and good response to endocrine treatment.