Mucinous (colloid) carcinoma

Mucinous (colloid) carcinoma accounts for approximately 5% of breast carcinomas and typically occurs in older women. It is associated with a better prognosis than the invasive ductal carcinoma NOS. Mucinous carcinomas can be divided into pure, mixed and signet ring cell carcinoma, with the latter having the worst prognosis. Mammography usually shows a smoothly outlined to lobulated mass, rather than the stellate-shaped infiltrating pattern of the usual type of breast cancer, and a soft mass is palpated.

Cytological diagnostic features

• Scattered three-dimensional groups of mildly to moderately atypical ductal cells.
• Abundant extracellular mucinous (gelatinous) material
• Usually grade 1-2 nuclei
• Cellular monorphism
• Relatively low tumour-to-stromal ratio

Mixed types of mucinous carcinoma show feature of the pure variant along with cytological findings of the conventional invasive ductal carcinoma. Special stains can highlight the mucinous material, but this is not usually needed. The prognosis of mixed types is the one of invasive ductal carcinoma NOS. The presence of mucinous material in breast FNA is not a definitive feature of mucinous carcinoma, as mucin can be present in several breast lesions, including fibroadenoma with stromal myxoid degeneration and benign breast cysts.