Mucinous adenocarcinoma of the breast is characterized by clusters of generally small and uniform cells floating in a lake of extracellular mucin. The mucous lake is compartmentalised by delicate fibrous septae. The cell clusters vary in size and shape and occasionally assume a tubular or, rarely, a papillary arrangement. Atypia, mitoses and microcalcifications are rare. 30-75% of the tumours contain an intraepithelial component with a micropapillary to solid pattern. The tumour usually presents as a palpable mass ranging in size from <1 to >20 cm (average diameter 2.8 cm). Mammography shows a well defined, lobulated lesion.
Pure mucinous carcinomas may be cellular or hypocellular. The cellular variant is more likely to contain intracytoplasmic mucin and argyrophilic granules. When there are other, not mucinous components, the lesion is classified as a mixed tumour. Regular invasive duct carcinoma is the most common admixture.
Mucinous carcinomas are typically estrogen receptor positive, while <70% are progesterone receptor positive.
Only about 2% of breast carcinomas are pure mucinous carcinomas. The tumour affects women over a wide age range (mean age >60 years). In general, the prognosis for pure mucinous carcinoma is favourable, with a 10-year survival of 80-100%. Mixed tumours have a less favourable prognosis. Axillary node metastases occur in 3-15% of pure mucinous carcinomas and in 33-45% of mixed tumours
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