Ductal adenoma of the breast—a lesion exhibiting a myoepithelial/epithelial phenotype

A detailed immunocytochemical study has been carried out on a case of ductal adenoma of the breast. The lesion was oestrogen receptor positive and contained focal areas of apocrine metaplasia and marked nuclear atypia. The tumour had a pseudo-infiltrative growth pattern and showed good evidence of both epithelial and myoepithelial differentiation. Islands of tumour cells were surrounded by a clearly defined basement membrane as demonstrated with an anti-type IV collagen antibody. The heterogeneous staining pattern produced by monoclonal antibody BA17, which recognizes keratin 19, was similar to that seen in normal lobules and raises questions concerning the site of origin of this lesion in the breast.