Signet-ring cell carcinoma of the breast. The mucinous variant of infiltrating lobular carcinoma?

Five cases of the type of mammary carcinoma that has been designated "signet-ring cell carcinoma" are presented. This tumor is characterized by the presence of numerous cells containing intracellular mucin, without large amounts of extracellular mucin as is seen in colloid (gelatinous, mucinous) carcinoma of the breast. Although such cells may be seen in many mammary carcinomas, they are never as frequent as in the variant described. Ultrastructurally, the most characteristic finding is the presence of numerous intracellular lumina containing material which appears to represent the mucin identified with the light microscope. This finding differs from that in colloid carcinoma, in which the scantier intracellular mucin occurs in the form of intracytoplasmic membrane-bound vesicles. The five tumors in the present series were all associated with either in situ lobular carcinoma or a "sinus catarrh"-like pattern of nodal metastases, or both. On the basis of these light and electron microscopic data, the signet-ring cell carcinoma is suggested as a variant of infiltrating lobular carcinoma, clinically and pathologically distinct from colloid carcinoma.