The purposes of this study were to investigate whether the biological characteristics or outcomes of patients with metaplastic carcinoma, invasive ductal carcinoma, or invasive lobular carcinoma of the breast differ; to determine whether the metaplastic carcinoma subtypes have similar malignant potentials; and to identify accurate predictors of outcome in patients with metaplastic carcinoma. The subject comprised 6137 invasive ductal carcinoma patients, 301 invasive lobular carcinoma patients, and 46 metaplastic carcinoma patients of the breast. The metaplastic carcinomas were classified according to the World Health Organization classification. Multivariate analyses clearly demonstrated that the metaplastic carcinoma patients had a significantly poorer outcome than the invasive ductal carcinoma patients or the invasive lobular carcinoma patients independent of the nodal status or age not exceeding 39 years, whereas patients with triple-negative metaplastic carcinomas or triple-negative invasive lobular carcinomas had a poorer outcome than those with triple-negative invasive ductal carcinomas. Although no significant differences in clinical outcome were observed among the metaplastic carcinoma subtypes in multivariate analyses, an age not exceeding 39 years, the presence of skin invasion, and the presence of a squamous cell carcinoma component in nodal tumors were significant outcome predictors for metaplastic carcinoma patients. In conclusion, the results of this study clearly demonstrated that metaplastic carcinoma is more aggressive than invasive ductal carcinoma or invasive lobular carcinoma. Although the metaplastic carcinoma subtypes had no prognostic significance, an age not exceeding 39 years, the presence of skin invasion, and the presence of a squamous cell carcinoma component in nodal tumors were significant predictors of outcome among metaplastic carcinoma patients.
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