We report 20 examples of a distinctive form of intraductal papillary carcinoma frequently associated with both mucinous carcinoma and infiltrating ductal carcinoma, not otherwise specified. All the patients were in the seventh decade of life or older. The clinical presentation and macroscopic appearance suggested a benign lesion in most cases. The tumors grew in a solid papillary pattern and displayed low-grade cytological features as well as intracellular and extracellular mucin. Endocrine differentiation was demonstrated by the Grimelius stain in 11 of 17 cases and by the chromogranin immunohistochemical technique in eight of 14 cases. Lymph node metastases were not observed, but pulmonary metastasis occurred in one case. All the tumors were positive for estrogen receptors. We postulate that these lesions are the preinvasive counterpart of mucinous carcinomas with endocrine differentiation.
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