Mucoid carcinoma of the breast on fine-needle aspiration biopsy sample: cytology and ultrastructure.

16 cases of mucoid carcinoma of the breast were diagnosed in specimens obtained by fine-needle aspiration biopsy. The subsequent tissue examination confirmed the cytologic diagnosis in all 16 cases but revealed in 1 case an association with an endocrine component and in 3 cases the presence of an invasive ductal component. The diagnostic and prognostic necessity of distinguishing pure forms of mucoid carcinoma from those associated with invasive ductal carcinoma or endocrine differentiation means that investigations other than fine-needle aspiration biopsy are also required. In every case, the cytologic diagnosis of mucoid carcinoma should be considered as an aspect of the tumor and not representative of the entire neoplasm.