Previous attempts at subclassifying pure mucinous carcinomas of the breast based on architectural patterns and other associated features are on record. A distinctive micropapillary variant with most tumor cells arranged in micropapillae/pseudoacini has not been described previously.
The author reviewed the histologic slides from all 556 patients who underwent wide excision/mastectomy for mammary ductal carcinoma, either in situ or invasive, at Pamela Youde Nethersole Eastern Hospital, Hong Kong, during an 8-year period from early 1994 to the end of 2001. Five patients with pure mucinous carcinoma with diffuse micropapillary architecture were noted. The cytologic features (if available) were correlated with clinical, radiologic, and pathologic findings.
Among the five patients with pure mammary mucinous carcinoma (micropapillary variant), three patients underwent preoperative fine-needle aspiration biopsies and had specimens available for review. The direct smears and cytospin preparations were of moderate cellularity and showed cohesive clusters and micropapillae of mildly pleomorphic tumor cells among a mucoid background. True tumor papillae with fibrovascular cores were not present. Nuclear hobnailing was observed commonly, and occasional psammoma bodies were found. There were also scanty isolated tumor cells scattered around. The pseudoacinar pattern was appreciated more readily in the cell block sections. Histologic examination of the surgical specimens showed features of pure mucinous carcinoma with diffuse micropapillary architecture. The micropapillary arrangement was confirmed further by the demonstration of a reverse polarity immunostaining pattern for epithelial membrane antigen and the identification of microvilli rimming the periphery of tumor cell clusters under the electron microscope. Peritumoral lymphovascular permeation and ipsilateral axillary lymph node metastasis was found in one of the patients.
The micropapillary variant of mammary mucinous carcinoma demonstrates characteristic cytologic and histologic features that warrant special attention. It may represent the mucinous counterpart of invasive micropapillary carcinoma. Further analysis of a larger series of patients, however, will be required to delineate its prognostic significance, especially its propensity for lymph node metastasis. Cancer (Cancer Cytopathol) 2002;96:280–8. © 2002 American Cancer Society.