Mammary mucinous carcinoma with marked cytoplasmic hyalinization. A report of 2 cases with emphasis on fine needle aspiration cytologic findings.

Background: Cytoplasmic eosinophilia in breast tumors is due mainly to abundance of secretory granules, lysosomes and/or mitochondria. Diffuse cytoplasmic hyalinization caused by intermediate filaments has not been described before.

Cases: Two cases of pure mammary mucinous carcinoma occurred with marked cytoplasmic hyalinization among 556 wide excision/mastectomy specimens of mammary ductal carcinoma, either in situ or invasive, encountered at Pamela Youde Nethersole Eastern Hospital, Hong Kong, during the period from 1994 to the end of 2001. Preoperative fine needle aspiration cytology was available in 1 case, showing cohesive clusters and micropapillae of tumor cells in abundant background mucin. The tumor cells had low mitotic activity and possessed round to oval nuclei, solitary macronucleoli; ample, eosinophilic cytoplasm and discrete cell borders. Cytoplasmic granularity, intracytoplasmic vacuoles or "cometlike" cells were not found. Histologic examination of the surgical specimens in both cases revealed pure mucinous carcinoma with diffuse and marked cytoplasmic hyalinization. There was no evidence of peritumoral lymphovascular permeation or regional lymph node metastasis. Ultrastructural examination showed minimal secretory activity, with abundant, loose aggregates of intermediate filaments in the cytoplasm. There was a relative paucity of other organelles.

Conclusion: Diffuse cytoplasmic hyalinization is a peculiar morphologic change in mammary ductal carcinoma and apparently unique to pure mucinous carcinoma. Recognition of this phenomenon may help to avoid misdiagnosis of other types of ductal carcinoma with cytoplasmic eosinophilia, such as apocrine carcinoma and ductal carcinoma with neuroendocrine differentiation, which sometimes are also associated with pools of extracellular mucin.