Clear cell carcinoma of the breast with immunohistochemical evidence of divergent differentiation.

Primary clear cell carcinoma of the breast is a rare tumor. The clear cell morphology of the neoplastic population in these tumors has been ascribed to the presence of intracellular lipid, mucin or glycogen, or to myoepithelial, apocrine, or neuroendocrine differentiation. However, a clear cell neoplasm exhibiting evidence of a range of differentiation has not been previously reported. We describe a case of a glycogen-rich primary clear cell breast carcinoma occurring in a 59-year-old woman that showed positivity for apocrine and neuroendocrine markers, as well as possible myoepithelial differentiation. The tumor was a 4-cm mass composed predominantly of periodic acid-Schiff-positive clear cells arranged in a solid, infiltrative pattern. Immunohistochemical staining of the tumor cells was variably positive for cytokeratin, progesterone receptors, gross cystic disease fluid protein-15, neuron specific enolase, chromogranin, and S-100 protein and negative for estrogen receptors, smooth muscle actin, CD31, and CD34. The patient refused any form of further investigation or treatment, but shows no evidence of recurrence or metastatic disease after 18 months of follow-up.