Answer: Rosai-Dorfman disease

Histology: This lesion is primarily in the subcutaneous tissue but does extend into the breast parenchyma. It consists of sheets of polygonal epithelioid cells with abundant cytoplasm. The nuclei have vesicular chromatin and fairly prominent nucleoli. The lesional cells label for S100 protein and CD68, but not for cytokeratin or CD1a. Several of these cells demonstrate intraepithelial lymphocytes and plasma cells, which is called “emperipolesis”.

Discussion: Histiocytoid invasive lobular carcinoma may simulate histiocytes, but is consistently cytokeratin positive. This would be a very unusual lesion in a 19 year-old and would have a more infiltrative growth pattern. Abundant epithelioid histiocytes after chemotherapy is a great mimic of mammary carcinoma, particularly on frozen section. The absence of a clinical history, the labeling for S100 protein but not cytokeratin, and the presence of emperipolesis argue against this diagnosis. Langerhan’s Cell Histiocytosis would feature nuclei with more reniform, grooved appearance, would label for CD1a, and would not show emperipolesis.

Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy) may be entirely extranodal. Extranodal cases typically have a more storiform pattern, are more spindled, have more collagen deposition, and have less emperipolesis, so the diagnosis becomes more difficult.

Reference(s):
– Am J Surg Pathol 1992;16:122-129.