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The tumor is derived from Schwann cells of peripheral nerves. It is composed of sheets or nests of polygonal cells with abundant eosinophilic granular cytoplasm. The cytoplasmic granules are PAS positive and diastase resistant. Nuclei are round to oval and may display mild cytologic atypia. Bundles of tumor cells permeating the collagenous breast stroma may be mistaken for apocrine carcinoma in needle core biopsies.