Case: AP199
Contributor's Comment: Primary Granulocytic Sarcoma presents less commonly in the breast and is most frequently misdiagnosed as lymphoma, sarcoma or poorly differentiated carcinoma of the breast, especially to the unwary. A good rule of the thumb in the breast is that whenever the biopsied tumor is considered to be malignant lymphoma, and particularly if difficult to classify, one should always exclude granulocytic sarcoma first. Look out for scattered cells with eosinophilic cytoplasm. "Single file" or targetoid patterns with extensive sclerosis mimicking invasive lobular carcinoma is a known pitfall, as evidenced by 4 respondents who proffered this diagnosis without other considerations. Use of appropriate immunohistochemical stains for myeloid series is indispensable.