| Summary: | Clear cell hidradenoma (CCH) is an uncommon benign dermal-based tumor that typically presents on the head, neck, and upper extremities as a solitary firm nodule. The clear cytoplasm can resemble benign and malignant clear cell neoplasms from multiple sites; thus, a large differential diagnosis is often considered. When CCH is seen in the breast or axilla, glycogen-rich clear cell carcinoma (GRCCC) of the breast enters the differential diagnosis. Although GRCCC is rare, it is important to recognize as a breast carcinoma variant because most reports have suggested that it has a more aggressive course than typical invasive ductal carcinoma. We report a case of CCH in the upper axilla of a 64-year-old woman who also happened to have a remote history of invasive GRCCC to highlight the potential diagnostic pitfalls when evaluating these two histologically similar clear cell tumors. Although immunohistochemical studies can be helpful, overlapping staining patterns can lead to potential confusion and misclassification. Both of our patient's tumors were negative for estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and PAX8. ER and PR are typically negative in hidradenomas and can be negative in up to 50% of GRCCC. Although considered a breast marker, GATA-3 can be negative in GRCCC and positive in skin adnexal tumors. p63 can be especially helpful, as it is expressed in skin adnexal tumors, but lost in most (Thike et al., 2010) invasive breast carcinomas. Finally, periodic acid-Schiff (PAS) with and without diastase highlights intra-cytoplasmic glycogen in GRCCC.
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