Utility of Immunohistochemistry in Diagnosis of Papillary Eccrine Adenoma/Tubular Apocrine Adenoma (PEA/TAA).

Papillary eccrine adenoma/tubular apocrine adenoma (PEA/TAA) is a rare sweat gland neoplasm. Because PEA/TAA shares morphologic features with other sweat gland tumors, including digital papillary adenocarcinoma, histologic diagnosis may be challenging. We report a case of PEA/TAA presenting as a nodule on the left foot of a 43-year-old man. Histopathologic examination revealed a well-circumscribed dermal proliferation of variably sized glands composed of ductal cells with an outer layer of myoepithelial cells. By immunohistochemistry, the ductal tumor cells showed strong, diffuse expression of BRAF V600E, and the myoepithelial cells were positive for p63. This case illustrates the utility of immunohistochemistry in the diagnosis of this rare neoplasm. Particularly when PEA/TAA is located on an acral site, distinction from digital papillary adenocarcinoma is critical, because these tumors show significant differences in clinical behavior.