Utility of immunohistochemistry in diagnosis of papillary eccrine adenoma/tubular apocrine adenoma (PEA/TAA)

Summary: Papillary eccrine adenoma/tubular apocrine adenoma (PEA/TAA) is a benign neoplasm that can pose diagnostic challenges due to its rarity and overlapping morphology with other adnexal tumors. We report a case of PEA/TAA presenting as a nodule on the left foot of a 43-year-old man. Histologic examination revealed a nodular proliferation of variably sized and layered tubular glands with minimal cytologic atypia, on a background of stromal fibrosis with inflammation, extending from the superficial to deep dermis. By immunohistochemistry, the neoplastic cells demonstrated strong, diffuse positivity for BRAFV600E; p63 highlighted the outer myoepithelial cell layer. Particularly when involving a digit, the differential diagnosis of PEA/TAA incudes digital papillary adenocarcinoma (DPA), which can show similar morphologic features. However, BRAFV600E mutations are characteristic of PEA/TAA (reported in up to 100% of cases) but are exceedingly rare in DPA. In contrast, HPV42 has been implicated in the tumorigenesis of DPA. Notably, PEA/TAA is a benign neoplasm usually cured by excision, while DPA can show an aggressive clinical course, highlighting the importance of accurate diagnosis. We report this case to illustrate the value of ancillary studies such as BRAFV600E immunohistochemistry to support morphologic diagnosis of this rare adnexal neoplasm.