Lymph node location of a clear cell hidradenoma: report of a patient and review of literature.

Cutaneous clear cell hidradenoma is an uncommon benign adnexal tumor which is not supposed to metastasize, contrary to its rare malignant counterpart, hidradenocarcinoma. We report the case of a 49-year-old man, who had had a stable inguinal lymph node enlargement for 6 years. An excision was performed and revealed an intra-nodal tumor, made of large clear cells with abundant cytoplasm and round nuclei without atypia or mitosis. The immunohistochemical staining showed diffuse positivity for keratin AE1/AE3, keratin 5/6 and p63, and focal staining with keratin 7, epithelial membrane antigen (EMA) and carcinous epithelial antigen (CEA), which underlined some ductular structures. Tumor cells were negative for renal markers PAX8 and CD10. Ki67 stained less than 1% of tumor cells. A translocation involving MAML2 gene was evidenced by fluorescence in situ hybridization (FISH) analysis. No primary cutaneous tumor was found after extensive examination. Altogether, these results are in favor of an isolated nodal hidradenoma, for which we discuss two hypothesis: a primary nodal lesion, or a 'benign metastasis' of a cutaneous tumor. Cases of morphologically benign hidradenoma with lymph node involvement are exceptional. Our case, similar to every other reported case, was associated with an excellent prognosis, supporting the idea that these patients should not be overtreated.