Malignant clear cell hidradenoma of the vulva: report of a unique case and review of the literature.

Clear cell hidradenoma (CCH) is a rare tumor derived from eccrine sweat glands with a predilection for the head, face, and upper extremities. Its biologic behavior is unpredictable, although frank malignant transformation is reportedly rare (prevalence rate, 6.7% in a review). Malignant CCH (MCCH) exists only as case reports or very small series in the literature. We present a unique case of MCCH of the vulva with completely bland cytological features. A previously healthy 39-year-old woman underwent marsupialization of a presumed left-side vulval Bartholin gland cyst. Microscopy revealed a tumor with the features of CCH; no atypia, necrosis, or mitoses were observed. Ten months later, she developed enlarging left groin nodes, one of which contained a metastatic clear cell tumor. Radiological examination did not reveal any other primary source, and the diagnosis of MCCH was confirmed through an expert review. A reexcision of the vulval primary site, which contained residual tumor, was performed. The patient is currently free of any further recurrence 10 months after the excision of the lymph node metastasis. The case highlights the difficulty in predicting the behavior of CCH on the basis of histological examination alone. It also highlights the importance of considering MCCH in the differential diagnosis of a lymph node containing a metastatic clear cell tumor or an apparent metastatic clear cell lesion in the skin. We review the current literature on MCCH and discuss the problems in the differential diagnosis and treatment of this rare tumor.