Apocrine and eccrine hidradenomas have numerous histopathologic features in common. A histopathologically typical apocrine clear-cell hidradenoma with unusual clinical features is presented.
A 22-year-old woman consulted us with a tumor on the upper part of the left elbow. The lesion had evolved over two years and had been asymptomatic in the early stages of development but during pregnancy it had enlarged and became slightly painful. The patient did not report any relevant previous medical history. Clinically, a rounded exo-endophytic tumor with telangiectasias on the surface was seen. It showed some solid areas mixed with bluish cystic ones resembling a turgent varicocele on palpation (Figs. 1 A-C).
Clinical appearance of a huge, soft, solid-cystic neoplasm. The exoendophytic tumor was located above the elbow.
On histopathology, the lesion was an adnexal epithelial neoplasm located mainly in the dermis and extending throughout the subcutis (Fig. 2A-G). It was benign by silhouette, it being rather symmetric and well circumscribed. Proliferations of cells presented with a biphasic solid-cystic aspect, cells in solid areas being arranged in lobules. Some of the cells had an eosinophilic cytoplasm (Fig. 2B) and others a clear cytoplasm (Fig. 2C). Still others showed intraepithelial mucin (Fig. 2D). In foci, squamous differentiation exhibiting keratohyalin granules was seen (Fig. 2E). Atypical mitoses were lacking. Some ductal structures lined by a cord of cuboid cells were encountered (Fig. 2H). The stroma was hyalinized in some areas (Fig. 2F), in other zones granulomatous infiltrates with cholesterol clefts were present (Fig. 2G). No definite signs of decapitation secretion were found.
Histopathologic findings are diagnostic of a clear-cell hidradenoma. Although signs of decapitation secretion were lacking, the prominent clear- and pale-cell population in some foci is consistent with the diagnosis of apocrine hidradenoma.
The lesion was removed by surgery without recurrence to date.
Little is known about the clinical appearance of many of the adnexal epithelial neoplasms because they often are excised under the clinical diagnosis of a cyst or a lipoma, and the correct diagnosis is made only on histopathology. That being so, hardly ever is a clinical photograph taken prior to excision of the lesion. The most common clinical aspect of clear-cell hidradenoma is that of an asymptomatic solitary papule or nodule which is localized most commonly on the arms, face, scalp, and trunk of a middle-aged patient. In general, the lesion is firm with fluctuant areas if a cystic component is prominent. The overlying skin can be red or blue, occasionally showing central ulceration.  In the patient described here, a clear-cell hidradenoma presented as a huge mass, which had not been asymptomatic but had been painful. Interestingly, the lesion enlarged during pregnancy implying a hormone-sensitive lesion. Pregnancy-related growth has also been reported for other adnexal epithelial neoplasms such as poroma and syringoma. [2-6]
Hidradenomas are glandular neoplasms with eccrine or apocrine derivation. If the hidradenoma shows “pinching off” secretion, the diagnosis of an apocrine hidradenoma can be made with surety. Even if decapitation secretion is absent, the distinction between apocrine and eccrine differentiation can be made with certainty. Clear-cell hidradenomas may show various cytomorphologic features. Pale or clear cells are usually encountered in apocrine hidradenomas, which for that reason also are designated pale- or clear-cell hidradenomas. In contrast, the term hidradenoma is also applied to one type of poroma, namely, poroid hidradenoma. It consists of poroid and cuticular cells that form one or few nodules in the dermis or subcutis without connection to the epidermis. Poroid hidradenomas can also be either apocrine or eccrine, but often sure differentiation between both is sometimes hard especially when frank decapitation secretion is absent.
Clear-cell hidradenoma can be differentiated from poroid hidradenoma because cells in the latter are much more monomorphous. Moreover, cuticular cells, even though being pale too, have much less cytoplasm than the pale and clear cells in clear-cell hidradenoma. In addition, clear-cell hidradenoma often shows areas of squamous or mucinous differentiation of cells.
Both types of hidradenoma have the silhouette of a bening neoplasm characterized by neoplastic cells that lack frank pleomorphism and both may be solid or solid-cystic. 
Clear-cell hidradenoma is a very rare neoplasm in young patients, but a few cases have been reported. [8,9] The malignant counterpart, cutaneous hidradenocarcinoma, is even less common. However, a series of 14 cases has been published in the literature.  In the lesion presented here, circumscription and benign cytopathologic features enabled us to exclude a diagnosis of hidradenocarcinoma.
This report presents clinical and histopathologic features of a clear-cell hidradenoma. The lesion was unique because of its size, its rapid growth during pregnancy, and its being painful. Histopathologic changes were typical of apocrine clear-cell hidradenoma.
Eduardo Pittaro, M.D., and Ricardo Achenbach, M.D., are dermatologists at the Pirovano Hospital in Buenos Aires, Argentina. Graciela Sanchez, M.D., is a dermatopathologist at the same hospital. This article was reviewed by Friederike Kauer, M.D., and Almut Böer-Auer, M.D. Contact corresponding author via email: firstname.lastname@example.org .
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