Abstract

Hidradenomas are benign tumors of eccrine or apocrine derivation. They typically present as well-circumscribed nodules with a broad anatomic distribution but are most commonly seen in the head and neck. We present an unusual case of a hidradenoma on an acral location. A 36-year-old male presented with a 2 cm well-circumscribed mass on the extensor surface of the first toe. This lesion developed over the course of one month and was painful. Excision showed a well-circumscribed solid and cystic tumor confined to the dermis with no epidermal connection. The majority of the tumor showed poroid cytologic features including polygonal cells with scant eosinophilic cytoplasm, vesicular ovoid nuclei, nuclear grooves and small nucleoli. Areas of squamous and ductular differentiation were present as well as numerous areas of ghost cells. The tumor cells were diffusely positive for p40 and p63 and negative for S100 and SMA. EMA and CEA highlighted focal ductal differentiation. No recurrence was seen at 4 months follow up post excision. The unusual location of this tumor raises an important differential diagnosis ranging from benign to malignant. Amongst the benign entities, a poroma or dermal duct tumor could be considered given the poroid differentiation of the tumor. This tumor also showed the unusual feature of numerous ghost cells mimicking a pilomatrixoma. In addition, when confronted with a digital, solid and cystic tumor in the dermis, one should consider the malignant, yet often low-grade appearing, digital papillary adenocarcinoma, for which papillary structures are morphologic clues to the diagnosis as both can display mitotic activity and cytologic atypia. Immunohistochemistry highlighting the myoepithelial layer in digital papillary adenocarcinoma may also be of value in recognizing this potential diagnostic pitfall. Careful inspection of this rare tumor presenting at an usual site is necessary to exclude both clinical and histopathologic mimics.

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