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Sir,
Poroid hidradenoma is a benign adnexal tumour with eccrine or apocrine differentiation, described originally by Abenoza and Ackerman in the year 1990.[1] It is usually a solitary asymptomatic neoplasm that rarely becomes malignant. The onset ranges from 28 to 77 years, with a peak of incidence in the seventh decade and equal sex predilection.[2] Although benign, poroid hidradenomas can recur if they're not adequately excised.
Most variants of hidradenoma, including poroid hidradenoma, exhibit a varied morphological spectrum and differentiation, and consequently, there is a great likelihood of being mistaken for other benign or malignant cutaneous and noncutaneous tumours.[3]
Histomorphology of this neoplasm is described as being composed of small basophilic poroid cells and larger pale cuticular cells with clear cytoplasm and no connection to the overlying epidermis.[4] We present a case of poroid hidradenoma of the scalp, with adipocytic metaplasia, mimicking a lipoma on clinical presentation.
A forty-eight-year-old man presented with a palpable lesion on the scalp, which had appeared three years ago and had gradually increased in size.
Family history and past medical history were not significant.
Physical examination revealed a 2.3 × 2.1 cm painless, soft, and movable nodule, with clinically distinct margins and covered by apparently normal skin. The lesion was clinically suspected as being a lipoma, and the patient was posted for simple surgical excision after clinical and radiological evaluation. The specimen was sent to us for histopathology.
The excised nodule was well circumscribed, with no surrounding soft tissue, covered with skin and measured 2.2 × 2 × 1.7 cm.
Cut surface was grey white and showed yellow areas and foci of congestion.
Histopathology revealed a well-circumscribed tumour with a thinned out fibrous capsule.
Tumour was lobulated, and the tumour cell nests were separated by a fibromyxoid matrix containing occasional macrophages, plasma cells and lymphocytes.
Tumour cells were monomorphic, cuboidal to polyhedral with scant eosinophilic cytoplasm and large round bland nuclei. There were areas of coagulative necrosis, keratinisation, hyaline change and cystic change. Features of malignancy were not present [Figure 1] and [Figure 2].
The novel feature seen was nested and scattered mature adipocytes between the tumour cells [Figure 3].
No special stains or immunohistochemistry were performed, and the diagnosis was made on histomorphology.
Poroid hidradenoma is a poroid neoplasm, a variant of the eccrine poroma. There are two groups of hidradenomas, the first being neoplasms with eccrine differentiation, and the second being neoplasms with apocrine differentiation.[5]
In 1990, Ackerman and Abenoza described four variants of poroid neoplasms according to the location of the neoplastic cells: dermal duct tumour, hidroacanthoma simplex, eccrine poroma and poroid hidradenoma.[6]
The entity typically presents as a solitary, tender papule or nodule, solid or solid cystic, well circumscribed and located intradermally. The most common sites of involvement are the head and neck regions with a predilection for centro-facial location, with less frequent sites including axilla, trunk and extremities.[7],[8] The age of onset ranges widely, with a peak incidence in the middle age to elderly age group[7] and no sex or ethnic predilection.
Clinically these lesions may mimic lipomas, sebaceous cysts or pyogenic granulomas.[7]
Microscopic examination reveals a tumour with solid and cystic components, in which neoplastic poroid cells are all located within the dermis and without connection to the epidermis. These cells are arranged in lobules and nests, with few clear cells and areas of cystic change. Though areas of degeneration are noted, metaplastic changes are uncommon, and adipocytic metaplasia has never been reported to date.
The differential diagnosis of poroid hidradenoma includes hidroacanthoma simplex, dermal duct tumour and eccrine poroma.
Hidroacanthoma simplex is made of nests of discrete, small, rounded cells within the normal epidermal cells.
In dermal duct tumours, the nests of tumour cells are located mainly in the dermis with no epidermal connection.
Eccrine poroma may have an epidermal connection but also show lumina formation and mainly a single population of basaloid cells.
Some cases of poroid hidradenoma coexist with eccrine poroma, dermal duct tumour or hidroacanthoma simplex.[9]
There are no known risk factors for the entity; two cases have been reported in immunocompromised individuals.[10]
There is no reported malignancy with this lesion but if misdiagnosed, this lesion can be confused with a benign subcutaneous neoplasm. The above scenario would lead to an excision without margins, and a wide excision procedure is usually recommended in this case as there is a propensity for local recurrence.[10]
Hence, radiology and/or fine-needle aspiration cytology (FNAC) may be advisable for ensuring an accurate surgical procedure.[11]
Eccrine gland-derived lesions make up a large and common group of appendage tumours.
However, poroid hidradenoma is a lesser-known entity, with adipocytic metaplasia having never been described.
We would like to describe this rare variant, which could mimic a lipoma, to aid a better understanding of the entity as they need to be excised with clear margins to prevent recurrence.
Acknowledgements
Department of Histopathology, Plus Care Internationals Private Limited
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