Poroma (poroid tumor) is a benign adnexal tumor that usually originates from the terminal duct of the sweat gland.[1] Initially, in 1956 Pinkus et al. described poroma and its poroid (terminal ductal) differentiation and had been thought to be from the eccrine origin.[2] Further reports have shown cases with apocrine, sebaceous, and follicular differentiation.[3] Some authors used to describe this neoplasm with the term of acrospiroma. Others consider this as a broad group of neoplasms, including nodular hidradenomas, clear cell hidradenomas, hidroacanthoma simplex, dermal duct tumors, and hidradenoma.[1] Poroma has a potential degenerative progression, and its malignant counterpart is referred to as porocarcinoma developing after several years from a pre-existing poroma.[4]
Causes of Poroma
The exact etiology of poroma is unknown. Unlike other adnexal follicular lineage neoplasms, family predilection has not been identified for the development of poromas. Long-term radiation exposure has been suggested to trigger the development of poroma and porocarcinomas.[5] In a case report, multiple eccrine poromas were reported in areas with chronic radiation dermatitis.[6]
Porocarcinoma is known to result from a possible malignant progression of a benign poroma. Its pathogenesis is still not clear, and there is not a definite timeline in which a poroma advances and becomes a porocarcinoma.[4] Immunosuppression, exposure to chemical agents, and chronic light exposure are thought to be the factors that increase the susceptibility to develop eccrine porocarcinoma.[7]
Occasionally, poroma has been reported with other conditions, including Bowen’s disease and hypohidrotic ectodermal dysplasia.[8] Porocarcinomas occurred more frequently in association with xeroderma pigmentosum, extramammary Paget disease, Hodgkin’s lymphoma, chronic lymphocytic leukemia, pernicious anemia, sarcoidosis, and HIV-infection.[9]
Pathophysiology
Poroma is a benign adnexal tumor that occurs on the skin and does not affect any other tissue. It shows differentiation toward glandular ductal cells. Its malignant counterpart is known as porocarcinoma, which also shows clear poroid differentiation. It only occurs in the skin and is not known to affect any other body organ.[12]
Diagnosis of Poroma
Histopathology
Poroma is a very well-circumscribed tumor.[13] Depending on the location of the tumor cells, poroid tumors were divided into four subtypes, namely eccrine poroma, poroid hidradenoma, hidroacanthoma simplex, and dermal duct tumor.[10] These tumors can be entirely intraepidermal, a pattern known as hidroacanthoma simplex. Dermal duct tumors are poroid tumors with a wholly, or nearly so, intradermal localization. Additionally, poroma can occur in continuity with the epidermis, and so named juxta-epidermal poroma. Poroid hidradenomas are completely in the dermal layer.[1]
Poroid cells are cuboidal keratinocytes, and the pattern is non-palisading and monomorphous ovoid nuclei with discrete nucleoli. The cytoplasm is usually eosinophilic and stains positively with periodic acid–Schiff (PAS). Interestingly, some atypical features of malignant tumors can be observed in poroma, including a variable number of mitosis, highly vascularized stroma, and foci of necrosis.[1] A particular type of poroid tumor has been reported to be associated with the presence of dendritic melanocytes along with poroid cells scattered within the intraepidermal tumor nests. This histological feature correlates clinically with a pigmented variant of poroma named “pigmented hidroacanthoma simplex.”[14]
The degree of ductal differentiation varies greatly between poromas. Dermatopathologists can be aided by carcinoembryonic antigen immunostaining, which is a tool to confirm the presence of ductal differentiation. This staining labels the luminal surface of both apocrine and eccrine ducts.[1] In a review of the literature, Kamiya et al. proposed four histopathological features distinguishing apocrine from eccrine poroma:
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Presence of elongated tubules lined by columnar cuticular or polygonal cells that usually exhibit hints of apocrine secretion along the luminal border and also contain an amorphous, eosinophilic material in their lumina
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The connection of aggregates of neoplastic cells to preexisting infundibula seem like connections between excretory ducts of apocrine glands
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Presence of follicular differentiation in the form of epithelial lobules similar to the tumor of follicular infundibulum or trichoblastoma
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Presence of isolated sebocytes and their small clusters in neoplastic the cells[15]
Porocarcinoma is composed of anaplastic cells that contain large irregular and hyperchromatic nuclei and are glycogen-rich.[1] It also shows areas of necrosis and mitotic figures. Porocarcinoma has usually a first intraepidermal proliferation and an extension into the dermis. Dermal lymphatics are subsequently invaded, and the tumor shows regional and distant metastasis.
Diagnosis of Poroma
History and Physical
Poroma typically presents as an asymptomatic mild solitary slow-growing papule, nodule, or plaque with colors varying from skin color to red to brown or bluish.[14] Rarely it can present with mild tenderness. It commonly displays a vascularized feature, with a clinical appearance mimicking pyogenic granuloma. Indeed, Betti et al. found the reddish color to be the most common color of poroma in a 101 case series. The most common locations of poroma are palms and soles. Nevertheless, it can be found on any cutaneous portion of the entire body surface. Unusual locations are head and neck, trunk, armpits, upper limbs, buttocks, and lower limbs.[10]
Although it is extremely rare, poroma arising within nevus sebaceous was also reported.[16] The apocrine poroma clinically presents in a similar fashion like its eccrine correspondent.[1] Nevertheless, none of the reported cases of apocrine poroma was located on palms or soles. They only involved the face, the body, and limbs.[17]
Multiple poromas are known as “poromatosis,” which can occur following chemotherapy and/or radiotherapy.[18] They can be either in an acral or in a widespread distribution. Porocarcinoma may arise as a result of transformation or may arise as a new lesion (de novo). It is suspected if there is spontaneous recurrent ulceration, bleeding, explosive growth, or sudden pain that occurred in a preexisting tumor.[1] The head and neck (39.9%) are the most common sites followed by lower extremity (33.9%), according to a meta-analysis.[11]
Evaluation
A biopsy is the only way to make the diagnosis of poroma. If the diagnosis of poroma is considered, one must be attentive to the risk of malignancy in the future. For porocarcinoma, the evaluation of sentinel lymph node biopsy has been used as a staging tool. Screening for distant metastases is also indicated.
Treatment of Poroma
Treatment for poroma is optional but curative as it is a benign adnexal neoplasm. Deeper lesions may be cured with simple excision, but electrosurgical destruction may be the cure for superficial lesions.
Treatment modalities for porocarcinoma have included Mohs micrographic surgery, standard surgical excision with broad tumor margins, radiation therapy, and chemotherapy. The Mohs micrographic surgery affords, for some authors, the greatest likelihood of cure in the absence of regional and distant metastases and clear margins.[9]