45-year-old woman with bleeding plantar papule
Poromas are glandular neoplasms that belong to the spectrum of acrospiromas, along with nodular and clear cell hidradenomas. Historically they have been considered only of eccrine lineage. However, analyses by many investigators suggest that poromas can be of either eccrine or apocrine lineage, and the latest may be even more common. Their malignant counterpart is porocarcinoma.
From the many immunohistochemical studies performed to determine their cellular derivation, it is concluded that poromas arise from the basal keratinocytes of the glandular duct ridge and lower acrosyringium (since poroid cells of the tumor express cytokeratin (CK) 14 and do not express CK1, CK6, CK7, CK10, CD34, or CK77). The scattered larger cells within the tumor express CK1 and CK10 suggesting a ductal differentiation.
While the specific causes of poromas are unknown, there have been reports associated with sites of trauma, chronic radiation dermatitis, or burn injury. In contrast to other adnexal neoplasms, they are not related to genodermatosis and are circumscribed to the skin.
Poromas most commonly present in the middle-aged and elderly population with no gender or race predilection.
Poromas classically manifest as skin-colored papules or nodules smaller than 2 cm in diameter. They occasionally may present pigment and, if they protrude, erosion or ulceration of their surface. Although most poromas are asymptomatic, slow-growing, or stable nodular lesions, pain can be a presenting sign. If it develops, it is important to inquire about a history of bleeding, pruritus, and growth as these qualities are more common in porocarcinomas.
Although they have been historically described as acral neoplasms, they can arise in non-acral skin with even higher incidence. Poromas can manifest in association with nevus sebaceous as a secondary neoplasm.
The clinical differential diagnosis is comprised of both benign and malignant neoplasms such as pyogenic granuloma, clonal seborrheic keratosis, angioma, dermatofibroma, amelanotic melanoma, Merkel cell carcinoma, squamous cell carcinoma, basal cell carcinoma, and porocarcinoma.
Hematoxylin and eosin show a circumscribed tumor composed of uniform basaloid or poroid cells extending from the epidermis into the upper dermis, with few pale larger cells and small ducts and/or cysts scattered within. Although poromas can usually be considered benign based on circumscription and benign cytologic features, is not uncommon to see small foci of necrosis and a highly vascularized (granulation tissue-like) stroma, the combination of which can create an unnecessary concern for malignancy.
Poromas manifest several different variations. Hidroacanthoma simplex has been described as a purely intraepidermal poroma, while the dermal duct tumor has been described as an intradermal poroma, and not completely different entities. These tumors are similar both clinically and histologically with the same derivation simply found at different locations within the skin, and studies have shown that serial sections of dermal duct tumors do reveal a connection to the epidermis.
Variations in the histology have been reported. Poromas may show scattered sebocytes among the poroid cells, which raises the question of apocrine differentiation in this subtype. While eccrine and apocrine glands have distinct functions, they both arise from down growths of the epidermis around the fourth month of gestation and therefore do have some common cellular precursors.
Prognostic and molecular features
The prognosis of poromas is favorable because the lesions have no known clinical significance. Even poromatosis (multiple poromas) is not known to be associated with other anomalies. The risk of malignant transformation of a poroma is minimal and is thought to be similar to that of normal skin. Multiple poromas may be of cosmetic concern and are sometimes disabling, especially if involvement of the sole is noted.