Poroma (poroid tumor) is a benign adnexal tumor that usually originates from the terminal duct of the sweat gland. Initially, in 1956 Pinkus et al. described poroma and its poroid (terminal ductal) differentiation and had been thought to be from the eccrine origin. Further reports have shown cases with apocrine, sebaceous, and follicular differentiation. 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. Poroma has a potential degenerative progression, and its malignant counterpart is referred to as porocarcinoma developing after several years from a pre-existing 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. In a case report, multiple eccrine poromas were reported in areas with chronic radiation dermatitis.
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. Immunosuppression, exposure to chemical agents, and chronic light exposure are thought to be the factors that increase the susceptibility to develop eccrine porocarcinoma.
Occasionally, poroma has been reported with other conditions, including Bowen’s disease and hypohidrotic ectodermal dysplasia. Porocarcinomas occurred more frequently in association with xeroderma pigmentosum, extramammary Paget disease, Hodgkin’s lymphoma, chronic lymphocytic leukemia, pernicious anemia, sarcoidosis, and HIV-infection.
In primary skin lesions, sweat gland tumors account for approximately 1% cases. Eccrine and apocrine poromas are believed to account for approximately 10% of these. Poromas are not known to have any ethnic and racial predilection. There is also no sex predilection reported in the distribution. It equally affects males and females. It can develop at any age, but onset is typically noted in adulthood.
The incidence of eccrine porocarcinoma is lower than poroma, representing only 0.005% of epithelial cutaneous neoplasms, and it is more common in elderly patients. In a meta-analysis, published in 2017, eccrine porocarcinoma was diagnosed in 453 patients. In this study, 49% of cases were male, and 51% were female, and age ranging from 6 months to 97 years.
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.
Poroma is a very well-circumscribed tumor. 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. 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.
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. 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.”
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. In a review of the literature, Kamiya et al. proposed four histopathological features distinguishing apocrine from eccrine poroma:
Porocarcinoma is composed of anaplastic cells that contain large irregular and hyperchromatic nuclei and are glycogen-rich. 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.
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. 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.
Although it is extremely rare, poroma arising within nevus sebaceous was also reported. The apocrine poroma clinically presents in a similar fashion like its eccrine correspondent. Nevertheless, none of the reported cases of apocrine poroma was located on palms or soles. They only involved the face, the body, and limbs.
Multiple poromas are known as “poromatosis,” which can occur following chemotherapy and/or radiotherapy. 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. The head and neck (39.9%) are the most common sites followed by lower extremity (33.9%), according to a meta-analysis.
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 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.
Poromas are often misdiagnosed as other skin neoplasms because their clinical presentations are nonspecific and variable. In the study of Chen et al., the preoperative diagnoses of eccrine poroma were: pyogenic granuloma (five cases), soft fibroma (four cases), verruca vulgaris (three cases), hemangioma (two cases), pigmented nevus (one case), and basal cell carcinoma (one case). Other differentials may include squamous cell carcinoma, seborrheic keratosis, hidradenomas, trichilemmoma, and other adnexal tumors.
The long-standing or chronic tumors of the limbs and head, namely squamous cell carcinoma, Paget’s disease, basal cell carcinoma, hidradenocarcinoma melanoma, can mimic inflamed poroma and metastatic cancer porocarcinoma. Porocacinoma may also resemble an ingrown toenail if it occurs in the nail fold.
Histologically, recognizing apocrine poroma maybe sometimes difficult as it manifests a mixture of apocrine, sebaceous, and follicular differentiation in otherwise typical poroid tumors. A typical case of apocrine poroma was reported by Kamiya et al. and discussed the coexistence of basal-cell epithelioma and apocrine poroma, eccrine poroma, in association with trichoblastoma and also basal-cell epithelioma having sweat gland differentiation.
The prognosis of poromas is good as the lesions have minimal clinical significance. Poroma clinically presents as a solitary nodule or papule. The lesions are typically asymptomatic, and few lesions have mild tenderness. The patient sometimes develops multiple poromas at the same time, a phenomenon called porokeratosis. Poromatosis is of cosmetic concern for the patient.
The malignant transformation risk of poroma is very minimal, and it transforms into porocarcinoma. In the study of Salih et al., metastasis of porocarcinoma was detected at a presentation in 31% of cases. The most common sites in the body for the metastases of porocarcinoma were the neighboring lymph nodes (57.7%) followed by respiratory tract/lungs (12.8%),brain (9%), liver (9%), skin (5.8%), bone (3.2%), stomach (0.6%) and breast (0.6%). Metastasis was disseminated in 1.3% of cases.
As it is a benign disease, no specific complication is associated with poroma itself. Some common complications may arise after a surgical procedure, including infection and hemorrhage.
Poromas are benign tumors not clinically distinctive are difficult to diagnose clinically. However, an experienced clinical dermatologist can often identify poromas, in particular, one with a vascular appearance on a volar surface.
Poroma is a benign tumor, mostly asymptomatic, and the prognosis is usually good. Patient education regarding malignant transformation, i.e., porocarcinoma is not necessary because its risk is similar to normal skin. Poromatosis can be the cause of cosmetic disfigurement, and when involving soles, it can cause some disability.
Poroma is a benign tumor with mild clinical symptoms, and rarely it can progress to porocarcinoma. An interprofessional approach is important. A proper collaboration and communication between dermatologists, histopathologists, and surgical specialists are very important. Skin biopsy for histopathology is indicated in all cases with suspected poroma, in order to eliminate the diagnosis of its malignant variant. Porocarcinoma is a rare malignancy but is curable if accurately diagnosed and properly treated.
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