Porocarcinoma

Porocarcinoma (PCA) (also termed malignant poroma, eccrine porocarcinoma, and malignant eccrine poroma)^[1] is a rare form of skin cancer that develops in eccrine sweat glands, i.e. the body's widely distributed major type of sweat glands, as opposed to the apocrine sweat glands which are located primarily in the armpits and perineal area.^[2] This cancer typically develops in individuals as a single cutaneous tumor in the intraepidermal spiral part (termed the acrosyringium) of these sweat glands' ducts (i.e. channels) at or near to where they open on the skin's surface.^[3] PCA tumors are classified as one form of the cutaneous adnexal tumors;^[4] in a study of 2,205 cases, PCA was the most common (11.8%) form of these tumors.^[5]

Porocarcinomas are malignant counterparts to the far more common benign tumors of the eccrine sweat gland's acrosyringium, i.e. poromas. As currently viewed, there are 4 poroma variants based on their predominant cell types and extent of their tumor tissues presence in the epidermis and dermis: 1) Hidroacanthoma simplexe poromas are confined to the epidermis, i.e. uppermost layer of the skin. 2) Dermal duct poromas are confined to the dermis, i.e. layer of skin between the epidermis and subcutaneous tissues.^[1] 3) Hidradenomas have recently been sub-classified into two groups; 95% are termed clear cell hidradenomas and have features suggesting that they derive from apocrine sweat glands while the remaining 5% are termed poroid hidradenomas and have features suggesting that they derive from eccrine sweat glands.^[6] And 4) eccrine poromas are eccrine sweat gland tumors that consist of three cell types (see the histopathology section of Poromas) and are primarily located in the epidermis and superficial dermis. Poromas may have 2 or more of these variants in the same tumor tissue and the variants typically have histopathology findings that are not clearly distinguishable from each other.^[7][8] PCA tumors may arise from one of these longstanding poromas^[8] (in one study this occurred in 18% of cases^[9]) but more commonly appear to develop independently of any precursor poroma.^[1]

PCA are locally invasive tumors^[1] that have been treated by surgical resection but often recur at the site of their surgical removal and metastasize to distant tissues before or after their removal.^[1] Repeatedly recurrent, unresectable, and metastatic PCA have been treated with chemotherapy and/or radiotherapy.^[10][11] However, metastatic PCA has responded poorly to these treatments and carries a poor prognosis.^[12]

Because of their rarity and lack of distinct clinical features and variable physical and microscopic histological appearances, the diagnosis of porocarcinomas is often challenging.^[1] PCA has commonly been either mis-diagnosed or over-diagnosed.^[4][9] Under-diagnosis may have been responsible for a recent study conducted in a United Kingdom single center that reported that the number of PCA cases had increased 3-fold over the previous 4 years and was expected to rapidly rise further during the next decade.^[9]