Porocarcinoma (PCA) (also termed malignant poroma, eccrine porocarcinoma, and malignant eccrine poroma) 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. 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. PCA tumors are classified as one form of the cutaneous adnexal tumors; in a study of 2,205 cases, PCA was the most common (11.8%) form of these tumors. 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. 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. 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. PCA tumors may arise from one of these longstanding poromas (in one study this occurred in 18% of cases) but more commonly appear to develop independently of any precursor poroma. PCA are locally invasive tumors 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. Repeatedly recurrent, unresectable, and metastatic PCA have been treated with chemotherapy and/or radiotherapy. However, metastatic PCA has responded poorly to these treatments and carries a poor prognosis. Because of their rarity and lack of distinct clinical features and variable physical and microscopic histological appearances, the diagnosis of porocarcinomas is often challenging. PCA has commonly been either mis-diagnosed or over-diagnosed. 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.
Presentation In a large study reviewing all literature reports of PCA up to December 1, 2016, patients (including those with a poroma that later progressed to a PCA) presented with a single epidermal nodule or mass (71.2% of cases), ulcerated mass/nodule (18.3%), plaque (9.8%), swelling (1.3%), wart (0.6%), papule (0.6&) or nevus (i.e. mole, 0.6%). The patients were aged 6 months to 97 years old (average age 67.57 years). The lesions presented in the head & neck (39.9% of cases), legs (33.9%), arms (8.8%), back (5.1%), chest wall (4.6%), genitalia (4.0%), abdomen (2.6%), or around the anal area (0.6%) and had been present for as little as 4 days or as long as 60 years (average 5.6 years). Metastasis (most commonly involving the lymph nodes close to the primary lesion) were diagnosed at presentation in 31% of cases. Other studies report that: a) PCA tumors presented with average sizes of 2.53 cm (range of 0.3–7 cm) in largest diameter; b) PCA tumors at presentation were commonly red to violet in color, usually <2 cm in maximum diameter; and typically asymptomatic but may have been called to attention because of spontaneous bleeding, ulceration, sudden itching, pain, or rapid growth; c) in 37 cases, metastases were present in 16.2% of cases at presentation and occurred in two cases (5.4% of cases) 3 and 17 months after diagnosis; d) no metastatic disease was found at presentation or after a 3-year follow-up in 7 cases; e) PCA have metastasized to nearby or distal skin sites, local lymph nodes, or to the bones, bladder, breast, retroperitoneum, ovary, liver, lung, brain, or stomach; and f) PCA have been reported to occur in skin areas previously traumatized or exposed to radiation, excessive sunlight, or chronic lymphedema; in a sebaceous cyst; and in individuals with extramammary Paget's disease, sarcoidosis, chronic lymphocytic leukemia, pernicious anemia, Hodgkin's disease, nevus sebaceous, HIV/AIDS, xeroderma pigmentosum, immunosuppression caused by a disease or chemotherapy, pernicious anemia, and xeroderma pigmentosa.
Histopathology
The microscopic histopathology of benign poroma tumors stained with hematoxylin and eosin dyes consists of poroid cells (i.e. small, round cells with oval nuclei and little cytoplasm) admixed with cuticular cells (i.e. epithelium-like cells) with centrally placed nuclei and abundant eosinophilic (i.e. colored pink or red due to uptake of the eosin stain) cytoplasm. Porocarcinomas differ from poromas by their content of cells that are more irregular in size and contain misshaped nuclei that often have conspicuous nucleoli. Unlike the tumor cells in poromas, PCA tumor cells often appear to invade nearby normal tissues. are rapidly proliferating as evidenced by their increased mitotic activity, and may have differentiated (this process is termed metaplasia) to appear as squamous cells, clear cells, mucous cells, or spindle cells. PCA tissues may contain areas of necrosis (i.e. dead cells).
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