Carcinomas of the eccrine sweat gland represent a rare group of tumors with potential for local destruction and metastasis. High recurrence rates have been reported following conventional surgical excision of eccrine carcinomas. See the image below.
The specific classification of eccrine carcinomas is both complex and nebulous, mostly because of the paucity of reported cases but also because many of these tumors show little histologic resemblance to mature eccrine glands. Histogenetic association is based primarily on histochemical, immunochemical, or ultrastructural features. Nevertheless, eccrine carcinomas may be taxonomically segregated into two main groups, as follows:
Those that are histologically similar to certain benign appendage tumors (eg, sclerosing sweat duct carcinoma, porocarcinoma, malignant chondroid syringoma, malignant nodular hidradenoma, malignant eccrine spiradenoma)
Those that show a diverse array of histologic features, not encapsulating any aspects of a benign counterpart
A slightly different method of eccrine carcinoma classification is suggested by Galadari et al,  who divide these tumors into those that arise de novo in normal skin and those that originate within preexisting benign sweat gland tumors. Precise identification based on histology is of significant importance because treatment and prognosis vary according to the microscopic appearance (see Histologic Findings).
There is damage to sweat glands and suppression of sweating, which disrupts thermoregulation.
Eccrine carcinoma may be derived de novo from any portion of the normal eccrine apparatus or result from the transformation of an existing benign eccrine tumor.
A 2000 study by Takata et al  examining the incidence of cytogenetic abnormalities in malignant eccrine tumors showed low incidences of loss of heterozygosity (LOH) or TP53 alterations in a mixed group of these neoplasms, in contrast to the frequent and multiple genetic abnormalities seen in tumors arising from epidermal keratinocytes. The authors hypothesize that this difference may be partly explained by the fact that the bulk of eccrine sweat glands lie deep in the dermis, an environment relatively protected from the sun and environmental mutagens. The precise role of ultraviolet radiation (UVR) on sweat glands remains to be elucidated. However, research shows UVR is known to suppress sweating, which disrupts body thermoregulation, and a study analyzing TP53 mutations in 16 sweat gland carcinomas identified 3 G:C → A:T transitions at dipyrimidine sequences on the antisense strand. 
Primary eccrine carcinomas are exceedingly rare, accounting for roughly 1 of 13,000 specimens submitted to a dermatopathology laboratory. The more common subtypes include MAC, eccrine porocarcinoma, and hidradenocarcinoma. The less common subtypes include eccrine mucinous carcinoma, malignant eccrine spiradenoma, malignant mixed tumor, malignant cylindroma, and papillary eccrine adenoma.
Only several hundred cases of eccrine carcinoma have been reported in the literature worldwide. No specific data are available regarding United States versus international incidence of eccrine carcinoma.
Sex incidence appears to be equal for eccrine carcinoma. Exceptions to this are the malignant chondroid syringoma and primary cutaneous adenoid cystic eccrine carcinoma, both of which occur more commonly in females than in males.
Eccrine carcinomas most commonly are diagnosed in patients in their sixth through eighth decades of life.
Given the paucity of data on eccrine carcinomas, the prognosis remains unknown; the tumor appears to be locally aggressive but with a low probability of metastases (3.2%). [7, 8] Because eccrine ductal carcinoma is such a rare tumor, it has been included in the tumor, node, metastasis (TNM) staging for squamous cell carcinoma.