Microcystic adnexal carcinoma (MAC) is a rare, malignant appendage tumor commonly classified as a low-grade sweat gland carcinoma that typically occurs on the head and neck, particularly the central face. Microcystic adnexal carcinoma shows aggressive local invasion but has little metastatic potential. Goldstein et al  described the first series in 1982, emphasizing the importance of its distinction from the benign histologic mimics termed syringoma, desmoplastic trichoepithelioma, and trichoadenoma. Note the image below.
Despite subsequent widespread recognition of microcystic adnexal carcinoma as a discrete clinicopathologic entity, its precise relationship to and histologic discrimination from other putative locally aggressive sweat gland carcinomas (reported under a variety of names, including sclerosing sweat duct carcinoma, syringoid eccrine carcinoma, syringomatous carcinoma, and eccrine epithelioma) remains unresolved and has provoked considerable nosologic and diagnostic confusion. The authors regard microcystic adnexal carcinoma as synonymous with group 1 sclerosing sweat duct carcinomas as described by Cooper et al.  In addition, the tumor described as syringomatous adenoma of the nipple is regarded by some authorities as microcystic adnexal carcinoma.
The differentiation pathway of microcystic adnexal carcinoma (MAC) has provoked considerable debate. Goldstein et al  initially suggested that it showed dual pilar (the superficially located keratocysts resembling follicular infundibula) and eccrine differentiation. Others have supported this notion, noting, in particular, that the keratocysts often expressed pilar-type keratins and that occasionally, trichohyalin granules are present. However, some authors have suggested that microcystic adnexal carcinoma shows only eccrine differentiation.
More recently, based on the ontogenetic relationship between hair follicles and apocrine glands, microcystic adnexal carcinomas have been theorized to display folliculo-apocrine or sometimes folliculo-sebaceous-apocrine differentiation rather than folliculo-eccrine differentiation. Support for this premise is provided by occasional cases showing focal apocrine decapitation secretion and the demonstration of sebaceous foci in some tumors.
Little information is available on the molecular pathophysiology of microcystic adnexal carcinomas, although 1 report documented a deletion on arm 6q in 1 case. A case of microcystic adnexal carcinoma occurring in siblings also suggests a genetic role. 
Microcystic adnexal carcinoma is a rare tumor, with only slightly more than 300 cases reported worldwide.  Thomas et al  reported a mean number of 1.63 cases per year in a rural northeastern area of the United States.
Most cases (90%) of microcystic adnexal carcinoma occur in whites, but it may occur in persons of other races such as Latinos and Asians. [6, 7] Fifty-two cases have been reported in Japan. [8, 9] In African Americans, 9 cases in total have been reported in 8 patients. [4, 10, 6, 11, 12, 13, 14] The overall clinical behavior of microcystic adnexal carcinoma appears to be similar in all races.
Overall, the prognosis is good, especially when margin-control techniques such as Mohs micrographic surgery (MMS) are used, with a modest tendency for recurrence and a very low rate of metastases. Ten-year survival of patients with MAC is equivalent to other patients of similar age without MAC. 
The largest series of cases reports an overall (combining results from simple excision and MMS) 10-year recurrence rate of 18%. Others have reported a 12% recurrence rate at mean follow-up of 39 months.  Recurrence in one case occurred 30 years after initial resection, making routine clinical follow up necessary. 
Although microcystic adnexal carcinoma is considered a nonmetastasizing tumor, rare reports detail histologically confirmed lymphatic spread. Of the more than 300 reported cases of microcystic adnexal carcinoma, only 9 have been metastatic. [21, 22, 23, 24, 25, 26, 27, 28, 29]
Two deaths have been attributed to metastatic microcystic adnexal carcinoma. One patient presented with mediastinal lymphadenopathy and later died of the disease. However, no histologic evidence from the lymph nodes confirmed the cause of death.  The other death occurred from a microcystic adnexal carcinoma of the right-upper eyebrow that lead to perineural spread though the orbit with central nervous system dissemination.  In another reported case of distant metastatic disease, the patient eventually died of aspiration pneumonia, not from the microcystic adnexal carcinoma itself. 
Morbidity is high because of the deeply infiltrating nature of the tumor, which can invade into bone, muscle, blood vessels, cartilage, and nerves. In one study, the mean clinical lesion size was 3 cm2, but the final defect size was 18 cm2, highlighting its occult extension.  Exquisitely rare orbital extension can occur, and a primary orbital presentation has also been reported. 
Mortality from metastatic disease is rare, with only 6 local [21, 22, 23, 24, 25, 26] and 3 distant metastases reported. [27, 28, 29] Local metastatic disease may, in most cases, be occult contiguous extension along neurovasculature bundles. [24, 25, 26] Considering that more than 300 cases have been reported in the medical literature worldwide,  only 2 deaths have been attributed to microcystic adnexal carcinoma, [26, 10] both due to metastatic disease.
Of possible relation, Fernandez-Figueras et al reported a case of a high-grade carcinosarcoma with an architectural pattern similar to microcystic adnexal carcinoma but also exhibiting nuclear pleomorphism, hyperchromasia, and large nucleoli histologically. The patient developed metastatic lung disease from the tumor and died within 6 months.  The authors further noted histologic high-grade features in their case that were similar to other cases reported with metastatic disease, [25, 28, 10] and they suggested this may be an indicator for more aggressive disease. Whether this particular tumor represents an undefined adnexal carcinoma with an architectural pattern similar to microcystic adnexal carcinoma or a morphological variant of microcystic adnexal carcinoma with high-grade histologic features remains unknown.