Microcystic adnexal carcinoma is a rare tumor of the skin that most often develops in the head and neck region, particularly in the middle of the face, though it may occur in the skin of other parts of the body as well. The average age of diagnosis is 56. This tumor is often first noticed as a bump or yellowish spot in the skin. Though microcystic adnexal carcinomas frequently grow into and disturb nearby tissues and is therefore considered an invasive cancer, this type of tumor rarely spreads to more distant parts of the body (metastasizes). The main treatment for microcystic adnexal carcinoma is Mohs micrographic surgery, which is thought to improve the chances that all of the tumor cells are removed during surgery.

The underlying cause of this condition is not fully known. Risk factors appear to include radiation treatment, ultraviolet exposure, immunosuppression, and possibly genetics.

A case of microcystic adenxal carcinoma occurring in siblings suggests a genetic component. While a specific risk has not been formulated (there are too few cases to speculate), a positive family history may suggest an increased risk for an immediate family member developing a tumor.

Microcystic adnexal carcinoma appears as a smooth bump or patch that is slightly raised from the surrounding skin. It may be flesh-colored or yellowish, and it increases in size over time. A microcystic adnexal carcinoma may grow into nerves nearby, which can cause discomfort, numbness, tingling (paresthesia), burning, or itching.

Unfortunately, because microcystic adnexal carcinoma is a rare cancer, there is currently not enough information to determine if radiation therapy is an effective treatment for this disease. There are no guidelines for the use of radiation therapy as treatment for microcystic adnexal carcinoma, and little is known about the dose of radiation that might be needed to be effective. Two articles studying a total of 17 patients receiving radiation therapy for microcystic adnexal carcinoma have suggested that there may be a benefit of radiation therapy for decreasing the chances of the tumor regrowing (recurrence), and the authors suggest that this treatment could be considered if there is evidence that cancer cells remain after surgery, or in situations where additional surgery cannot be done. However, another article raises concerns about the use of radiation therapy considering the small number of patients studied, the report of one patient's disease potentially worsening after radiation therapy, and the belief that exposure to radiation may be a risk factor for the initial development of this tumor.