Adnexal carcinomas are a diverse group of malignant skin tumors that are derived from the various components of the skin appendageal structures. These tumors are extremely rare and comprise well less than 1% of all skin cancers diagnosed annually. They are difficult to diagnosis clinically because they can all mimic the more common types of skin cancer, particularly basal cell carcinoma and squamous cell carcinoma. They can be diagnosed with certainty only after histological examination. These tumors are believed to be derived from hair follicle, sebaceous gland, apocrine gland, or eccrine gland epithelium. They are thought to arise de novo and can also arise from a preexisting benign precursor. An example is an eccrine porocarcinoma developing within an eccrine poroma.
Clinical Findings: These tumors are very rare, and one is unlikely to consider them in the differential diagnosis when evaluating an individual with an undiagnosed skin growth. There are few clues to their origin, which makes diagnosis of these cancerous tumors almost impossible based on clinical findings alone. Most manifest as a solitary papule, plaque, or dermal nodule. Most are asymptomatic, but pruritus, bleeding, and pain may be present.
The diagnosis of these tumors requires tissue sampling. A punch or excisional biopsy is the best method to biopsy these lesions, because it allows the pathologist to get a large enough piece of tissue to evaluate. A punch biopsy is especially important to help differentiate microcystic adnexal carcinoma from a benign syringoma. The latter is very superficial in nature, whereas the microcystic adnexal carcinoma displays a deep infiltrative growth pattern that will not be appreciated with a superficial shave biopsy.
Pathogenesis: The pathogenesis of these tumors is poorly understood. In contrast to basal and squamous cell carcinomas, they are unlikely to be caused by ultraviolet light exposure. The rarity of the tumors makes them difficult to study. There appears to be no genetic inheritance to these malignant tumors, with the lone exception of the sebaceous carcinoma. Sebaceous carcinoma can be seen in the Muir-Torre syndrome, which is inherited in an autosomal dominant pattern.
Histology: Each tumor is unique histologically. The tumors can be subdivided according to the type of epithelium from which they are derived: sebaceous, hair follicle, eccrine, or apocrine. The pathologist is able to differentiate these tumors based on certain criteria. The tumors show varying amounts of cellular atypia and an invasive growth pattern. They are usually poorly circumscribed with varying amounts of mitotic figures, necrosis, and abnormal-appearing cells. Various gland-like structures can be seen in some tumors, which can be helpful in making the diagnosis. Often, special immunohistochemical stains are used to help differentiate the subtypes of these tumors.
Treatment: These tumors should all be surgically excised with clear surgical margins. The Mohs surgical technique has been used successfully to treat these tumors, as has a standard wide local excision. Sentinel node removal and evaluation is not routinely performed, but some clinicians advocate its use, especially in some of the more aggressive subtypes such as the eccrine porocarcinoma. Sentinel node removal and evaluation has not shown any survival benefit to date. Mohs surgery may lead to a decrease in recurrence rate and is tissue sparing. Because of the rare nature of these tumors and the lack of prospective randomized studies, it is difficult to determine the best removal method. For the same reasons, the ultimate prognosis and the recurrence rate of these tumors are unknown. After diagnosis and removal of these tumors, the patient should have long-term follow-up to evaluate for recurrence.
Adnexal tumors that have metastasized are treated with chemotherapy with or without radiotherapy. The prognosis is poor for patients who develop metastatic adnexal carcinoma.