Microcystic adnexal carcinoma (MAC) represents a rare and slow-growing type of skin cancer. This tumor originates from the adnexal structures of the skin, specifically the sweat ducts and hair follicles. MAC is predominantly found on the head and neck, although it can appear in other body regions. Understanding MAC involves its subtle presentation and the specialized approaches needed for its identification and management.
What is Microcystic Adnexal Carcinoma?
Microcystic adnexal carcinoma often appears as a subtle, firm, skin-colored, or yellowish plaque or nodule. These lesions typically grow slowly over several years, often taking years to develop noticeable size. While most commonly found on the head and neck, especially on the central face, including the upper lip and periorbital areas, cases have also been documented on the scalp, tongue, trunk, upper extremities, and genitals.
The characteristic growth pattern of MAC is slow and infiltrative, spreading deeply into surrounding tissues rather than metastasizing widely. This infiltration can extend into fat, muscles, and nerves, which can lead to complications such as numbness, tingling, or pain, especially with perineural invasion. Despite its invasive local growth, distant metastasis is rare, though rare. The average tumor size is typically less than 2 cm, but larger cases have been reported.
How is Microcystic Adnexal Carcinoma Diagnosed?
Diagnosing microcystic adnexal carcinoma begins with a thorough clinical examination, though its subtle appearance can make it challenging to identify. MAC can often be mistaken for other benign or less aggressive skin conditions, such as basal cell carcinoma or desmoplastic trichoepithelioma, due to its subtle presentation. This makes a high index of suspicion and specialized knowledge important for accurate identification.
The primary diagnostic tool for MAC is a biopsy, a deep incisional or excisional biopsy extending into the subcutaneous fat. Superficial shave biopsies are often insufficient because the tumor frequently lies deep within the dermis and can show benign features at the surface, leading to misdiagnosis in many cases. Subsequent histopathological examination of the biopsy specimen is performed by a specialized pathologist. This examination looks for specific microscopic features, such as small basaloid cells and keratocysts in the upper dermis, along with infiltrating cords and ductular structures in a dense fibrous stroma.
Imaging studies, such as MRI or CT scans, are not typically used for the initial diagnosis, but they can be valuable in assessing the full extent of the tumor if deep invasion into nerves, muscle, cartilage, or bone is suspected. These imaging techniques help map out the tumor’s reach before surgery, providing a clearer picture of the local invasion. While dermoscopy and optical coherence tomography are emerging tools, their specific features for MAC are still limited, and they are not yet primary diagnostic methods.
Treatment Options
Surgical excision is the primary treatment for microcystic adnexal carcinoma. Due to MAC’s infiltrative growth and tendency to extend beyond visible margins, complete tumor removal with clear margins is a priority. This approach aims to minimize the risk of recurrence.
Mohs micrographic surgery (MMS) is the preferred surgical method for MAC due to its high cure rates and tissue-sparing nature. During MMS, the surgeon removes thin layers of cancerous tissue one at a time. Each layer is immediately examined under a microscope to check for cancer cells at the margins. This process continues until no cancerous cells are detected, ensuring complete removal while preserving healthy tissue. The ability of MMS to meticulously track the tumor’s infiltrative extensions, including perineural invasion, makes it particularly effective for MAC, which often has subclinical spread.
Traditional wide local excision, which involves removing the visible tumor with a predetermined margin of surrounding healthy tissue, is another surgical option. However, this method has a higher reported recurrence rate for MAC compared to MMS. This higher recurrence rate is largely due to the difficulty in accurately determining the full extent of MAC’s infiltrative growth, leading to potentially incomplete removal.
Radiation therapy has a limited role in the treatment of MAC and is not considered a first-line treatment. It may be considered when surgical removal is not feasible due to the tumor’s size or location near vital structures, or for residual disease if surgical margins cannot be cleared. However, the effectiveness of radiation therapy for MAC is inconclusive, and some reports suggest it may even increase tumor size.
Prognosis and Recurrence
The prognosis for individuals diagnosed with microcystic adnexal carcinoma is favorable, especially when the tumor is treated effectively with surgical methods. Mohs micrographic surgery yields high cure rates and contributes to a positive long-term outlook. The 10-year overall survival rate for MAC patients is reportedly high.
Despite the good prognosis, local recurrence is a concern with MAC. This tendency stems from the tumor’s infiltrative nature, which can make it challenging to ensure all cancer cells are removed during initial treatment. Even after seemingly complete excision, microscopic tumor extensions may remain, leading to the tumor reappearing in the same area. The longest reported latent period for recurrence is 30 years, highlighting the tumor’s indolent nature.
Long-term follow-up care is important for individuals treated for MAC. Regular monitoring allows for the early detection of any recurrence, enabling timely intervention. While local recurrence is a recognized risk, distant metastasis of MAC is rare, occurring only in advanced or neglected cases.