Microcystic adnexal carcinoma (MAC), a rare and low-grade malignant skin tumor, is characterized by a high rate of misdiagnosis and a preponderance for local recurrence, but seldom seen nodal or distant metastasis. Although MAC typically occurs almost in the head and neck region, primary eyelid or orbital MAC is very rare. To explore the unique characteristics of the eyelid and orbital MAC, we reviewed the relevant literature. Based on its distinctive anatomical location and the aggressive behavior, eyelid or orbital MAC not only exhibit a high rate of misdiagnosis and local recurrence, but also lead to serious complications such as disfigurement after orbital exenteration, paranasal sinuses or intracranial invasion, even death. Misdiagnosis of MAC commonly result from its rarity and nonspecific clinical and histopathological presentation. To reduce or avoid misdiagnosis, it is important to increase awareness for MAC and obtain a full-thickness biopsy specimen in histopathological analysis. Due to its extensive invasive growth pattern, MAC has a high rate of local recurrence, so complete excision with clear margins and long-term follow-up of patients with MAC are necessary. About those serious complications of the eyelid and orbital MAC, early and accurate diagnosis, complete excision is very important. Moreover, an interprofessional team consisting of ophthalmologist, otolaryngologist, neurologist, dermatologist, pathologist, radiologist is needed to evaluate and treat this disease. In summary, increasing awareness, early and accurate diagnosis, complete excision, long-term follow-up, and a multidisciplinary team is crucial for management of the eyelid and orbital MAC.

Merkel cell carcinoma (MCC), a rare primary cutaneous neuroendocrine neoplasm, is extremely aggressive and has a higher mortality rate than melanoma. Based on Merkel cell polyomavirus (MCPyV) status and morphology, MCCs are often divided into several distinct subsets: pure MCPyV-positive, pure MCPyV-negative, and combined MCC. MCPyV-positive MCC develops by the clonal integration of viral DNA, whereas MCPyV-negative MCC is induced by frequent ultraviolet (UV)-mediated mutations, that are characterized by a high mutational burden, UV signature mutations, and many mutations in TP53 and retinoblastoma suppressor gene ( RB1 ). Combined MCC consists of an intimate mix of MCC and other cutaneous tumor populations, and is usually MCPyV-negative, with rare exceptions. Based on the existing subsets of MCC, it is speculated that there are at least 4 stages in the natural history of stem cell differentiation: primitive pluripotent stem cells, divergent differentiated stem cells, unidirectional stem cells, and Merkel cells (or epidermal/adnexal cells). In the first stage, MCPyV may integrate into the genome of primitive pluripotent stem cells, driving oncogenesis in pure MCPyV-positive MCC. If MCPyV integration does not occur, the stem cells enter the second stage and acquire the ability to undergo multidirectional neuroendocrine and epidermal (or adnexal) differentiation. At this stage, accumulated UV-mediated mutations may drive the development of combined MCC. In the third stage, the stem cells differentiate into unidirectional neuroendocrine stem cells, UV-mediated mutations can induce carcinogenesis in pure MCPyV-negative MCC. Therefore, it has been speculated that several subsets of MCCs arise from different stages of differentiation of common stem cells.