Primary cutaneous myoepithelial carcinoma is very rare, with fewer than 15 cases reported in the literature to date. In our patient, primary cutaneous myoepithelial carcinoma was discovered incidentally during Mohs micrographic surgery of a basal cell carcinoma (BCC). Without awareness to such rare entities, this tumor could have easily been missed during Mohs surgery, leading to a delay in diagnosis and treatment.
An 80-year-old Caucasian male patient with a history of multiple nonmelanoma skin cancers presented for Mohs micrographic surgery for a biopsy-proven nodular BCC on his left forehead (Figure 1). Frozen sections from stage 1 revealed no residual BCC at the deep or lateral margins; however, an atypical proliferation of spindle cells with prominent nuclei was noted in the deep margin on hematoxylin-eosin (H&E) stain (Figures 2 and 3). Due to this finding, a second opinion of the Mohs slides was obtained intraoperatively with the staff dermatopathologist before closure was intiated. The exact etiology of these spindle cells required the residual specimen to be sent for permanent sections and immunohistochemistry stains for verification. Thus, the resultant Mohs defect was repaired primarily in the interim. Analysis of permanent sections revealed myoepithelial carcinoma invading the dermis, subcutis, and skeletal muscle. These cells were immunoreactive for CK5/6, P63, P40, S100 , and CD10. There was focal re- activity for smooth muscle actin (SMA), and the specimen was negative for CK7 and CEA. There was no residual BCC seen.
The patient was referred to ear, nose, and throat (ENT) for further management. A computed tomography of the head, neck, and chest and positron emission tomography scans were performed and were negative for salivary gland, bone, and nodal involvement with no evidence of metastasis. A wide local excision with margins was performed with negative intraoperative frozen sections, though permanent sections showed tumor near a margin. The plan was to monitor the patient clinically and with interval imaging, but unfortunately, the tumor recurred. The patient underwent another wide local excision with 1-cm margins to the cranium. While the intraoperative frozen sections were negative, permanent sections showed tumor in the deep margin with perineural invasion. Because of this, a multidisciplinary tumor board including radiation oncology, oncology, and ENT recommended radiation therapy to the region, which the patient is awaiting to begin.
Myoepithelial cells are epithelial cells that also have smooth muscle qualities and are normally found around eccrine and apocrine glands in the skin; in salivary, mammary, and lacrimal glands; and in the respiratory tract. Myoepithelial neoplasms are rare and primarily arise from salivary glands, though they have been reported in the breast, lungs, and soft tissues. Metastatic disease can present on the skin, but a primary cutaneous myoepithelial carcinoma is extremely rare. As there was no additional involvement of the tumor noted elsewhere by imaging in our patient, the working diagnosis is that the dermis is the primary site.
Myoepithelial neoplasms are thought to exist on a spectrum that includes benign mixed tumors of the skin, but their existence as a unique entity has been debated. These lesions showcase a broad range of morphologic features, including spindled, stellate, epithelioid, and plasmacytoid characteristics. They typically lack apparent ductal differentiation commonly seen in mixed tumors. Due to the varied morphologic characteristics, immunohistochemistry can help lead to the diagnosis of a myoepithelial neoplasm. These lesions typically stain positive for epithelial membrane antigen, cytokeratins, S-100 protein, glial fibrillary acid protein, and calponin, and it can also stain positive for muscle specific actin, SMA, vimentin, p40, and p63. Desmin is negative.
Given the rarity of myoepithelial carcinomas, there are no standardized workup or treatment guidelines. Initial assessment should include thorough imaging to identify a potential primary source in salivary or parotid glands or metastases elsewhere. Complete surgical excision with margins is recommended, but there is no consensus on the size of margins. Some authors recommend sentinel lymph node biopsy (SLNB) if there is concern for extensive or aggressive disease. Due to our patient’s negative imaging, a SLNB was not pursued. Radiation and chemotherapy have been used as primary and adjuvant treatments with variable results. A BRAF/MEK in- hibitor was used to successfully treat a patient with recurrent cutaneous myoepithelial carcinoma after her tumor tested positive for BRAF V600E mutations. Mohs surgery has been performed in benign myoepithelial lesions but not in malignant disease. Recurrence after surgery is not uncommon, as seen in our case, and has been reported to be around 30%, with the risk of metastases to be approximately 15%. Due to local recurrence and persistent positive deep margins in our patient, the surgical and oncology teams recommended adjuvant radiation to the affected area.
Our case expands the literature on a rare entity and is even more unusual by the incidental discovery of this cancer during Mohs surgery of an unrelated BCC. Our case also highlights the importance of examining the entire tissue specimen on histologic slides and not solely focusing on identifying the initial skin biopsy diagnosis.
Dr Papastavros is the chief resident in the department of dermatology at Howard University Hospital in Washington, DC. Dr Nootheti is a Mohs surgeon in the division of dermatology at the Veterans Affairs Medical Center and is also affiliated with the department of dermatology at Howard University Hospital and the department of dermatology at The George Washington University Hospital, all in Washington, DC.
Acknowledgement: The authors would like to thank Wen Chen, MD, of the division of pathology at Veterans Affairs Medical Center in Washington, DC, for her assistance reviewing the histopathology for this case and for providing the photomicrographs.
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