Melanoma in situ represents an intraepidermal proliferation of atypical melanocytes. The cells usually have abundant cytoplasm with melanin pigment and intranuclear pseudoinclusions. On immunohistochemistry, the neoplastic cells express melanocytic markers, including S100 protein, melan-A, HMB45 antigen, and SOX10, and are negative for cytokeratin. As a possible pitfall, numerous melanomas may express neuroendocrine markers.
Squamous cell carcinoma in situ reveals more epithelioid cells with desmosomes between the tumor cells. The cells express squamous cell markers, such as high-molecular-weight cytokeratins (eg, CK5/6), p40, and p63, and are negative for neuroendocrine markers and cytokeratin 20. Remembering that squamous carcinoma in situ and Merkel cell carcinoma can be concurrent is important.
In extramammary Paget disease, the atypical cells are thought to arise from intraepidermal sweat gland cells and demonstrate glandular features, including abundant basophilic cytoplasm, which can be highlighted by periodic acid-Schiff with diastase and mucin special stains. The cells express cytokeratin 7, carcinoembryonic antigen, and variably epithelial membranous antigen. Secondary extramammary Paget disease from the genitourinary tract may express also CK20, but not with a dot-like pattern.
Sebaceous carcinoma cells have multiple cytoplasmic vacuoles that contain lipid droplets and nuclear scalloping. The intracytoplasmic lipid droplets can be highlighted with an adipophilin stain. The cells are also negative for cytokeratin 20 and the neuroendocrine markers.
One substantial differential diagnosis is cutaneous T-cell lymphoma. Histologically, the infiltrating lymphocytes typically have clear cytoplasm and marked atypia. The cells express general lymphocytic markers (such as CD45) and T-lymphocytic markers, such as CD3 and CD4 (or CD8). The atypical lymphocytes are immunonegative for all cytokeratin and neuroendocrine markers.
The main treatment for cutaneous lymphoma is chemotherapy-based without surgical intervention. Therefore, recognizing cutaneous lymphoma is crucial to avoid unnecessary surgical procedure and provide the appropriate treatment.
Some important points to be remembered include the following:
Pure epidermal Merkel cell carcinoma (Merkel cell carcinoma in situ) is rare.
Identification of epidermal Merkel cell carcinoma should prompt reexcision for evaluation of a possible dermal component.
Immunohistochemistry is essential for histologic diagnosis. Depending on the morphology and location, the recommended panel includes cytokeratin 20; adipophilin; and squamous, neuroendocrine, melanocytic, glandular, and lymphocytic immunohistochemical markers.
Merkel cell carcinoma in situ can coexist with other skin lesions (especially squamous cell carcinoma in situ and actinic keratosis).
The patient in this case underwent wide local excision with two-stage tarsal conjunctival flap (Hughes flap procedure), without sentinel lymph node sampling. The patient is still alive without evidence of recurrence or metastatic disease after 18 months of follow-up.
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Cite this: Nail Alouch, Doina Ivan, Phyu P. Aung, et. al. Oncology Case Challenge: A Construction Worker Who Drinks Daily Has an Eyelid Lesion - Medscape - Nov 02, 2022.