Photomicrographs of a typical<unk>  at a 4x, b 40x, and c–d 100x objectives. Hematoxylin and eosin staining demonstrates small, undifferentiated<unk> s with high N/C ratio and scanty cytoplasm. Typical immunopanel demonstrates positive staining with e cytokeratin AE1/AE3 (100x oil immersion), f CK 20 (100x oil immersion), and neuroendocrine markers such as g chromogranin (100x oil immersion). Diagnosis of<unk>  begins with a clinical examination of the skin and lymph nodes. Following clinical exam, definitive diagnosis of<unk>  <unk>  requires examination of biopsy tissue to identify its histopathologic features. An ideal biopsy specimen is either a punch biopsy or a full-thickness incisional biopsy of the skin including full-thickness dermis and subcutaneous fat. On light microscopy,<unk>  shows basaloid tumor nests with neuroendocrine features ("salt and pepper" chromatin, scarce cytoplasm, and brisk mitotic activity). In addition to standard examination under light microscopy, immunohistochemistry (IHC) is also generally required to differentiate<unk>  from other morphologically similar tumors such as small<unk>  lung cancer, the small<unk>  variant of melanoma, various cutaneous leukemic/lymphoid neoplasms, and Ewing's sarcoma. Neuroendocrine molecular markers such as synaptophysin or chromogranin A are characteristic of<unk>  and other neuroendocrine tumors, while other markers such as PAX5 or cytokeratin 20 can distinguish<unk>  from these tumors. Longitudinal imaging may also help in ruling out a diagnosis of metastatic small<unk>  lung cancer. Once an<unk>  diagnosis is made, a sentinel lymph node biopsy as well as other imaging is recommended as a part of the staging work-up needed to determine prognosis and subsequent treatment options.
Merkel-cell carcinoma
Basal-cell carcinoma
Lymphoepithelioma
Trichoepithelioma
Angiokeratoma
Onychomatricoma
Sebaceous carcinoma
Generalized trichoepithelioma