Merkel Cell Carcinoma In Situ Arising in a Trichilemmal Cyst: A Case Report and Literature ReviewSu, Wei MD*; Kheir, Sonia M MD†; Berberian, Brenda MD‡; Cockerell, Clay J MD*The American Journal of Dermatopathology: October 2008 - Volume 30 - Issue 5 - p 458-461 doi: 10.1097/DAD.0b013e31817a9896 Extraordinary Case Report Abstract Author Information A 73-year-old man presented with a 4-mm asymptomatic, smooth, dome-shaped lesion on the right fourth finger with a clinical impression of a cyst/dermatofibroma. Histological examination showed a trichilemmal cyst with 3 nests of small blue round cells within the basal layer of the cyst lining. There were many mitotic figures and apoptotic bodies. The stain for cytokeratin 20 “decorated” the tumor cells with an unequivocal perinuclear dot-like pattern, confirming their Merkel cell origin. Dermal Merkel cell carcinoma (MCC) arising in association with benign adnexal tumors or cysts, with or without epithelial involvement, is a rare event. MCC in situ in this context has not been previously reported. The immunostain for cytokeratin 20 is an important ancillary study in diagnosing MCC. Our case supports the view that a subset of MCCs is of intraepithelial origin and underscores the clinical importance of surveillance for changes in an “innocent” cyst. From the *Department of Dermatology, University of Texas Southwestern Medical Center, Dallas, TX, USA; †Mid-Atlantic Pathology Services Inc., Sterling, VA, USA; and ‡Private practice, Chevy Chase, USA. Reprints: Wei Su, MD, Department of Dermatology, University of Texas Southwestern Medical Center, Dallas, TX. © 2008 Lippincott Williams & Wilkins, Inc.