title: Reactivity with TdT in Merkel cell carcinoma: a potential diagnostic pitfall.
abstract: Merkel cell carcinoma (MCC) is a high-grade neuroendocrine carcinoma of skin characterized by cells with a "blastic" appearance, scant cytoplasm, and fine, evenly distributed chromatin. Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase present in thymic T cells, lymphoblastic lymphoma/leukemia, and some cases of acute myeloid leukemia. After observing TdT immunoreactivity in a case of MCC, we analyzed 26 tumors by immunohistochemical analysis to determine their spectrum of reactivity with TdT and identified TdT in 19 (73%) of 26 MCCs. Staining intensity was variable but was often moderate to strong and present in a significant percentage of cells. Because MCC has cytomorphologic features similar to those of lymphoblastic lymphoma and may manifest as metastatic disease, reactivity with TdT in MCC could represent a diagnostic pitfall in the differential diagnosis with lymphoblastic lymphoma, particularly because the latter may lack CD45 and/or CD20, yet both neoplasms may express PAX-5, a B-cell-associated marker.
authors.affiliation: Immunohistochemistry Division, ProPath Laboratory, Dallas, TX 75247, USA.
authors.forename: Cary J
authors.initials: CJ
authors.lastname: Buresh
authors.forename: Bahram Robert
authors.initials: BR
authors.lastname: Oliai
authors.forename: Rodney T
authors.initials: RT
authors.lastname: Miller
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D018951:Antigens, CD20 / Q000378:metabolism; D014408:Biomarkers, Tumor / Q000378:metabolism*; D015266:Carcinoma, Merkel Cell / Q000201:enzymology* / Q000473:pathology; D004253:DNA Nucleotidylexotransferase / Q000378:metabolism*; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D007124:Immunoenzyme Techniques; D017493:Leukocyte Common Antigens / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D051757:PAX5 Transcription Factor / Q000378:metabolism; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000175:diagnosis / Q000201:enzymology; D012878:Skin Neoplasms / Q000201:enzymology* / Q000473:pathology
chemical_list: D018951:Antigens, CD20; D014408:Biomarkers, Tumor; D051757:PAX5 Transcription Factor; C497281:PAX5 protein, human; D004253:DNA Nucleotidylexotransferase; D017493:Leukocyte Common Antigens; C516769:PTPRC protein, human