Reactivity with TdT in Merkel cell carcinoma: a potential diagnostic pitfall.

2008 
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
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