Merkel cell carcinoma is an uncommon aggressive primary cutaneous neuroendocrine carcinoma. Histologically, the differential diagnosis includes the ‘small round cell' tumor group, particularly metastatic small cell carcinoma and blastic hematological malignancies involving skin/soft tissues. Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase, which is a sensitive and specific antibody for acute lymphoblastic lymphoma with a small proportion of acute myeloid leukemia showing positivity. This study investigates the expression of TdT in 20 cases with initial diagnosis of Merkel cell carcinoma. Archival blocks and slides were retrieved and reviewed and clinical information obtained from patient charts. Immunohistochemistry was performed and graded as: 0, no staining; 1+, less than 50% staining in the cells; 2+, 50% or more staining in the cells. After review, 15 cases were confirmed as Merkel cell carcinoma. Immunohistochemical positivity was as follows: 8/15 cases were positive for TdT with strong nuclear staining, morphologically resembling ‘blasts', AE1AE3, CAM5.2 (15/15) (both membrane and paranuclear dot positivity), CD56 and BCL-2 (15/15), Synaptophysin (13/15), Chromogranin A (11/15), NSE (15/15), CK20 (14/15), CK7 (3/15), both CK7 and CK20 (3/15), CD117 (8/15), CD99 (2/15), CD10 (1/15). One case was negative for CK7/CK20. All 15 cases were negative for thyroid transcription factor-1, LCA, CD20, CD3 and CD34. Expanded immunohistochemical panel with positive staining for epithelial/neuroendocrine markers, CK20, negative staining for hematolymphoid markers and awareness of TdT expression and other markers that show overlap with blastic hematological malignancies avoids misinterpretation in the diagnosis of Merkel cell carcinoma. This aids in further diagnosis of Merkel cell carcinoma, avoiding the potential diagnostic pitfall with other small round cell tumors and hematological malignancies primary or metastatic to the skin.
