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TDT Positive in Blastic Dermal Infiltrate: Think of Blastic Plasmacytoid Dendritic Cell Neoplasm.

Author: Sara Chaabi, Asmaa El Kebir, Farida Marnissi, Meriem Regragui, Asmae Mazti, Samira Benayad, Nisrine Bennani - Guebessi, Mehdi Karkouri

Category: JMSR Pathology


Background: Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase expressed in lymphoblastic lymphomas/leukemias but also in other undifferentiated tumors. An expression of Tdt in a blastic dermal infiltrate may constitute a diagnostic pitfall. Methods: We present here a case diagnosed in our institution illustrating this diagnostic difficulty. Results : We report the case of an 81-year-old man, with no particular medical history, who had consulted for a purplish nodular dorsal mass accompanied by several diffuse infiltrated papular lesions that had evolved over several months. The biopsy showed a diffuse tumor infiltrate in the dermis, made of medium-sized cells with reduced cytoplasm and dense chromatin and separated from the epidermis by a grenz zone (Figure 1). The initial immunohistochemical study showed absence of CD3, CD8 and CD20 expression and expression of CD4, CD56 and TDT (Figure 2). Other lymphoid markers were requested to rule out lymphoma/lymphoblastic leukemia (PAX5, CD79a, CD5) as well as myeloid markers to rule out myelomonocytic leukemia (MPO, CD34, CD68, CD15), cytotoxic markers to rule out NK lymphoma (granzyme B and TIA1) and epithelial and neuroendocrine markers to rule out Merkel carcinoma. These markers were all negative, pointing to a blastic plasmacytoid dendritic cell neoplasm.

Figure 1: HEx20 Figure 2: Tdt

Conclusion: Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare hematologic malignancy with usually poor outcomes and high risk of progression to acute leukemia. Histological diagnosis is challenging and immunohistochemical analysis is mandatory. This entity represents a diagnostic challenge for pathologists given its rarity and possible TdT expression.

Keywords: Terminal deoxynucleotidyl transferase, blastic dermal infiltrate