Seminomas are the most frequent testicular tumors and in spite of specific markers some histological subtypes can be diagnostically challenging due to the potential overlap of morphologic features and a variant antigen expression. Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase present in hematogones, thymic T cells, lymphoblastic lymphoma/leukemia (LBL), and in some cases of acute myeloid leukemia but so far has not been described to be expressed in seminomas. After observing a reactivity of TdT in one case of seminoma, we analyzed ten additional tumors by immunohistochemistry to determine their spectrum of reactivity for TdT. In all seminoma cases investigated (10/10) as well as in two tumor-associated germ cell neoplasias in situ (2/2) the TdT staining intensity was variable but was often moderate to strong and restricted to the nucleus. We conclude that TdT expression in seminomas could represent a diagnostic pitfall in the differential diagnosis of LBL, particularly because both may lack CD45 and/or CD20 expression and-concerning relapse in long-term survivors of testicular cancer-LBL is the most frequent secondary neoplasm in the patients.
Keywords: Aberrant; Acute leukemia; Pitfall; Seminona; TdT; Testis.