Incidence of TdT positivity in cases of leukemia and lymphoma

The expression of the enzyme marker terminal deoxynucleotidyl transferase (TdT) was examined by immunofluorescence assay in the cells from 333 cases with various types and subtypes of leukemia or lymphoma. More than 90% of cALL and T-ALL, 70% of Null-ALL and 80% of pre-B-ALL were TdT-positive. One case in the commonly TdT-negative group of B-ALL showed TdT-positive cells. All cases of mature B-cell malignancies (B-CLL, hairy cell leukemia, B-cell lymphoma) have been TdT-negative. In the group of mature T-cell malignancies, T-CLL and mycosis fungoides were negative and 2 out of 6 mature T-cell lymphomas were TdT-positive. 13% of acute myeloid leukemias and 36% of CML in blast crisis expressed TdT. Therefore, these TdT-positive cases of CML in blast crisis also carrying the common ALL-antigen belong to the lymphoid subtype. CML and erythroleukemia were invariably TdT-negative. TdT has become an indispensable indicator of immature lymphoid leukemia cells and is particularly valuable as part of the panel of markers used in leukemia phenotyping.