CD56⁺ TdT⁺ blastic natural killer cell tumor of the skin: A primitive systemic malignancy related to myelomonocytic leukemia

BACKGROUND. An unusual cutaneous tumor that has blastic morphology and coexpresses CD56 and terminal deoxynucleotidyl transferase (TdT) has been recently recognized and termed blastic natural killer cell lymphoma. METHODS. The authors identified seven cases of such CD56⁺TdT⁺ blastic tumors presenting in skin at their institution. The authors correlated clinical course with histomorphology and immunophenotype. RESULTS. All 7 patients (6 men, 1 woman, 52-85 years) presented with rapidly growing, frequently multiple cutaneous nodules. All patients had low level bone marrow involvement at diagnosis and frequently had lymph node involvement. Tumor cells were of intermediate size with irregular nuclear contours, fine chromatin, and indistinct small nucleoli. The expression of TdT varied between 5% and over 90% of the neoplastic cell population. Tumor cells were negative for surface CD3, CD5, and CD20 in all cases, but some patients showed expression of CD2 (three out of five), cytoplasmic CD3 (two out of seven), CD4 (six out of seven), and CD16 (three out of seven). Molecular studies showed absence of T-cell receptor gene rearrangements in all cases. All seven patients had rapid progression of disease, and six patients have died of their disease or complications. Three patients developed progressively increasing numbers of bone marrow blasts that had a myeloid immunophenotype and were negative for TdT and CD56. Two patients met criteria for acute myeloid leukemia at 11 and 22 months after presentation, respectively. CONCLUSIONS. CD56⁺ TdT⁺ blastic tumor presenting in skin is a systemic malignancy likely of primitive/undifferentiated hematopoietic origin. Patients might subsequently develop tumors of myeloid or myelomonocytic phenotype, indistinguishable from acute myelogenous leukemia.