TDT (-), KIT (+), CD34 (+), CD99 (+) precursor T lymphoblastic leukemia/lymphoma

Although the definition of precursor T lymphoblastic lymphoma (T-LBL) is based only on histopathology, most cases cannot be diagnosed only by HE sections. Since 95% of T-LBL expresses TdT and TdT is expressed only in lymphoblasts, immunohistochemical demonstration of TdT is mandatory for the diagnosis of TLBL. However, little is known about the expression of other precursor cell molecules. A 58-year-old woman with myelodysplastic syndrome (RAEB) became overt acute myelogenous leukemia (AML). She was treated twice with allogeneic peripheral blood stem cell transplantation from her son. Nine months later, imaging modalities detected a soft tissue tumor around the left ileal bone. A biopsy was performed. Histologically, the tumor cells were malignant polymorphic lymphoid cells with hyperchromatic nuclei and inconspicuous nucleoli. Immunohistochemically, the tumor cells are positive for CD45, CD45RO, CD34, KIT (CD117), CD99 (MIC-2), p53, CD10, PDGFRA, and Ki67 (labeling=60%). They were negative for pancytokeratin AE1/3, pancytokeratin CAM5.2, TdT, CD3, CD20, CD79α, CD43, CD56, CD57, CD30, bcl-2, κ-chain, λ-chain, cytokeratin (CK) 7, CK20, synaptophysin, chromogranin, smooth muscle actin, p63, MPO, CD68, lysozyme, and ASD esterase. Although TdT was negative, other precursor cell markers (KIT, CD34, and CD99) were positive and the lymphoid cells showed T-cell lineage, the diagnosis was T-LBL. The patient died of lymphoma/ leukemia 11 months after the diagnosis. The author stress that TdT, KIT, CD34 and CD99 should be included in panels of precursor T-cell neoplasms. In addition, the author think that KIT, CD34 and CD99 are helpful for the diagnosis of T-LBL in cases negative for TdT. Further, it is unique that this case was not myeloid sarcoma but precursor T-cell neoplasm, and that T-LBL develops during AML.