Case 8 -

Precursor T-Cell Lymphoblastic Lymphoma D. Ashley Hill, M.D. Mihaela Onciu M.D.

Case History: 12-year-old boy with a large anterior mediastinal mass and left cervical lymphadenopathy. A needle-biopsy of a left cervical lymph node was performed. Further work-up showed normal bone marrow and peripheral blood. Microscopic Findings: Histologic sections showed a malignant lymphoid neoplasm with diffuse growth pattern. The neoplastic cells were uniform and medium-sized, with regular nuclear outlines, finely clumped chromatin and small inconspicuous to absent nucleoli. Frequent mitotic figures were noted. Differential Diagnosis: Lymphoblastic lymphoma (precursor B-cell or precursor T-cell) Other blastic neoplasms (myeloid sarcoma) Burkitt lymphoma Blastic variant of mantle cell lymphoma (In adults only) Immunophenotypic Findings (Immunohistochemistry): The lymphoma cells were weakly positive for CD45, CD3, CD1a, and Tdt, and were negative for CD79a. Molecular / Cytogenetic Analysis: Not performed. Diagnosis: Precursor T-cell lymphoblastic lymphoma/leukemia. Precursor T-cell / Precursor B-cell Lymphoblastic Lymphoma / Leukemia Definition: Precursor B-cell and T-cell tumors are neoplasms composed of blasts (immature lymphoid cells) committed to the B-cell or T-cell lineage, respectively. They may present with exclusive tissue infiltration (lymphoblastic lymphoma - LBL) or with tissue as well as peripheral blood and bone marrow involvement (designated as acute lymphoblastic leukemia - ALL). The arbitrary cut-off between LBL with marrow involvement and ALL has been established at 25% bone marrow involvement by blasts. This cut-off is not based on biologic differences and is used at the current time only as an arbitrary measure of tumor bulk (for staging purposes). While tumors diagnosed in the tissue as precursor B-cell LBL are most often a manifestation of ALL, it is not unusual for precursor T-cell LBL to be limited to peripheral involvement (most commonly anterior mediastinum) with no evidence of bone marrow involvement. Epidemiology: ALL is the most common childhood malignancy and up to 75% of ALL occur in children under 6 years of age. In Western countries the majority of cases (80-85%) have a precursor B-cell immunophenotype, while only 15-20% are of T-cell lineage. The prevalence of T-cell ALL is higher in Mediterranean countries, North Africa and the Middle East. Lymphoblastic lymphoma (i.e. disease limited to the peripheral tissues with <25% bone marrow involvement) is predominantly of T-cell lineage with a propensity towards anterior mediastinum as a primary site. Only ~10% of lymphoblastic lymphomas are of B-cell lineage. The latter cases appear to have a somewhat distinct clinico-pathologic spectrum (see below). Clinical Features: Children with precursor T-cell LBL/ALL often present with signs of mediastinal compression ranging from shortness of breath and coughing to superior vena cava syndrome, resulting from a bulky anterior mediastinal mass. Precursor B-cell LBL often involves the skin (scalp), lymph nodes, soft tissues and bones of the head and neck, and appears to have a distinctively better prognosis than B-cell ALL. The presenting features in ALL often reflect the degree of bone marrow failure and extra-medullary involvement by the tumor. Thus, these patients may show signs and symptoms of anemia, thrombocytpenia, neutropenia, or marked leukocytosis. In addition, ALL patients will commonly present with fever, fatigue, bone or joint pain, abdominal pain, anorexia, and variable degrees of hepatosplenomegaly and lymphadenopathy. Significantly increased LDH levels in this context may signify extensive bone marrow necrosis. Following chemotherapy, LBL/ALL of any lineage may relapse at sites such as cerebrospinal fluid/central nervous system, testes and ovaries. Molecular Biology and Cytogenetics: LBL/ALL are clonal proliferations of precursor B-cells or T-cells "frozen" at an early stage of maturation. The genetic lesions associated with malignant transformation are poorly understood on this disorder, but they seem to confer the neoplastic cells a growth advantage over the normal marrow cells. Cytogenetics and molecular analysis have defined several cytogenetic and molecular groups in precursor B-cell ALL that correlate highly with sensitivity to chemotherapeutic agents, response to therapy and outcome. (See Table 8.1) Gene expression profiling experiments have demonstrated that these cytogenetic and molecular lesions define dramatically distinct tumor subtypes. In precursor T-cell ALL such heterogeneity has been less well defined, although recent insights into correlations between immunophenotype, certain genetic lesions, gene expression profiles, and outcome seem to suggest that clinically significant disease subtypes do exist within this tumor category as well. These important lesions which appear to be mutually exclusive can be demonstrated for clinical purposes using conventional cytogenetics, molecular analysis (RT-PCR), and FISH. Of note, some of these lesions, most notably the t(12;21) can be cryptic by conventional cytogenetics and therefore at least one other method of analysis should be employed in precursor B-cell ALL with apparently normal diploid karyotype. Morphologic Features: Lymphoblastic lymphoma of any lineage is characterized by a diffuse pattern of growth sometimes associated with a "starry-sky" appearance in at least a portion of the infiltrate. Associated fibrosis may be present in some cases. The lymphoma cells are usually small to intermediate in size, with scanty cytoplasm, round to irregular nuclear outlines, finely clumped nuclear chromatin and inconspicuous or single small ('pinpoint') nucleoli. Mitotic figures may be frequent. In rare cases, especially in precursor B-cell ALL involving bone marrows, the lymphoma cells may be large, with pleomorphic nuclei, mimicking large B-cell lymphoma. Of note, in bone marrow aspirate samples, the ALL have been divided morphologically in L1 and L2 subtypes, depending on blast size, amount of cytoplasm, pattern of chromatin and presence of nucleoli (FAB classification). These categories do not correlate with biologically significant entities and appear to be of no use at the present time. Immunophenotype: Precursor B-cell LBL/ALL analyzed by flow cytometry typically expresses B-lineage antigens, including CD19, CD22, CD24, and cytoplasmic CD79a. CD20 expression ranges from negative to weakly positive. In addition, most tumors express CD10, CD34, and Tdt. Immunoglobulin expression, when present, is restricted to the mu heavy chain (Ig mu), with no associated light chain expression or restriction. The Ig mu may be present only in the cytoplasm, or in a small subset of cases ('transitional pre-B ALL') may be weakly expressed on the blast surface. Immunohistochemical analysis of paraffin-embedded tissue typically shows these tumors to be positive for Tdt, CD10, CD34, CD79a and negative for CD20 and CD3. Stains for T-cell antigens usually highlight only scattered mature T-cells. Importantly, while most neoplasms show weak CD45 expression which can be demonstrated by both modalities, a subset of tumors (most commonly those associated with hyperdiploid karyotypes) are negative for CD45. Precursor T-cell LBL/ALL analyzed by flow cytometry shows a range of antigen expression profiles. All neoplasms show expression of weak CD45, cytoplasmic CD3, and surface CD2, CD7 and often CD5. There is variable expression of surface CD3 (usually weaker than that seen on normal T-cells), Tdt, and CD34, with a significant subset of Tdt negative, CD34 negative tumors. T-cell LBL mimicking the cortical thymocyte phenotype is also CD1a+, CD4+, CD8+, CD21+. Immunohistochemical staining of paraffin-embedded tissues shows these neoplasms to be positive for Tdt, CD45 and CD3 (the latter two weaker than the normal T-cells in the infiltrate), as well as CD2, CD5, and CD7. Some of the tumors are positive for CD10 and/or CD1a. Differential Diagnosis: Precursor B-cell/Precursor T-cell LBL/ALL (also See Table 8.2): Myeloid sarcoma Monoblastic sarcoma Burkitt lymphoma/leukemia. Non-hematopoietic small blue-cell tumors (in CD45-negative cases) Mantle cell lymphoma (blastoid variant) – In adult patients only Precursor B-cell LBL/ALL Benign precursor B-cells (hematogones) seen in reactive conditions or following chemotherapy or bone marrow transplantation (in bone marrow, lymph nodes and tonsils) Precursor T-cell LBL/ALL in mediastinal masses (see Table 10.1): Benign (normal) thymus associated with another type of tumor Thymoma Hodgkin lymphoma Diffuse large B-cell lymphoma, primary mediastinal. Table 8.1: Cytogenetic and Molecular Groups Important in Risk Stratification in Precursor B-cell Acute Lymphoblastic Leukemia Cytogenetic abnormality Molecular lesion Prognostic Significance t(9;22)(q34;q11.2)* BCR-ABL ('ALL-type') Unfavorable t(12;21)(p13;q22) TEL-AML1 Favorable t(1;19)(q23;p13.3) PBX-E2A Unfavorable** t(4;11)(q21;q23) AF4-MLL Unfavorable Hyperdiploid (>50 chromosomes) Not known Favorable Hypodiploid (<46 chromosomes Not known Unfavorable * The Philadelphia chromosome ** With the current therapies the outcome of this type of translocation has improved to standard risk. Table 8.2: Differential Diagnosis of Hematopoietic Neoplasms with Blastic Morphology Neoplasm CD45 Tdt CD79a CD3 MPO Lysozyme CD43 CD20 CyclinD1 Prec. B-LBL/ALL -/ weak+ + + - - - -/+ -/ weak+ - Prec. T-LBL/ALL weak+ +/- -/weak+ weak+ - - + - - Myeloid sarcoma weak+/ negative - - - + -/+ + - - Monoblastic sarcoma + - - - - + + - - Mantle cell lymphoma, blastic + - + - - - + + ++ Abbreviations: LBL/ALL – lymphoblastic lymphoma/leukemia.