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nal mass, low white blood cell count).327 Pediatric ALL patients with this immunophenotype have good clinical outcomes.283

The Pre-B ALL stage is characterized by the presence of intracytoplasmic |HC in the absence of LC. This immunophenotype is associated with unfavorable features, such as higher leukocyte counts at presentation, higher serum lactic dehydrogenase (LDH) levels, hepatomegaly, and above all the (1;19)(q23;p13) translocation, which results in in the creation of the E2A/PBX1 fusion gene.322,328 Lymphoblasts containing this translocation often lack CD34 expression. Monoclonal antibodies specific to the E2A/PBX-encoded chimeric protein have been developed and used to distinguish t(1;19)-positive from t(1;19)-negative leukemia.329 Pre-B ALL with or without t(1;19) has been associated with poor outcome in some clinical studies but not others and may have benefited from improved therapies 320,322,328,330

Transitional Pre-B ALL has been identified in a small subset of children or adults with ALL.320,322 This recently established subtype of B-lineage ALL demonstrates both intracytoplasmic and surface |HC but not LCs. It represents an intermediate stage between Pre-B and mature B-ALL. Its distinction from mature B ALL is important, since transitional Pre-B ALL carries an excellent prognosis.320,322

CD10-negative and CD10-positive Early Pre-B, Pre-B and transitional Pre-B ALL all express TdT. Immunofluorescent TdT determination on cytospin preparations is an easy and reliable tool for monitoring central nervous system (CNS) leukemia in TdT+ ALL subtypes.331-333

Mature B-ALL is uncommon. It shares its morphology (FAB L3), immunophenotype, clinical presentation (bulky extramedullary disease, lymphadenopathy, CNS involvement), and cytogenetic abnormalities with Burkitt's lymphoma. In the prototype (8;14)(q24;q32) translocation, the c-myc protoonco-gene is translocated from chromosome 8 onto chromosome 14 and brought under the transcriptional control of the IgHC locus. The variant translocations, t(2;8)(p12;q24) and t(8;22)(q24;q11), result in the positioning of portions of the K

Table 15-4. Subclassification of T-Lineage ALL

Stage Tia Stage Tib Stage Tn -Pre-T-

Pre-T Early T -Early Thymocyte—

-Immature T-

Cortical T

Common Thymocyte

Stage Tiii T

Mature T Late T

MedullaryThymocyte

Pre-T Early T -Early Thymocyte—

-Immature T-

Cortical T

Common Thymocyte

TdT

TdT

TdT

(TdT)

cCD3

cCD3

cCD3

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