About 85% of ALLs are B cell tumors and typically manifest in children less than 15 years of age. ALLs are the most common cancer of children.
Children typically are affected by B cell ALLs with bone marrow involvement. In these cancers, the bone marrow is typically hypercellular and packed with lymphoblasts, which replace the normal elements within the bone marrow. Therefore, common symptoms include fatigue due to anemia and bleeding due to thrombocytopenia. Neoplastic infiltration of bone marrow can also cause bone pain from marrow expansion and infiltration of the subperiosteum.
The less common T-cell ALLs tend to present in adolescents as a thymic lymphoma in the mediastinum, and can lead to a large, widening mediastinal mass.
Immunostaining for terminal deoxynucleotidyl transferase (TdT) a specialized DNA polymerase that is expressed only in pre B and pre T lymphoblasts is positive in more than 95% of cases.
Most B-cell acute lymphoblastic leukemias express the common acute lymphoblastic leukemia antigen (CALLA) which is an integral membrane protein.
A fraction of B cell tumors contain a translocation of genes 12;21 involving the genes TEL and AML1, two genes required in very early hematopoiesis. This translocation is associated with better prognosis.
ALLs are known to spread to the CNS and testes. CNS manifestations include headache, vomiting, and nerve palsies and involvement of the testes can cause testicular enlargement.
These tumors are highly responsive to chemotherapy. With aggressive chemotherapy, about 95% of children with ALL obtain complete remission.
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