Definition:

Acute lymphocytic leukemia (ALL) is a clonal stem cell malignancy of excessive lymphoblast proliferation. It is now understood that ALL and lymphoblastic lymphoma are the same disease entities at the morphologic and immunophenotypic levels and classified as either B- and T-cell lymphoblastic leukemia/lymphoma (B-ALL and T-ALL). In the case of B-ALL, numerous reports have demonstrated that recurring genetic abnormalities are associated with sufficiently unique clinical, immunophenotypic, and/or prognostic features so that they can be considered as distinct entities. The most common rearrangements observed in B-ALL are the t(12;21) (p13;q22) rearrangement resulting in expression of the ETV6-RUNX1 fusion (TEL-AML1); the t(1;19) (q23;p13) translocation that results in expression of the TCF3 (E2A) fusion partner, (also known as TCF3) TFPT-PBX1 fusion (E2A-PBX); the t(9;22) (q34;q11.2) "Philadelphia" chromosome resulting in expression of the BCR-ABL1 fusion; and rearrangements of MLL (also known as KMT2A) at 11q23 to a diverse array of fusion partners. If none of these specific genetic abnormalities are found, the designation of "B lymphoblastic leukemia/lymphoma, not otherwise specified," is appropriate.