Updates in the Pathology of Precursor Lymphoid Neoplasms in the Revised Fourth Edition of the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissues.

Purpose Of Review: Acute lymphoblastic leukemias (ALL) are malignant disorders of immature B or T cells that occur characteristically in children, usually under the age of 6 (75%). Approximately 6000 new cases of ALL are diagnosed each year in the USA, 80-85% of which represent B-ALL forms. Most presentations of B-ALL are leukemic, whereas T-ALL presents with a mediastinal mass, with or without leukemic involvement.