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. The revised fourth edition of the World Health Organization (WHO) classification (2017) has introduced some changes in both B and T-ALL. Here, we summarize the categories of lymphoblastic leukemia/lymphomas as defined by the WHO and recent developments in the understanding of this group of hematologic malignancy.

RECENT FINDINGS

Two provisional categories of B-ALL have now been identified including B-ALL, BCR-ABL1-like, and B-ALL with iAMP21. The Philadelphia chromosome-like B-ALL includes forms of the disease that shares the expression profiling of B-ALL with t(9;22) but lack such rearrangement. The second one shows amplification of part of the chromosome 21. Both entities are associated with worse prognosis. Within the T-ALL group, an early precursor T cell form has now been introduced as a provisional category. Such group demonstrates expression of stem cell and myeloid markers in conjunction with the T cell antigens. The current review summarizes the recent updates to the WHO classification.