New WHO classification of malignant hematological diseases

May 8, 2002 (abbreviated version)

A new classification system has been worked out for malignant hematological conditions by the WHO. The part which concerns the myeloid malignant diseases only recently started to appear in the literature. The majority of the textbooks available discusses only the old FAB classification system. The differences of the old and new classification systems are summarized below.

Contents

• General remarks
• Classification of myeloid malignancies
• Chronic myeloproliferative diseases (MPD)
• Myelodysplastic/myeloproliferative diseases
• Myelodysplastic syndromes (MDS)
• Acute myeloid leukemias (AML)
• Classification of lymphoid malignancies

General remarks

The new WHO classification will be published in book form in the series "International Histological Classification of Tumors" titled "WHO classification of neoplastic diseases of hematopoietic and lymphoid tissues".(It was quoted as "in press" in 1999, but it has now come out yet, as far as I know.) The classification is discussed in a 1999 issue of the Journal of Clinical Oncology (Vol 17, No 12, December, pp 3835-3849). The online abstract is available. A paper copy can be borrowed at the 17th floor in the Institute of Pathophysiology. This paper is mainly useful for teachers, but students may borrow it as well, if they feel they need it.

Classification of myeloid malignancies

The WHO classification system puts a few diseases that show characteristics of both myeloproliferative and myelodysplastic conditions into a new, separate group (myeloproliferative/myelodysplastic diseases).

Chronic myeloproliferative diseases (MPD)

Malignant stem cell disorder of clonal origin with a chronic course. It is characterized by splenomegaly and a high cell count in one or more cell lines. The marrow is hypercellular with signs of differentiation, there is no dysplasia.

CML

The most important change is, that only the Ph+ cases are called CML by the WHO. The Ph- cases (which show myelodysplastic signs, and are known to have significantly worse prognosis) are called aCML (atypical CML), and belong to the newly created myelodysplastic/myeloproliferative group. The aCML term is somewhat misleading, because it is not CML at all, but it was kept, having no better alternative.

Myelodysplastic/myeloproliferative diseases

CMML belonged to the MDS in the FAB classification. About one half of the cases show proliferative, the other dysplastic signs, but it looks like these are just different forms of the same disease.

Myelodysplastic syndromes (MDS)

Major changes: the RAEB-T of the FAB is now considered to be acute leukemia, not an MDS type.

CMML of the FAB was put into the new MPD/MDS group by the WHO.

A new group was formed from those cases previously classified as RA or RARS where the dysplasia affected more than one cell line, because the prognosis has been found worse for these cases (refractory cytopenia with multilineage dysplasia).

A new subgroup is 5q- syndrome (the loss of the long arm of chromosome 5).

Acute myeloid leukemias (AML)

The changes are easily summarized: the leukemias with consistent cytogenetic abnormalitites and those that are MDS related were taken into separate groups, the rest of the old FAB classification was put under the "AML not otherwise categorized" entry.

"REAL" classification of lymphoid malignancies