Using the Lymphoma & Leukemia Sections
The Use of Synonyms
In the Second Edition of ICD-O, cases could be coded using terms from any of the current classifications, as well as a number of archaic terms. This made comparison of datasets very difficult, especially where terms from multiple classifications were used in the same dataset. ICD-O-3 incorporates terms from the WHO systems as preferred terms for hematological malignancies, but terms from older systems are retained to permit universal coding and analysis of historical data. In some cases a synonym may not be an exact equivalent of the preferred (WHO) term, but in the judgment of experts in this field the majority of cases would lie within the category concerned.
Compatibility with ICD-10
In order to ensure compatibility with ICD-10, there are a number of ways in which the Third Edition of ICD-O difers from the structure of the WHO classification of hematological malignancies. Separate codes have been allocated to B-cell chronic lymphocytic leukemia and B-cell small lymphocytic lymphoma. These are now recognized to be exactly the same entity, and for presentation of data these categories may therefore be combined. The same argument applies to lymphoblastic lymphoma and acute lymphoblastic leukemia, which are now regarded as the same disease but for which separate codes are provided.
The use of cell marker studies has transformed hematopathology and is a major element in achieving a high standard of diagnostic accuracy. In the WHO classification, the lineage of the tumor is almost always implicit in the diagnostic term used. For example, a follicular lymphoma is by definition a B-cell malignancy. The only instance where this does not apply is lymphoblastic leukemia and lymphoblastic lymphoma, for which the lineage (T-cell or B-cell) must be specified. This was not the case in the Second Edition of ICD-O, where many of the terms were ambiguous with respect to cell lineage. In the Third Edition, the cell lineage is implicit in the four-digit morphology code, and an additional (6th) digit is not required. However, registries may wish to retain the additional digit to identify cases in which the diagnosis is supported by immunophenotypic data.
Cytogenetics and molecular biological data are now of key - and increasing - importance in the diagnosis of many types of hematologic malignancies. In ICD-O-3, an important change has been the introduction of subcategories of acute myeloid leukemia described according to cytogenetic abnormalities. Where these abnormalities are included in a laboratory report, they take precedence in classification over other data such as the FAB morphology type.