Review: ICD-O Structure & Format

Here is what we have learned from ICD-O Structure & Format:

  • ICD-O is a dual classification with coding systems for both topography and morphology.
  • In ICD-O, the topography code describes the site of origin of the neoplasm, and the morphology code describes the cell type of the tumor and its biologic activity.
  • ICD-O consists of five main sections:
    1. Instructions for Use
    2. Topography-Numerical List
    3. Morphology-Numerical List
    4. Alphabetic Index
    5. Differences in Morphology Codes between ICD-O-3 and ICD-O-2
  • The topography section has been adapted from the malignant neoplasm section of Chapter II of ICD-10.
  • ICD-O topography terms have four-character codes that run from C00.0 to C80.9. A decimal point (.) separates subdivisions of the three-character categories.
  • Each topographic and morphologic term appears only once in the numerical list, and the first listed term, printed in bold type under a particular code, is the preferred term.
  • Morphology terms have five-digit codes ranging from M-8000/0 to M-9989/3 with the first four digits indicating the specific histologic term and the fifth digit used as a behavior code.
  • Following the behavior code, a separate one-digit code is provided for histological grading or differentiation.
  • The alphabetic index is used to code topography and morphology, which also includes selected tumor-like lesions and conditions.
  • The terms are listed under both the noun and the adjective in the alphabetic index.
  • In the alphabetic index, any word that appears as part of three or more terms is in bold type.
  • Lymphomas and leukemias are exceptions to the rule of listing conditions under all parts of the terms; There is only one list for "lymphoma, malignant" and one for "leukemia."
  • ICD-O-3 incorporates terms from the WHO systems as preferred terms for hematologic malignancies, but terms from older systems are retained to permit universal coding and analysis of historical data.
  • 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.
  • The code for a term followed by "NOS" should be used when:
    1. a topographic or morphologic term is not modified;
    2. a topographic or morphologic term has an adjective that does not appear elsewhere; and
    3. a term is used in a general sense.


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