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Structure & Format of ICD-O, Third Edition

ICD-O is a dual classification with coding systems for both topography and morphology of tumors. The topography code describes the site of origin of the neoplasms and uses the same 3-character and 4-character categories as ICD-10 for malignant neoplasms (C00-C80); this allows greater specificity for coding sites of non-malignant neoplasms than is possible in ICD-10. The morphology code describes the cell type of the tumor and its biologic activity, in other words, the characteristics of the tumor itself.

Main Sections of ICD-O, the Third Edition

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 Second and Third Editions

Abbreviations Used in ICD-O

The following abbreviations are used throughout ICD-O, the Third Edition:

American and British Spelling

In order to avoid repetitions caused by differences in spelling, the American spelling of words has been used, for example "leukemia" and "tumor" rather than "leukaemia" and "tumour." These examples do not present a serious problem in alphabetization. However, when the differences in spelling, such as "esophagus" and "oesophagus," result in an appreciable separation of the two forms in the alphabetic index, the reader seeking the British spelling under the letter "O" is referred to the American spelling by the entry, "Oesophagus (see Esophagus)."