The International Classification of Diseases for Oncology (ICD-O) has been used for more than 35 years, principally in tumor or cancer registries, for coding the site (topography) and the histology (morphology) of the neoplasm, usually obtained from a pathology report.
The topography code describes the anatomical site of origin of the neoplasm and, while it uses the same categories as in the neoplasm section of Chapter II of the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10), some of the individual codes are different. The code always has a prefix of “C”, followed by a three-digit number that indicates the site (two digits) and the subsite (one digit), separated by a decimal point. For example, in C18.4, the C18 indicates that the site is the colon and the 4 indicates that the subsite is the transverse colon.
The morphology code describes the characteristics of a tumor itself, including its cell type and biological activity. The code is composed of four digits that indicate the cell type or histology and one digit that indicates the behavior. The first four digits are separated from the last (behavior) digit by a forward slash (/). The behavior digit can be 0 (benign), 1 (uncertain behavior), 2 (carcinoma in situ), 3 (malignant, primary site), 6 (malignant, metastatic site), or 9 (malignant, uncertain whether primary or metastatic site).
The International Classification of Diseases for Oncology, Second Edition was published in 1990, followed by the International Classification of Diseases for Oncology, Third Edition in 2000. The topography section of the third edition remained the same as in the second edition, which was based on the neoplasm section of the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10). However, the morphology section was revised. New classifications, especially for lymphomas and leukemia, were introduced, and new codes assigned to accommodate them. Although one of the prime commitments of the editors was to change as few terms as possible, to add new terms at empty spaces, and not to reuse previously assigned codes, this was not always possible. In order to keep groups of similar entities together, the codes for some terms had to be changed. Furthermore, the sequence or grouping of terms may not always be as logical as possible because of the limitations of available code numbers.
In developing the previous editions and the present edition of ICD-O, a particular effort was made to use the nomenclature appearing in the World Health Organization International Histological Classification of Tumours series (WHO “Blue Books”). This series covers all the principal sites of cancer and includes the morphology codes of ICD-O for each neoplasm.
Since the initial publication of the third edition of ICD-O (ICD-O-3) in 2000, updates to the WHO Blue Book series have continued. During the development of the fourth edition of the Blue Book volumes, chapter authors worked with the International Agency for Research on Cancer/International Classification of Diseases for Oncology (IARC/ICD-O) Committee for ICD-O-3 to review recently identified neoplasm entities and assign morphology codes. This updated version of ICD-O-3 (ICD-O-3 First Revision, or ICD-O-3.1) includes the new terms, codes, synonyms, related terms, morphology, and behavior code changes from the WHO Blue Books published between 2007 and 2010 on tumors of haematopoietic and lymphoid tissues, the central nervous system, and the digestive system.