ICD-O

The International Classification of Diseases for Oncology (ICD-O) is derived from the International Classification of Diseases for oncological diagnoses. The third revision of the ICD-O is currently in use. Tumors are coded in the following axes:
 * topography
 * morphology
 * biological behaviour
 * differentiation

The classification is expressed by a combination of a three- to four-digit alphanumeric code of topography (e.g. C01.1) b a five-digit code XXXX/Y''', where the first four digits indicate the morphological type of the tumor and the digit after the slash biological behavior. The next (optional) number (sometimes listed second after the slash in data collections) indicates the degree of differentiation of the tumor. Morphological codes ( XXXX ) are always four-digit numbers from the interval 8000 to 9989.'''

The biological behavior code ( Y ) can take on the following values:
 * 0 – benign tumor
 * 1 – uncertain behavior, it is not certain whether it is a benign or malignant tumor
 * 2 – tumor in situ, non-invasive
 * 3 – malignant tumor, primary
 * 6 – malignant tumor, metastatic (not used in oncology registries)
 * 9 – malignant tumor, origin cannot be determined (not used in oncology registries)

The differentiation code has the meaning of histological grading and differentiation:
 * 1 – well differentiated
 * 2 – moderately well differentiated
 * 3 – poorly differentiated
 * 4 – undifferentiated, anaplastic
 * 9 – differentiation not determined, not stated or cannot be used

In the case of lymphomas and leukemias, instead of the differentiation code, the gross immunophenotype is given in the National Cancer Registry:


 * 5 – immunoprofile corresponds to T lymphocytes
 * 6 – immunoprofile corresponds to B lymphocytes
 * 7 – immunoprofile of null cells (non-B non-T cells)
 * 8 – immunoprofile corresponds to NK cells
 * 9 – cell type unspecified, not specified or unusable

The classification is derived from the ICD-10 classification. Therefore, the coding of topography/localization for malignant neoplasms usually corresponds to the ICD-10 code. An exception is, for example, codes that already take into account the morphological type of neoplasm in ICD-10. For example, the code C43.5 Malignant melanoma of trunk is coded in ICD-O as C44.3 (Skin of trunk) 8720/3 (Malignant melanoma NS).

Example of interpretation of coding C24.1 M-8160/32:
 * C24 – (malignant) neoplasm of other and unspecified parts of the biliary tract
 * C24.1 – Vater papilla tumor
 * C24.1 8160 – cholangiocarcinoma of Vater's papilla
 * C24.1 8160/3 – (malignant) cholangiocarcinoma of Vater's papilla
 * C24.1 8160/32 – moderately well-differentiated cholangiocarcinoma of Vater's papilla

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