Structure and Format of Primary Site in ICD-O

The topography code indicates the site of origin of a neoplasm; the code describes where the tumor arose. The ICD-O topography codes (or rubrics) C00-C80 are based on the malignant neoplasm section of Chapter II of ICD-10. All neoplasms, whether in situ, malignant, benign, or uncertain whether benign or malignant, are coded with the same set of topography codes in ICD-O.

The ICD-O-3 lists topography codes in two sections; the first is a numeric listing by code number, the second is an alphabetic listing by anatomic site. The topography code consists of a lead character (the letter ‘C’) followed by two numeric digits, a decimal point, and one additional numeric digit. Generally, the decimal point is not entered as part of the code in a cancer abstract.

The topographic site of a neoplasm may be described by using the noun or its related adjective, for example "glioma of pons" or "pontine glioma." In general, noun forms appear in the numerical list and alphabetic index of ICD-O; for example, "pons" is listed but "pontine" is not. Only a few of the commonly encountered adjectives, such as "uterine" and "gastric," have been listed in ICD-O for the convenience of coders. When there is doubt, the coder should consult a medical dictionary to determine the correct noun to which the adjective corresponds.

Main Sections of ICD-O

ICD-O consists of five main sections:

  • Instructions for Use
  • Topography-Numerical List
  • Morphology-Numerical List
  • Alphabetic Index
  • Differences in Morphology Codes between Second and Third Editions

Abbreviations Used in ICD-O

The following abbreviations are used throughout ICD-O:

  • M - Morphology
  • NOS - Not Otherwise Specified
  • ICD-O - International Classification of Diseases for Oncology

Meaning of "NOS" & How It Is Used

"NOS" is printed in both the numerical lists and the alphabetic index to indicate that other modifiers of the term appear elsewhere in ICD-O.

Use the code for a term followed by "NOS" when:

  • a topographic or morphologic term is not modified
  • a topographic or morphologic term has an adjective that does not appear elsewhere
  • a term is used in a general sense

American and British Spelling

In order to avoid repetitions caused by differences in spelling, ICD-O uses the American spelling of words.

For example:

American British
leukemia leukaemia
tumor tumour
esophagus oesophagus

Some 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)."

Updated: December 13, 2023