The morphology code records the type of cell that has become neoplastic and its biologic activity; in other words, it records the kind of tumor that has developed and how it behaves. There are three parts to a complete morphology code:

  • 4 digits cell type (histology)
  • 1 digit behavior
  • 1 digit grade, differentiation or phenotype

In ICD-O morphology codes, a common root codes the cell type of a given tumor, while an additional digit codes the behavior. The grade, differentiation, or phenotype code provides supplementary information about the tumor.

Cancer and Carcinoma

The words "cancer" and "carcinoma" are often (incorrectly) used interchangeably, for example "squamous cell cancer" is used for "squamous cell carcinoma." To code the former as the latter would be reasonable. However, "spindle cell cancer" could refer either to "spindle cell sarcoma" or to "spindle cell carcinoma." In ICD-O, the word "cancer" is listed only once, as a synonym of the nonspecific term "malignant neoplasm," M-8000/3. Obviously, ICD-O cannot provide specific code numbers for all the instances in which the word "cancer" is used loosely and imprecisely as a part of a histologic diagnosis.


The behavior of a tumor is the way it acts within the body. Pathologists use a variety of observations to determine the behavior of a tumor. Figure 18 shows the spectrum of behaviors. A tumor can grow in place without the potential for spread (/0, benign); it can be malignant but still growing in place (/2, noninvasive or in situ); it can invade surrounding tissues (/3, malignant, primary site); or even disseminate from its point of origin and begin to grow at another site (/6, metastatic).

Figure 18. 5th Digit Behavior Code for Neoplasms
Code Neoplasm
/0 Benign
/1 Uncertain whether benign or malignant
Borderline malignancy
Low malignant potential
Uncertain malignant potential
/2 Carcinoma in situ
/3 Malignant, primary site
/6* Malignant, metastatic site
Malignant, secondary site
/9* Malignant, uncertain whether primary or metastatic site

* Not used by cancer registries (used by some pathologists in some parts of the world)

Most cancer registries collect data only on malignant and in situ neoplasms, that is, /3 or /2 of the behavior code. Behavior codes /6, malignant, metastatic site, and /9, malignant, uncertain whether primary or metastatic site, are not generally used by cancer registries. For example, if a person has a carcinoma that has spread to the lung and the site of origin is unknown, the appropriate code is C80.9 (unknown primary site) M-8010/3 (carcinoma). The /3 signifies the existence of a malignant neoplasm of a primary site.

Carcinoma in situ and CIN III

Most cancer registries record carcinoma in situ arising at any site. By far the largest number of in situ carcinomas are diagnosed in the cervix uteri. In recent years, several other closely related terms have been used by cytologists and pathologists, notably intraepithelial neoplasia. The term cervical introepithelial neoplasia, grade III (CIN III), is often applied to the cervix. Unfortunately this description includes both carcinoma in situ and severe dysplasia.

Leading experts in this field in several different countries were consulted, and the majority felt that CIN III could be considered as comparable to carcinoma in situ whether severe dysplasia is mentioned or not. Severe dysplasia of the cervix uteri without mention of CIN III is coded as for all other sites of severe dysplasia according to SNOMED. Similar terms in the vagina (VAIN III), vulva (VIN III), anus (AIN III), and prostate (PIN III) should be treated in the same way.

Pathologists who do not believe that CIN III (unqualified) is equivalent to in situ carcinoma can apply the matrix system and change the behavior code to /1 (uncertain whether malignant or benign).

The "Bethesda" cytology reporting system (23) recognizes only two groups, low grade squamous intraepithelial lesion and high grade squamous intraepithelial lesion; the high grade group includes moderate dysplasia (CIN II), severe dysplasia, and carcinoma in situ (CIN III).

Use of Behavior Code in Pathology Laboratories

While most of the instructions provided in this part of the manual are aimed at coders and tumor or cancer registrars, this section considers the classification needs of pathologists. The primary difference between the two groups lies in the use of the behavior code. Pathologists are usually interested in "specimen coding" whereas the cancer registrar's main interest is identification of the primary tumor. A pathologist may receive several specimens from the same patient, for example (a) a biopsy, (b) the resected primary site, and (c) a metastatic site (Figure 19). The pathologist wants to keep track of all three of these specimens; the cancer registrar is only interested in the primary. Each specimen would be coded with the appropriate topography and morphology but in (b) the behavior would be /6 (metastatic), indicating that the associated topography code is not the site of origin. On the other hand, the cancer registrar would report only (b) -- the primary site and morphology with a behavior code/3.

Figure 19. Examples of Specimen Coding in a Laboratory
Specimen Code
a. Biopsy diagnosis: Supraclavicular lymph node, metastatic signet ring cell adenocarcinoma, most likely from stomach. C77.0 8490/6
b.* Primary site: Fundus of stomach, signet ring cell ademonarcinoma C16.1 8490/3
c. Metastatic site: Upper lobe bronchus, metastatic signet ring cell adenocarcinoma. C34.1 8490/6

* Codes for this case as recorded in registry