Code for Histologic Grading and Differentiation

Rule G. Assign the highest grade or differentiation code described in the diagnostic statement.

ICD-O includes, as the 6th digit of the morphology code, a single-digit code number designating the grade or differentiation of malignant neoplasms as listed in Figure 21. Only malignant tumors are graded.

The practice of grading varies greatly among pathologists throughout the world, and many malignant tumors are not routinely graded. In the grading code listed in Figure 21, the code numbers 1 to 4 are used to designate grades I to IV respectively. Words used to designate degrees of differentiation are listed in a separate column.

Figure 21. 6th Digit Code for Histologic Grading & Differentiation
Code Grade Differentiation
1 Grade I Well differentiated
Differentiated, NOS
2 Grade II Moderately differentiated
Moderately well differentiated
Intermediate differentiation
3 Grade III Poorly differentiated
4 Grade IV Undifferentiated
9   Grade or differentiation not determined, not stated or not applicable

Differentiation describes how much or how little a tumor resembles the normal tissue from which it arose. There is great variability in the use of descriptors by pathologists. In general, the adverbs "well," "moderately," and "poorly" are used to indicate degrees of differentiation, which approximate to grades I, II, and III. "undifferentiated" and "anaplastic" usually correspond to grade IV. Thus the diagnoses "squamous cell carcinoma, grade II" and "moderately well differentiated squamous cell carcinoma" would both be coded to the morphology code M-8070/32. When a diagnosis indicates two different degrees of grading or differentiation, the higher number should be used as the grading code. Thus "moderately differentiated squamous cell carcinoma with poorly differentiated areas" should be given the grading code "3." The complete code would therefore be M-8070/33.

The grading codes can be applied to all the malignant neoplasms listed in ICD-O if the diagnosis includes information about grade or differentiation. For example, complete coding of the diagnosis "anaplastic squamous cell carcinoma" requires addition of the grading code "4" to the morphology code M-8070/3, as M-8070/34. It would be incorrect to code this diagnosis to the morphology code M8070/39, which does not indicate grade.

It should be noted that words such as "anaplastic," "well differentiated," and "undifferentiated" are used as integral parts of approximately 15 histologic terms for neoplasms (in addition to those used to describe lymphomas). Examples are: "malignant [glossary term:] teratoma, anaplastic" (M-9082/34), "retinoblastoma, differentiated" (M-9511/31), and "follicular adenocarcimoma, well differentiated (M-8331/31). Coders should use the appropriate morphology code together with the proper grading code, as indicated in the examples.

This same 6th digit column may also be used to denote cell lineage for leukemias and lymphomas (Figure 22). This may be useful when comparing data coded according to the Third Edition of ICD-O with data coded according to the Second Edition. As noted in the section on lymphomas, in the Third Edition, the cell lineage is implicit in the four-digit histology code, and an additional grade or differentiation (6th digit) code is not required. However, some registries may wish to retain the additional digit to identify cases in which the diagnosis is supported by immunophenotypic data. In such instances, the immunophenotype code has precedence over other diagnostic terms for grade or differentiation, such as "well differentiated" or "grade III."

Figure 22. 6th Digit Code for Immunophenotype Designation for Lymphomas & Leukemias
Code Designation
5 T-cell
6 B-cell
7 Null cell
Non T-non B
8 NK cell
Natural killer cell
9 Cell type not determined,
not stated or not applicable