The Morphology Code Matrix Concept

Rule F. Use the appropriate 5th digit behavior code even if the exact term is not listed in ICD-O.

Refer to the matrix in Table 20 for the underlying structure and concept of the morphology codes for terms in ICD-O. In the first example (A) five terms appear with their morphology codes. Each of these five terms has the same four-digit morphology code, M-8140, indicating a neoplasm of glandular origin. "Adenoma, NOS" is a benign tumor and has the behavior code /0. "Adenocarcinoma, NOS" is the malignant equivalent of "adenoma, NOS" and has the behavior code /3. "Adenocarcinoma in situ" has the appropriate behavior code /2. "Bronchial adenoma" was originally described as a benign tumor but was later discovered to be malignant or potentially malignant. "Bronchial adenoma, NOS" has therefore been assigned the behavior code /1 to indicate that it is uncertain whether a particular bronchial adenoma will behave in a benign or malignant manner. "Metastatic adenocarcinoma, NOS" has the code M-8140/6. The code M-8140/9 is also part of the matrix even though it is not printed in the numerical list or alphabetic index of ICD-O. If a diagnosis of "adenocarcinoma of lung, uncertain whether primary or metastatic site" was reported in a clinical or pathology records, it could be coded to M-8140/9. It would not be used by cancer registrars who, as previously explained, normally only include /2 (in situ) and /3 (malignant neoplasm, primary site) in their registries.

In the second example (B), three terms are listed under the four-digit morphology code number M-9000. "Brenner tumor, NOS" is usually benign, so it is assigned the code M-9000/0. If a diagnosis of "malignant Brenner tumor" were reported, however, its correct code would be M-9000/3; similarly a diagnosis of "Brenner tumor, borderline malignancy" would be correctly coded M-9000/1. The codes M-9000/2, M-9000/6, and M-9000/9 have not been listed in ICD-O. They are available for use when appropriate; for example, M-9000/2 would be used for "Brenner tumor in situ" if such an entity were to be identified.

In the third example (C) only one term, "chordoma," is listed. "Chordoma" is usually considered to be a malignant neoplasm and is therefore assigned the morphology code M-9370/3. Other codes in the 9370 matrix also exist and could be used when appropriate, for example M-9370/0 for "benign chordoma," even though this term is not actually listed in ICD-O. It should be noted that some of the possible combinations probably do not exist or have not been recognized and defined; a "benign sarcoma" would contradict current concepts and usage.

Usually a histologic term carries a clear indication of the likely behavior of the tumor, whether malignant or benign, and this is reflected in the behavior code assigned to it in the ICD-O tabular list. Only a few histologic types of in situ neoplasms are actually listed in ICD-O. The behavior code /2 could be attached to any of the four-digit codes in ICD-O if an in situ form of the neoplasm is diagnosed.

It should be emphasized here that the matrix system was designed to give the pathologist the final say on whether a tumor is considered to be benign, malignant, in situ, or uncertain whether malignant or benign.

The behavior code assigned here is what most pathologists believe is the usual behavior. If the pathologist disagrees on the ICD-O code assignment or disagrees in a particular case, he or she can change the behavior code. For example, Paget disease of the nipple (breast) is a malignant disease in ICD-O. Recently some pathologists have felt, in the absence of a demonstrable tumor, it should be considered "in situ." In this event they should describe the tumor as "in situ" and code it accordingly.

Remember that ICD-O is a topography and morphology coding system (in other words, a coded nomenclature), not a system for coding stage or extent of disease. ICD-O has no relationship to the TNM classifications of the International Union Against Cancer (UICC) or the American Joint Committee on Cancer (AJCC). Coding is based on what the pathologist states. However, if the behavior is unclear or not stated, code the behavior as assigned in ICD-O.