Coding Rules for Topography & Morphology
Summary of Principal Rules for Using ICD-O, Third Edition
(See Corresponding Numbers in ICD-O-2, Second Edition)
Rule A. Topographic regions and ill-defined sites: If the diagnosis does not specify the tissue of origin, code the appropriate tissues suggested in the alphabetic index for each ill-defined site in preference to the "NOS" category. Ill-defined sites, such as "arm," have several component tissues. For example, "squamous cell carcinoma of the arm" should be coded to C44.6 (skin of arm) rather than to C76.4 (arm, NOS). There are a few exceptions to this, such as chin and forehead, because these regions are predominantly composed of skin, and the NOS category was therefore assigned to skin.
Rule B. Prefixes: If a topographic site is modified by a prefix such as peri-, para-, or the like, and is not specifically listed in ICD-O-3, code to the appropriate ill-defined subcategory C76 (ill-defined site), unless the type of tumor indicates origin from a particular tissue. This general rule also applies to imprecise phrases such as "area of" or "region of."
Rule C. Tumors involving more than one topographic category or subcategory: Use subcategory ".8" when a single tumor overlaps the boundaries of two or more categories or subcategories and its point of origin cannot be determined. Because more categories have been allotted to neoplasms in ICD-10 than in ICD-9, some previous three-digit categories have been replaced by two three-character categories.
Rule D. Topography codes for lymphomas: If the site of origin of the lymphoma is in the lymph nodes, code to C77._. If a lymphoma involves multiple lymph node regions, code to C77.8 (lymph nodes of multiple regions). Code extranodal lymphomas to the site of origin, which may not be the site of the biopsy. If no site is indicated for a lymphoma and it is suspected to be extranodal, code to C80.9 (unknown primary site). Lymphomas occur in specific sites, for example stomach, as well as in one or more lymph nodes and therefore are not assigned a site-specific topography code. Lymphomas occurring in specific sites are called extranodal.
Rule E. Topography code for leukemia: Code all leukemias except myeloid sarcoma (M-9930/3) to C42.1 (bone marrow).
Rule F. Behavior code in morphology: Use the appropriate 5th digit behavior code even if the exact term is not listed in ICD-O. Use of the 5th digit behavior code number that fits the diagnosis, even though the exact term is not listed in ICD-O. For example, "benign chordoma" as a diagnosis should be coded M-9370/0. If the pathologist states that the behavior differs from the usual behavior as given in ICD-O, code as the pathologist indicates.
Rule G. Grading or differentiation code: Assign the highest grade or differentiation code described in the diagnostic statement. If a diagnosis indicates two different degrees of grade or differentiation (such as "well and poorly differentiated" or grades II-III"), code to the higher grade.
The 6th digit may also be used for identifying cell origin for lymphomas and leukemias. In these lymphatic and hematopoietic diseases, T-cell (code 5), B-cell (code 6), Null cell(code 7), and NK cell(code 8) take priority over grade codes 1-4.
Rule H. Site-associated morphology terms: Use the topography code provided when a topographic site is not stated in the diagnosis. This topography code should be disregarded if the tumor is known to arise at another site. The appropriate site-specific codes are listed in parentheses after morphology terms for neoplasms that usually occur in the same site or tissue, for example "retinoblastoma" (C69.2). If no site is indicated in the diagnosis, the site-specific code should be used (in this case, the site should be coded to retina C69.2).
If a site is given that is different from the site indicated by the suggested code, use the site code appropriate to the diagnosis. This should be done only after thoroughly reviewing the case to ascertain that the neoplasm at the site mentioned is not a metastasis.
Only three-character codes are given for some sites, for example C44._ (skin), because the appropriate fourth-digit cannot be assigned in advance. Use the detail provided in the actual diagnosis to assign the sub site.
Certain neoplasms have names that could be interpreted as implying a topographic location (pseudotopographic morphology terms), but these entities should not necessarily be coded to that site. For example, bile duct carcinoma is a tumor frequently arising in intrahepatic bile ducts of liver (C22.1).
Rule I. Intentionally, there is no Rule I in ICD-O-3. There is a Rule 1 in ICD-O-1 and ICD-O-2, and to keep "Rule 1" separate from "Rule I," the editors of ICD-O-3 elected to intentionally not include a Rule I.
Rule J. Compound morphology diagnoses: Change the order of word roots in a compound term if the term is not listed in ICD-O.Not all forms of compound words are listed in ICD-O-3. For example, "myxofibrosarcoma" is not in ICD-O-3 but "fibromyxosarcoma" is. The coder must check various permutations of the prefixes if the first one is not found.
Rule K. Coding multiple morphology terms: When no single code includes all diagnostic terms, use the numerically higher code number if the diagnosis of a single tumor includes two modifying adjectives with different code numbers. If a term has two or more modifying adjectives which have different code numbers, code to the one with the highest code number, as it is usually more specific.