Although the purpose of the new edition was to include terms from new classification systems, it was also necessary that older terminology be retained for reference by pathology departments and cancer registries where data has been collected over several decades and changes in coding manuals. As a result, it has not always been possible to assign codes to new entities in the same sequence in which they appear in their original classification. For example, the term 'follicular lymphoma, grade 2' is actually a new synonym for the existing code 9691/3, 'malignant lymphoma, follicle center cell, mixed small cleaved and large cell.' Grade 1 follicular lymphoma is a new synonym for 9695/3, 'malignant lymphoma, follicle center cell, small cleaved.' It appears that the terms with grade descriptors are out of sequence in the numerical list, but in fact, these terms are now the preferred names for previously identified lymphomas. Additionally, in certain sections of the numerical list, there was no room to add new codes in sequence and codes had to be inserted where numbers were available. Thus despite its name, the International Classification, the third edition of ICD-O should be considered a coded nomenclature rather than a true classification of disease entities.

In many instances, there is more specific designation of the preferred behavior of an NOS term (Not Otherwise Specified or nonspecific) when the word benign, borderline, or malignant is not part of the diagnostic phrase. For example, if the diagnostic statement says simply 'teratoma,' this is to be coded as 9080 with a behavior code of /1. If the diagnostic statement were to say 'malignant teratoma,' the code would be 9080/3.

To reflect contemporary pathology practice and terminology, a number of revisions and enhancements have been made to ICD-O-3. More acronyms have been included as synonyms in the numeric list and in the alphabetic index, such as DCIS (ductal carcinoma in situ) and PNET (primitive neuroectodermal tumor). Table 4 lists some of the more lengthy or obscure acronyms and their complete definitions.

In addition to the very specific codes for newly-identified disease entities, several helpful codes have been added for non-specific diagnoses, such as 8046/3, non-small cell carcinoma and 9861/3, acute non-lymphocytic leukemia (ANLL). Although these are not "good" diagnoses according to the pathologists, registrars see them frequently on cytology reports and death certificates, so these terms were included in ICD-O-3. Also, having a code for these disease entities allows registrars to 'mark' these cases for case follow-back for a more specific diagnosis. For example, following back on a non-small cell carcinoma case coded to 8046/3 may afford the opportunity to code to a more specific non-small cell carcinoma such as adenocarcinoma or large cell carcinoma.

Some terms listed in ICD-O-3 have been marked as obsolete [obs]. This descriptor is intended to discourage the use of such a term for a new diagnosis when better diagnostic terms are available, yet to serve as a reference when such a diagnosis is noted during research using historical data. Some terms are older names for neoplasms that have been more specifically described, for example hepatoma [obs] which is now described as hepatocellular carcinoma with new codes for four subtypes. Others are truly archaic, such as lymphosarcoma (first described in the 1890s, although the term is still used in veterinary medicine). In many cases, obsolete terms which had specific codes in ICD-O-2 have been moved to the "Not Otherwise Specified" category for the disease.

Eponymic terms (diseases named after a person) will be listed in ICD-O-3 in their European format, without the apostrophe s ('s). Both the numeric and alphabetic lists will show Klatskin tumor and Hodgkin lymphoma, for example.