«Chapter 7. Classification and coding of neoplasms C. S. Muirl and C. Percy2 International Agency for Research on Cancer, 150 cours Albert-Thomas, ...»
Chapter 7. Classification and coding of neoplasms
C. S. Muirl and C. Percy2
International Agency for Research on Cancer,
150 cours Albert-Thomas, 69372 Lyon Cidex 08, France
2National Cancer Institute, National Institutes of Health,
Bethesda, MD 20892-4200, USA
Classification of neoplasms involves their arrangement or distribution in classes
according to a method or system. Neoplasms can be classified in many ways but, for
clinician and cancer registry alike, the two most important items of information are the location of the tumour in the body (synonyms: anatomical location; site;
topography) and the morphology, i.e., the appearance of the tumour when examined under the microscope (synonyms: histology, cytology), as this indicates its behaviour (malignant, benign, in situ, and uncertain). Cancer registries endeavour, as a minimum, to classify each neoplasm according to its topography, morphology and behaviour, as well as recording particulars of the host.
Sound classification requires an agreed nomenclature-a series of names or designations forming a set or system-so that, for example, all histopathologists agree to give a particular microscopic appearance the same name.
The custodians of a classification have a three-fold task: first, to ensure that the classification adapts to accommodate changes in concepts and user needs, otherwise the classification will fall into disuse; second, to ensure that such changes as are made avoid the inclusion of terms and concepts that are ephemeral, and third, to ensure that changes are made in such a way as to permit continuity of time series.
For convenience, most classifications assign numerical codes to their constituent entities so that a frequently complex series of pieces of information can be conveyed, stored and retrieved in the form of numbers. With the continual advances in electronic and computer techniques, it is possible today to eliminate manual coding and enter the descriptors directly, letting the computer assign code numbers, but this added convenience does not influence the basic concepts of disease classification.
At first glance the classification and coding systems currently used seem illogical and needlessly complex. This is due, in part, to the fact that cancer is but one of many diseases and is thus assigned a niche in the larger classification systems which have developed over time. The principal manual for classifying diseases is the International Classification of Diseases (ICD) published by the World Health Organization, the ninth revision of which (WHO, 1977) is in current use. It will be Classification and coding of neoplasms 65 described in detail below, but first it is useful to have some knowledge of the evolution of the classifications used, as this helps to explain their current format and structure.
Historical review of topographical and morphological classifications of neoplasms (1948-1 985) An excellent history of disease classification prior to 1948 is given in the introduction of ICD-7 (WHO, 1957). After the United Nations was established following the second world war, WHO was created as a specialized United Nations agency dealing with health, and took over the responsibility for the International Lists of Causes of Death. In 1948, WHO published the sixth revision of ICD (WHO, 1948) and the classification has been revised usually every 10 years thereafter (see Figure I).
Chapter I1 of the ICD, dealing with neoplasms, is primarily a topographic classification arranged according to the anatomical site of the tumour, except for a few histological types such as melanomas, lymphomas and leukaemias. Basically the structure of the neoplasms chapter has not changed for the past 40 years. Neoplasms were allotted 100 consecutive three-digit code numbers running from 140 to 239.
These numbers are also commonly called categories or rubrics. From ICD-6 onwards most organs (or categories) have also been subdivided with a fourth digit giving greater anatomical detail, e.g., in ICD-7, 141.0 was assigned to malignant neoplasms of the base of the tongue. Organs were arranged according to organ systems, for example ICD-7 rubrics 150-159 covered the malignant neoplasms of digestive organs and peritoneum. Neoplasms with a given behaviour were grouped into blocks designated malignant, benign, and of unspecified nature; beginning with ICD-9, blocks were also allotted to in situ neoplasms and to neoplasms of uncertain behaviour. The structure of ICD-9 is illustrated by the example in Table 1.
In the 1940s, the first cancer registries had already recognized the need for distinguishing between histologically different tumours of the same organ (Clemmesen, 1965). A histological classification of tumours was not furnished in ICD-6, which, for example, provided no way to distirguish between a squamous cell Table 1. Structure of chapter 11, neoplasms, of the International Classification of Diseases, Ninth Revision (ICD-9) categories 140-239
carcinoma of the lung and an adenocarcinoma of the lung; both were classified as malignant neoplasm of lung (ICD-6 162) (and still are in ICD-9). Therefore, in 1951, the American Cancer Society (1951) developed and published its first Manual of Tumor Nomenclature and Coding (MOTNAC). This had a three-digit morphology code, of which the first two digits gave histological type and the third the behaviour of Classification and coding of neoplasms 67 the tumour. Cancer registries at that time usually used the malignant neoplasm section of ICD-6 for coding topography and MOTNAC for morphology. This principle was later adopted by WHO when in 1956 it published a Statistical Code for Human Tumours (WHO, 1956), which consisted of a topography code based on the malignant neoplasms chapter of ICD-7 (WHO, 1957) and the morphology, including behaviour code, of MOTNAC (see Figure 1).
The College of American Pathologists (1965) published the Systematized Nomenclature of Pathology (SNOP). This included a two-digit (and a highly detailed four-digit) topography code to cover all anatomy (not just cancer sites) and a morphology code, of which sections 8 and 9 were assigned to neoplasms. In addition there were four-digit codes for the fields of etiology and function. It was agreed that the American Cancer Society could use sections 8 and 9 from SNOP for the morphology section of a revised MOTNAC, which appeared in 1968 (Percy et al., 1968). The revised MOTNAC had no relation to the original 1951 edition. Instead the topography section was based on the topographic structure of the malignant neoplasm section of ICD-8 (WHO, 1967) (see Figure I), while the four-digit morphology code provided (behaviour being the fourth digit) was taken from SNOP.
When the ninth revision of ICD was being developed, WHO asked the International Agency for Research on Cancer (IARC) to make recommendations concerning the content and structure of the neoplasms chapter (Chaper 2) in consultation with the Cancer and ICD units of WHO in Geneva. In the course of this work, the worldwide need for a logical, coherent and detailed classification for neoplasms was recognized. Thus, a working party was formed that developed the International Classification of Diseases for Oncology (ICD-0) (WHO, 1976b), which categorized a tumour by the three axes of topography, morphology and behaviour.
The topography section was based on the malignant neoplasms chapter of ICD-9, the morphology field on MOTNAC (Percy et al., 1968), which was expanded by one digit (from three to four), and finally a behaviour code following a slash or solidus (I).In addition, a grading code (degree of differentiation) was provided as the sixth digit of morphology.
At the same time, the College of American Pathologists (1977) revised SNOP as the Systematized Nomenclature of Medicine (SNOMED). SNOMED incorporated the ICD-0 morphology section for its morphology sections 8 and 9-Neoplasms. The SNOMED topography section on the other hand, as in SNOP, has no relation to ICDor ICD-0 topography, since it covers all anatomical structures and not just the sites where tumours occur.
Classifcation and coding A cancer registry is faced with a number of problems when deciding on the classification to be used for the coding of tumours. These include the degree of detail desirable, internal comparability of long time series (a particular problem for existing registries) and international comparability between registries.
The underlying principle of coding is to bring together in classes cases of cancer which have common characteristics. While classification by etiology, prognosis and response to treatment would be highly desirable, such information is frequently C.S. Muir and C. Percy obtained some time after diagnosis. Based on current knowledge tumours are still best delineated on the three axes of site of tumour, histopathological appearance and behaviour. The cancer registry should therefore code its tumours by an internationally accepted system, using all three axes, which easily allows the classification of tumours in more or less broad categories.
ICD-9 fulfils many of the requirements, but lacks the logic, flexibility and histological detail of ICD-0, which is recommended for use in cancer registration.
SNOMED shares many of the advantages of ICD-0, but lacks the international recognition attached to the ICD classification system. Although revision of SNOMED is planned by its publisher, the College of American Pathologists, only ICD and ICD-0 will therefore be described in detail in the following pages.
International classification of diseases, 1975 revision (ICD-9) (WHO, 1977) The ICD-9 manual is published as two volumes: Volume 1 gives a numerical listing; Volume 2 an alphabetical index. The manual is designed for the coding and classification of both mortality (death certificates) and morbidity (hospital and other medical diagnoses). A United Nations treaty engages 44 nations to code and report mortality from their countries using the current ICD, but the treaty does not include cancer registry data. Several rules for the coding of morbidity are included in the back of Volume 1, in addition to those dealing with the choice of underlying cause of death.
In ICD-9, the neoplasms chapter comprises the categories (rubrics) running from 140 to 239 inclusive. These rubrics are further divided as follows into six groups according to the behaviour of the neoplasms.
5. 235-238 Neoplasms of uncertain behaviour 6. 239 Neoplasms of unspecified nature The greatest anatomical detail is provided for the malignant neoplasms. Most three-digit rubrics are further subdivided by means of a fourth digit.
Although in essence topographical in axis, ICD-9 includes several morphological categories, sometimes mixed with topography, e.g., the distinction of malignant melanoma of skin (ICD-9 172) from the other forms of skin cancer (ICD-9 173). For several rubrics the axis is a tissue, no matter where located, e.g., connective and soft Classijication and coding o neoplasms f tissue, or lymphatic and haematopoietic tissue. The complete list of malignant
neoplasms of such "morphological" rubrics is as follows:
Malignant neoplasm of connective and other soft tissue: ICD-9 171 Melanoma of skin : ICD-9 172 Malignant neoplasm of placenta (choriocarcinoma) : ICD-9 181 Hodgkin's disease: ICD-9 201 Non-Hodgkin lymphoma: ICD-9 200, 202 Multiple myeloma : ICD-9 203 Leukaemias : ICD-9 204-208 While the benign neoplasms (ICD-9 210-229) are also classified for the most part on grounds of anatomical location, several of the rubrics are morphological or relate
to a connective or other soft tissue:
Lipoma: ICD-9 214 Other benign neoplasm of connective and other soft tissue: ICD-9 215 Uterine leiomyoma : ICD-9 218 Haemangioma and lymphangioma, any site: ICD-9 228 The diagnosis of carcinoma in situ (ICD-9 230-234) can only be made microscopically, as the critical feature is the lack of invasion of the malignant cells through the basement membrane of the epithelial tissue involved. Such neoplasms are classified topographically.
The neoplasms of uncertain behaviour (ICD-9 235-238) are those with a well defined histological appearance, but whose subsequent behaviour is difficult to forecast, e.g., granulosa cell tumours of the ovary (ICD-9 236.2).
The index of ICD-9 also contains all the morphological (histological) codes of the morphology (M) field of ICD-0 (see below).
International Classification of Diseases for Oncology (ICD-0), first edition (WHO, 1976b) ICD-0 is an extension or supplement of the neoplasms chapter, i.e., Chapter 11, of
ICD-9. It permits the coding of all neoplasms by:
(a) topography (T) (four digits), (b) histology (morphology) (M) (five digits) including be haviour (one digit following a I) i.e., malignant, benign, in situ, uncertain whether malignant or benign;
and (c) one digit for grading (grades I-IV) or differentiation (well differentiated to anaplastic).
A tumour is thus completely characterized by a ten-digit code, e.g., a well differentiated adenocarcinoma of the lung is coded as T-162.9 M-8 140131 (lung 162.9, adenocarcinoma 8140, malignant behaviour 13, well differentiated 1).
C.S. Muir and C. Percy Topography All topographic categories have the same code number within the range 140 (Lip) through 199 (Unknown site) as ICD-9 except for the categories 155.2: Liver, not specified whether primary or secondary, 172: Malignant melanoma of skin, and
197.J : Secondary malignant neoplasms of respiratory and digestive systems, and 198.-: Secondary malignant neoplasms of other specified sites. These categories were not used since they could be handled in ICD-0 by using the behaviour codes /6 (metastases), or 19 (uncertain whether primary or metastatic site), or by using the site category 173 for skin in conjunction with the morphology code numbers 872013which denote one of the forms of malignant melanoma. (It will be recalled that, in ICD-9, rubric 173denotes 'Other malignant neoplasms of skin', i.e., those that are not malignant melanomas).
ICD-0 contains a code number, 169, which does not appear in ICD-9. This provides a topographic point of reference for malignant neoplasms of the reticuloendothelialand haematopoietic systems, i.e., those neoplasms which would be coded to ICD-9 rubrics 200-208.
ICD-0 169.- Haematopoietic and reticuloendothelial system