Classification and Coding: Some Thoughts Concerning Bone Tumors

Path. Res. Pract. 166, 25-30 (1979) Orthopedic Clinic and Policlmic, University of Heidelberg (Chairman: Prof. Dr. H. Cotta ), FRG

Classification and Coding: Some Thoughts Concerning Bone Tumors! W. BECKER

Summary Any classification is a means of dealing with a group of interrelated entitles by subdividing them into subgroups of comparible or identical entities. A phylogenetic approach to classification has been adapted for tumors in general, arranging the different tumors according to their assumed tissue of origin. Classification by organs is another means of subdividing, and logically can not lead to the same results. The "organ" bone contains a number of cell types not unique m bone, and a classification of bone tumors based upon tissue of origin will necessarily overlap with other classifications dealing, for instance, with tumors of soft tissues. In addition features concerned with rhe quality of the rumor .ICe sought to be incorporared withm the classifrcanon . Whereas "tissue of ongin" pertams to the "birth" of a given lesion, "behavior" (benignity, malignancy, semimalignancy and others) pertains to the "natural history", which is quite another means of classification with no logical impact upon the classification based on tissue of origin. Since all classifications serve the purpose of definition and communication, it appears necessary to analyze the different goals being sought by defining a given classification. Once this is understood, apparent contradictions can be solved easily. Coding, on the other hand, primarily serves the purpose of developing a machine language where computerized studies are performed. Coding systems, therefore, should be oriented to the needs of research in a specific field. Codes are better understood if they correlate with certain forms of classification, but they are developed not as a substitute for classification but merely as a " language". Once designed unambiguously, they can be translated mto any other code language. Whereas new results in research or philosophy will demand rearrangements of classification, codes are neither true, false, nor indeterminate; they are simply labels for given entities.

Classifying and coding are quite different approaches, although both methods divide things or thoughts. All classifications serve the purpose of understanding what surrounds us, and communicate with a language arising from these considerations. Currently the most widely accepted classification of bone tumors, developed within the last hundred years and documented both in the WHO classification (Schajowicz et aI., 1972) and the fascicle of the ARMED FORCES INSTITUTE OF PATHOLOGY (Spjut et aI., 1971), is based on the idea that most information can be drawn from division by the tissue of origin, or at least what is thought to be the tissue of origin. New I

Dedicated to Prof. Dr. Dr. h.c. E. Uehlinger on occasion of his 80th birthday.

26 . W. Becker

names have been coined using this classification, though not in all cases (Becker, 1970). This system is inadequate in the case of quite a few tumors, merely because of lack of knowledge as to what tissue the tumor originates from. Such circumstances, however, should not-be an obstacle for the system. A black hole in a system can always be filled by new experience or simply left as it is as long as one appreciates the lack of knowledge at this particular point. Moreover, it should not be a problem that some of the names given for bone tumors do not describe our existing knowledge about the tissue of origin. Cases in point are the giant cell tumor, based on the description of a histologic pattern, and the aneurysmal bone cyst, based on the roentgenologic appearence (in most cases). Classification based on the - presumed - tissue of origin, however, is generally accepted as a useful one. The classification of bone tumors is even further complicated by the fact that there are a number of lesions which are not tumors at all but which appear and to a certain degree behave like bone tumors. Since classification does not involve a purely scientific subdivision of entities, and since these tumorlike lesions are important in differential diagnosis and therapy for the physician dealing with bone tumors, they are generally included within the label "bone tumors." Needless to say, these entities do not readily fit into a system where the tissue of origin of a tumor regulates the classification. Natural history, however, is of equal interest to the physician. Classification according to the tissue of origin now seems logical, practical, and in most cases feasable, and allows a precise discription and thus a means of communication between people dealing with these tumors. Natural history, and especially the question of benign or malignant behavior, is of main interest for those concerned with therapy. It is conventional to define tumors as either benign or malignant, based mainly on histologic evidence. Part of the definition, however, is the capability of the given tumor to produce metastases. This ability of the malignant tumor ranks highest as far as therapeutic consequences are concerned, though this property of the tumor is based not so much on histologic evidence as on clinical experience. Moreover, the actual outcome cannot be determined from histologic appearence; only statistical conclusions are possible, which do not apply to the individual. Therefore we have to decide our therapeutic action according to the accumulated statistical evidence of the possible outcome of the given disease. We have to relate our means of therapy to the risk of suffering or death of the patient. Since we know that the different tumors have very different natural histories, we can adjust our regimen to the least dangerous but still sufficiently safe procedure. There is no doubt that amputation of a benign tumor with a very small risk of malignant degeneration will save the patient's life. There is also no doubt that

Bone Tumors ' 27

hemipelvectomy with positive malignant tumor margins will not do so. For practical purposes, we need an ever increasing knowledge about the natural history of a given tumor and its response to the various modes of therapy. Divisions according to behavior must be viewed upon as superimposed on the first one (based on tissue of origin), and it is to be expected that this "second level" classification will not always match with the "first level" classification. A benign tumor can undergo malignant transformation as can any other healthy tissue. Some do this so rarely that they can be neglected; some transform with a substantial rate of risk. How can the probability of malignant degeneration be expressed within the classification of bone tumors? After demonstration of malignancy within a formally benign lesion (if that is really proven) the labelling is simple, and the term "secondary malignancy" has been coined. But how about prospective risks? The terms potential malignancy and semimalignancy have been used with different connotation. The term potential malignancy designates a tumor which is benign by histologic evidence but which is known to have a high risk of malignant degeneration. How high should that risk be to justify the designation? Giant cell tumor has been so designated, although it has become apparent that almost all proven cases had been treated by X-ray. This would make the physician semimalignant and not the tumor. On the other hand, the term has been used seldom or never for Ollier's disease, which has this risk in and of itself. Malignant transformation has been observed in some chondromyxoid fibromas, but one very seldom finds this tumor under this heading. It is necessary to gather much more information on natural histories of well documented cases in order to be able to calculate the actual risk - also with respect to a given therapy - and to render this information as additional qualification of the tumor in question. The term semimalignancy has been used for some bone tumors but with two separate connotations. First it allows for special tumors, such as giant cell tumor and chondroma, to avoid a clearcut designation concerning dignity. It often proves extremely difficult to judge dignity, especially in these tumors on purely histologic grounds. Other factors like localization and age of the patient must be considered. Very often in addition the surgeon expects a recommendation as what to do with a given tumor. As specialized bone tumor centers develop, the surgeon or orthopedic surgeon will- hopefully - arrive at a better understanding of the implications of a given diagnosis. The signal for the surgeon to deal properly with such a dangerous, but not necessarily malignant, tumor would then not longer have to be expressed by the designation "semimalignant". Recently Uehlinger (1976) has defined semimalignancy (in bone tumors) as the the property of local aggressivness without the ability to metastasize. This

28 . W. Becker

concept, therefore, also includes an entity such as pigmented vil1onodular synovitis, since this disease is highly destructive locally and can be cured only by comparatively aggressive methods. We should therefor be aware, that this term "semimalignancy" relates to the clinic, not the histology. Knowing that the histologically defined terms benign and malignant are qualifications superimposed upon a classification by tissue of origin, and that malignancy in addition includes a prospective quality, we will be less reluctant to accept further destinctions made in order to increase the information on the specific case. While classification serves as a means of communication for conveying information, coding is a measure taken to translate this information into machine language. The use of computers requires a highly explicit language and cannot deal with ambiguities. In most cases it is wise to use a numerical language, and it is probably also wise to use a logical language. It is possible to enumerate diseases at random, but it is advantageous for the user if the system has a logical basis. Once translated into one numerical language, a given entity may be translated into any other language, as long as it contains the proper word. In coding the main interest is who wants the information and for what purpose. Since coding is part of the preparation for a machine language, this procedure is clearly appropriate if a large group of entities is going to be studied, justifying the use of a computer. Are bone tumors such a large group? Compared with other tumors certainly not, but the diagnosis and treatment of bone tumors has long stimulated pathologists, radiologists, and surgeons who work as a team, because everyone in this therapeutic and diagnostic group depends largely on the cooperation of the other, probably more so than in most other fields of oncology. Bone tumor registers developed early, and the analysis of collected cases is the basis for the present valid classification. Many coding systems now exist and serve different purposes. On the international scale, the ICDO (International Classification of Diseaes - Oncology) is a coding system for all tumors (jacob et aI., 1978). As far as bone tumors are concerned, we must concede that by lumping together bone as an organ we have chosen an illogical system if tissue of origin is the basis for classification. Consequently ICDO accepts as bone tumors only such which develop from bone and cartilage. Bone, however, also contains marrow, blood vessels and nerves. Sarcomas arising in these structures but within bone behave quite differently from those in soft tissues and therefore are usually included under the heading of "bone tumors". In addition tumorlike lesions usually are included in bone tumors. For a group working constantly with bone tumors, such as the "Arbeitsgemeinschaft Knochentumoren" (Bone Tumor Study Group), which was founded and presided over for

Bone Tumors' 29

the first 5 years by Prof. Uehlinger, it seems logical to ask for a coding system which easily comprises all known and possible tumor entities involving bone, including tumorlike lesions and if possible the usual diagnostic problem cases (a sort of extension of tumorlike lesions: "tumor-simulating", as for example myositis ossificans). We have prepared a key for bone tumors and tumorlike lesions, which in the first position relates to the tissue of origin, in the second to the behavior of the tumor, i, e. benign, malignant, undetermined, tumorlike, etc., and in the third position to individual tumor entities (Becker, 1975). By including positions for "unknown" and "other", the system is open to new developments as they arise from further investigation. This coding system is easily convertible into any other system. Differring international coding systems might seem deplorable, the reason, however, lies in the different goals of various coding systems. For instance a key covering all tumors will necessarily skip some of the points which researchers in bone tumors have an interested in. The same holds true in a key designed to cover all sorts of so called "rheumatic diseases" (Mathies et al., 1978). Since those include bone diseases, bone tumors will be found there too. The main question is not to design a coding system for specific diseases, but to design it in such a way that it serves certain defined purposes of a given group of people. Coding therefore is not a substitute for classification but a labelling instrument for a specific purpose. Bone tumors are very rare diseases, in addition diversified into numerous categories and individual forms each demanding a different approach. The main problem in handling bone tumors, therefor, is limited experience. The team approach, involving cooperation between the orthopedic surgeon, radiologist, and pathologist, has long been recommended, and most centers have realized that only an established oncologic unit can achieve careful and intelligent management of bone tumors. The "Interdiszipliniire Arbeitsgemeinschaft Knochentumoren" (Interdisciplinary Bone Tumor Study Group) has therefore initiated such oncologic units for bone tumors in many places in West Germany, Switzerland, and Austria, and has emphasized the need for comparing documented experience using computer analysis. The .Jnterdisziplinare Arbeitsgemeinschaft Knochentumoren" was also involved in the planning and organization of the "Arbeitsgemeinschaft Knochentumoren" (Bone Tumor Study Group) in which pathologists and clinicians working with bone tumors act cooperatively to solve difficult diagnostic problems, develop scientific programs, and compile data for epidemiologic studies. The above-mentioned key has been prepared for the needs of this group, and it is essential to keep in mind that coding is merely an instrument for a specific purpose - here to define clearly every possible diagnosis which might be of interest for this group of investigators. Differences in codes result from the different goals which various groups may seek. Thus if codes are designed

30 . W. Becker

for all tumors or more generally for all diseases, they necessarily will differ from that for bone tumors and tumorlike 'lesions within bone. Classification means to order a group of entities by various definitions of qualitative differences (for instance tissue of origin, behavior, roentgenological appearance, localization, etc.), Coding, on the other hand, is a tool for accurate designation of such diseases to the extent required for a specific group of investigators for their individual goals.

References Becker, W.: Zum Nomenklaturproblem bei den Knochentumoren. Z. Orthop. 108, 476-490 (1970) Becker, W.: Knochentumorschliissel. Arbeitsgemeinschaft Knochentumoren, Deutsches Krebsforschungszentrum Heidelberg (1975) Jacob, W., Scheida, D., Wingert, F.: Tumorhistologieschliissel. Springer, Berlin-Heidelberg-New York (1978) Mathies, H., Otte, P., Villiaumey, J., Dixon, A. St.: Klassifikation der Erkrankungen des Bewegungsapparates Eular, Basel (1978) Schajowicz, F., Ackerman, 1. V., Sissons, K. A., Sobin, 1. H., and Torloni, H.: International Histologic Classification of Tumours Nr. 6. Histological Typing of Bone Tumours. World Health Organization, Geneva (1972) Spjut, H. J., Dorfman, H. D., Fechner, R. E. and Ackerman, 1. V.: Atlas of Tumor Pathology, 2nd series fascicle 5: Tumors of Bone and Cartilage. American Forces Insitute of Pathology, Washington, D. C. (1971) Uehlinger, E.: Primary Malignancy, Secondary Malignancy and Semimalignancyof Bone Tumors, in: Grundmann, E.: Malignant Bone Tumors, pp. 109-119. Springer, Berlin-Heidelberg-New York (1976)

Received February 7, 1979 . Accepted February 9, 1979

Key words: Bone tumors - Classification - Coding - Dignity Prof. Dr. med. Wolfgang Becker, Orthopadische Klinik und Pohklmik der Universitat, Schlierbacher Landstr. 200a, D-6900 Heidelberg-Schlierbach, FRG