The radiological features of osteosarcoma of the appendicular skeleton in dogs: a review of 74 cases

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1 J. smallanim. Pract. (984) 5, The radiological features of osteosarcoma of the appendicular skeleton in dogs: a review of 74 cases CHRISTINE GIBBS,* H. R. DENNY* AND D. F. KELLYtS Departments of Veterinary Surgery* and Pathologyt,'University of Bristol, School of Veterinary Science, Langford House, Langford, Bristol BS 8 7DU ABSTRACT The clinical features, sites of origin and radiological changes associated with 75 osteosarcomas of the appendicular skeleton in 74 dogs are reviewed. Clinical data and the areas affected were broadly similar to those in previously reported series of cases, but the precise sites of origin were found to vary considerably from the generally accepted predeliction site, which is the metaphysis. A wide spectrum of radiological changes was recorded. Only 6. per cent of tumours could be classified as characteristic. A further. per cent showed features suggestive of, but not specific for, osteosarcoma and were therefore classified as equivocal. In the remaining 7. per cent the radiological signs, although indicative of neoplasia and usually of malignancy, were quite uncharacteristic. Among 6 cases in which both radiographic and post-mortem examination of the lungs was performed the incidence of pulmonary metastases was per cent, with total correlation between their radiological and pathological detection. Of ten dogs which were treated surgically, one with a radiographically atypical lesion has survived for more than four years. INTRODUCTION The characteristic radiographic changes associated with osteosarcoma are well documented in the veterinary literature (Schnelle, 97; Douglas & Williamson. 970; Morgan, 97; Gillette et al., 977; Webbon & Clayton-Jones, 978; Kealy, 979). They include poorly marginated destruction of medullary bone, cortical erosion or destruction, sclerosis, periosteal new bone formation and the $ Present Address: Faculty of Veterinary Science, University of Liverpool. P.O. Box 47, Liverpool L69 BX /84/ % BSAVA 77

2 78 CHRISTINE GIBBS, H. R. DENNY AND D. F. KELLY production of spicular or disorganized tumour bone. These changes are usually accompanied by soft tissue swelling. In long bones, by far the most common site of origin is reported to be the metaphysis. Joints and adjacent bones are rarely invaded. Morgan (97) and Kealy (979) classify osteosarcomas into osteoclastic (lytic), osteoblastic (sclerotic) and mixed types, according to the relative predominance of destructive and proliferative changes. Metastasis to the lungs is widely considered to be common. Brodey, Sauer & Medway (96) quote a rate of approximately 50 per cent, but in later surveys (Ling et al., 974; Liu et a!., 977) the reported incidence is considerably lower. The development of osteosarcomas at sites of previous fracture has been reported and recorded cases were reviewed by Stevenson et al. (98). A radiographically and pathologically distinct form, the parosteal osteosarcoma, has also been recognized in the dog (Banks, 97 ). The problems of radiological differentiation of osteosarcoma from other types of malignant bone tumour are well recognized, to the extent that in a detailed report of cases of primary bone neoplasia, the radiographic features of osteosarcoma, fibrosarcoma and chondrosarcoma are described collectively (Ling et al., 974). The importance of distinguishing between malignant bone tumours and osteomyelitis has also continually been emphasized (Kirk, 945; Suter, 96; Ling et al., 974) and the differential radiographic features of the two conditions are summarized in standard texts (Douglas & Williamson, 970; Morgan, 97). Several large series of cases of osteosarcoma affecting the canine appendicular skeleton have been reported (Brodey et al., 96; Wolke & Nielsen, 966; Ling et al., 974; Liu el al., 977). Breed, age, sex and site incidences are meticulously recorded, but reference to radiographic changes is principally confined to descriptions of characteristic features with little mention of the variability of radiological signs or the frequency with which they are atypical. Furthermore, it is only recently that attention has been drawn to the importance of recognizing the radiographically atypical lytic form of human osteosarcoma (de Santos & Edeiken, 98). The purpose of this paper is to report the spectrum of radiographic changes recorded in a consecutive series of histologically confirmed osteosarcomas affecting the appendicular skeleton of 74 dogs referred to the University of Bristol Veterinary School between 97 and 98. In a later communication, the findings of the present study will be compared with those from a series of non-osteogenic malignant bone tumours recorded in this species during the same period. MATERIALS AND METHODS In 6 of the 74 cases under review, histological confirmation of osteosarcoma was obtained at necropsy. Otherwise, material became available as a result of amputation of the affected limb (six cases), excision of the portion of bone containing the lesion (four cases) or biopsy (one case). From the case records, breed (Table l), age (Fig. ) and sex incidences were

3 ~~ ~ RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOGS 79 TABLE. Breed incidence of 74 dogs with osteosarcoma of the appendicular skeleton Breed Alsatian Labrador Retriever Irish Wolfhound Great Dane Greyhound Golden Retriever Springer Spaniel Crossbred (<0 kg) Afghan Hound Boxer Irish Setter Mastiff No. Dogs Breed Rhodesian Ridgeback Crossbred (>0 kg) Airedale Borzoi Curley Coated Retriever Dobermann Flatcoat Retriever Standard Poodle Weimaraner Dachshund Cairn Terrier Yorkshire Terrier No. Dogs I Age lyears) a FIG.. Age incidence of osteosarcoma of the appendicular skeleton in 74 dogs. recorded together with duration of signs (Fig. ) and the results of clinical examination. For cases which had received treatment, survival times were also noted. Radiographs of the affected limb and chest were available in all cases. From the former, the bone and site of origin of the lesion was recorded as shown in Fig., the latter being defined as the area showing most severe disruption of bone structure.

4 80 CHRISTINE GIBBS, H. R. DENNY AND D. F. KELLY c Weeks Months Duration FIG.. Duration of signs in 7 dogs presented with osteosarcoma of the appendicular skeleton. TABLE. Radiological features of 75 osteosarcomas of the appendicular skeleton Scapula Humerus Radius Ulna Pelvis Femur Tibia Hock Radiological sign/s N=4 N=9 N=l N= N- N=0 N= N=l Bone destruction Medullary only Medullary + cortical Destruction + expansion Pathological fracture Sclerosis Margination Non-marginated Poorly marginated Distinctly marginated New bone formation Periosteal reaction ( brush or smooth Periosteal elevation (Codman s triangle) Spicular ( sunburst ) Disorganized Type equivocal Absent Spread to adjacent bone Subchondrial bone destruction Trans-articular spread Gross soft tissue swelling Mild to moderate soft tissue swelling No soft tissue swelling I I 6 4 I I

5 ~ ~~ ~~ RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOCS 8 The radiographic features were evaluated according to the criteria listed in Table, which include the extent and nature of bone destruction and new bone formation, the presence or absence of sclerosis, the degree of soft tissue swelling and evidence of involvement of adjacent bones or joints. From these criteria, each tumour was classified (Table ) as either characteristic, equivocal or atypical on the following basis: lesions were considered to be characteristic provided that non- or poorly-marginated medullary bone destruction was accompanied by erosion or destruction of the cortex and clearly abnormal (spicular or disorganized) new bone formation. Equivocal tumours include those in which destruction was confined to medullary bone (i.e. early lesions) or in which poorly-marginated medullary destruction associated with cortical erosion was accompanied by new bone formation which could not convincingly be classified as neoplastic. Atypical lesions showed features which were clearly uncharacteristic of osteosarcoma-i.e. (a) distinctly marginated bone destruction with no new bone formation, (b) bone destruction with cortical expansion and new bone formation either characteristic of TABLE. Classification of 75 osteosarcomas of the appendicular skeleton according to site and radiological signs Radiological signs Site Characteristic Equivocal Atypical Total Predeliction sites: Proximal Humerus I I t 6 Distal Radius 7 Distal Femur lo$ 6 Proximal Tibia 6-8 Distal Tibia - 4 Sub-total 7 56 Other sites: Scapular Blade - - Scapular Neck - ~ Humeral Shaft I - - Distal Humerus - - Proximal Ulna - - Distal Ulna I - Ilium Proximal Femur - Femoral Shaft I* - - Tibia Shaft I* - - Fibular Tarsal - - I Sub-total Total * Originated at site of previous fracture. t Including one originating in amputation stump. Including two parosteal osteosarcomas.

6 8 CHRISTINE GIBBS, H. R. DENNY AND D. F. KELLY periosteal proliferation or equivocal in type, (c) subchrondral bone destruction, and (d) trans-articular spread. Thoracic radiographs were examined and the incidence of pulmonary metastases recorded. For the 6 cases examined post mortem, the results were compared with the necropsy reports. HUMERUSIN = 9) Central metaphysis Cranial I N C a u d metaphysis a t I RADiUSlN = ) SCAPULA IN = 4 ULNAIN=) Cranial metaphyseal diaphysis - I metaphysis 5 h\ Central metaphyseal diaphysis 4 II Metaphyseai diaphysis ) Metaphysis 7 + I originating in amputation stump FEMUR IN = 0 Metaphysis () d Caicaneus I PELVIS IN = ) Mid-shaft lold fracture] I ) diaphysis I Metaphyseal diaphysis 8) lntercondyiar groove I Caudal distal diaphysis lparosteall ) Lateral condyle I Shaft (old - fracture) ) Metaphysis 4 FIG.. Sites of origin of 75 osteosarcomas of the canine appendicular skeleton. RESULTS Tumours were solitary, except in one dog in which lesions were found in two sites. The breed incidence is shown in Table. Giant breeds (0 per cent) are over-represented and small dogs (less than 0 kg adult weight) (8 per cent) are under-represented. There were 9 males and 5 females. The age incidence (Fig. ) shows a peak between six and nine years, but 8 per cent of the dogs were less than three years of age. The duration of signs before presentation was recorded in 7 cases and ranged between three days and more than one year (Fig. ). On clinical examination, lameness of the affected limb was the most consistent presenting sign and was recorded in all cases except one, in which the ulna was involved. Pain could usually be elicited by palpation of the affected area but was absent in eight dogs ( per cent). Although soft tissue swelling occurred relatively frequently, in 7 dogs ( per cent), none could be detected. Three dogs with massive proximal humeral tumours had radial paralysis.

7 RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOGS 8 The sites of origin of the 75 tumours, as determined radiographically, are shown in Fig.. In descending order of frequency, the bones involved were: femur (0 cases, 6.6 per cent), humerus (9 cases, 5. per cent), tibia ( cases, 7. per cent), radius ( cases, 6 per cent), scapula (four cases, 5. per cent), ulna and ilium (each three cases, 4 per cent) and fibular tarsal (one case, a per cent). In the dog with two lesions, the distal femur and ilium of the left limb were involved. The five well-recognized predeliction sites, namely proximal humerus ( 6 cases), distal femur (6 cases), distal radius ( cases), proximal tibia (eight cases) and distal tibia (four cases) accounted for 75 per cent of the tumours recorded. One had developed in the proximal humeral remnant 7 years after forelimb amputation. The remaining tumours affecting these four long bones originated in the proximal femur (four cases), the distal humerus (two cases) and the humeral, femoral and tibial mid-shafts (one case each). The last two occurred at the sites of previous fractures, sustained 5 years and months respectively before presentation. Among the five predeliction areas, the apparent site of tumour origin was variable (Fig. ). Although the metaphysis was the commonest location (5 cases, Fig. 4a), lesions originated fairly frequently in the region of the diaphysis-adjacent to, but distinctly beyond, the area of the epiphyseal scar which marks the position of the metaphysis (6 cases, Fig. 4b). The term metaphyseal diaphysis has been assigned to this region. The area which was originally the epiphysis was also involved in a significant number of cases (nine; Fig. 4c) although in the proximal humerus and distal tibia this region was spared. Three humeral tumours, although affecting the proximal end of the bone, quite clearly originated in the diaphysis well below the metaphyseal region (Fig. 4d). The two parosteal osteosarcomas had a common site of origin on the caudal aspect of the distal femoral diaphysis. Sites of origin of proximal humeral metaphyseal tumours could be distinctly separated into cranial ( case), central ( case) and caudal (5 cases). The two lesions recorded in the distal femoral epiphysis also originated from separate sites, one being the lateral condyle and the other the intercondylar groove. Of the proximal femoral tumours two involved the neck and one the upper diaphysis. One ulnar tumour originated in the olecranon and the remaining two in the distal diaphysis. The site of origin of the tumour arising in a humeral amputation stump could not be accurately determined. The radiological features of the 75 tumours recorded are summarized in Table and Table shows the categories into which they were divided on the basis of site of origin and radiological signs. Characteristic features were shown by 46 tumours (6 - per cent). Two-thirds of these occurred in the five predeliction sites which were also the sites of origin of three-quarters of the 6 equivocal lesions. By contrast, almost half the tumours showing atypical radiological features are among the 5 per cent of lesions which arose in less commonly affected sites. The radiological features classified as characteristic of osteosarcoma are illustrated in Fig. 4 and Fig. 5 shows the appearance of a parosteal osteosarcoma. Equivocal lesions fell into three distinct radiological categories and consisted of

8 84 CHRISTINE GIBBS. H. R. DENNY AND D. F. KELLY

9 RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOGS 85 (a) early lesions which showed only medullary destruction and new bone formation which was typically periosteal in appearance (nine cases: Fig. 6a); (b) more advanced lesions in which medullary and cortical destruction were accompanied by new bone which could not convincingly be classified as neoplastic but appeared rather more aggressive than would normally be expected from a benign periosteal reaction (five cases: Fig. 6b) and (c) two lesions in which bone destruction was uncharacteristic in that it was either expansile or had invaded articular cartilage, although associated with a small amount of disorganized new bone formation (Fig. 6c). The thirteen atypical lesions also fell into distinct radiological categories: seven were expansile, of which five showed no evidence of bone proliferation (Fig. 7a); two were entirely intramedullary, distinctly circumscribed and lytic, one of which was associated with pathological fracture (Fig. 7b); four were predominantly destructive and had invaded articular cartilage with little production of new bone, one of these had extended into adjacent bones (Fig. 7c); the remaining tumour originated at the site of a week-old fracture of the lateral humeral condyle (presumed to be pathological); it contained tiny areas of destruction which could not readily be distinguished from resorption following trauma. FIG. 4. Characteristic radiological features of osteosarcoma. (a) Boxer, 6 years; duration of signs 6 weeks; site of original caudal proximal humeral metaphysis. A poorly marginated area of bone destruction originates in the caudal segment of the proximal humeral metaphysis and spreads into the humeral head and greater tuberosity. The overlying cortex has been destroyed and irregular masses of amorphous new bone extend into the soft tissue beyond its margin. (b) Greyhound, 6 years; duration of signs 4 weeks; site of origin cranial proximal humeral metaphyseal diaphysis. An extensive area of non-marginated bone destruction originates in the cranial aspect of the bone just distal to the metaphysis. Within the lucent area is a small patch of sclerosis. A more diffuse area of sclerosis occupies the caudal medullary cavity of the proximal diaphysis. The cranial cortex is thin and irregular new bone with a somewhat spicular configuration extends into a soft tissue mass beyond it. (c) Standard Poodle, years; duration of signs months; site of origin distal radial epiphysis. An ill-defined area of destruction within the extreme distal portion of the radius is partly masked by patches of sclerosis and overlying paracortical new bone. The medial distal cortex has been destroyed and replaced by solid. highly proliferative new bone, the peripheral portion of which is horizontally laminated. More proximally, amorphous masses of new bone can be seen. Despite extensive involvement of the epiphysis, the subchondral bone remains intact. There is no evidence of involvement of the distal ulna. (d) Rhodesian Ridgeback, 8 years; duration of signs months; site of origin proximal humeral diaphysis. An enormous soft tissue mass surrounds the proximal humerus, the diaphysis of which shows extensive medullary destruction and sclerosis. The cranial proximal cortex has been completely destroyed and amorphous new bone lies in the soft tissue mass adjacent to it. The caudal cortex is intact but irregular and there is a brush-like segment of periosteal proliferation adjacent to it at the junction of the upper and middle thirds of the diaphysis. Amorphous new bone can also be seen caudally within the soft tissue mass.

10 86 CHRISTINE GIBES, H. R. DENNY AND D. F. KELLY FIG. 5. Parosteal osteosarcoma. Great Dane, 6 years; duration of signs 4 weeks. A large, fairly well marginated mass of amorphous new bone which shows a somewhat radiating pattern lies adjacent to the caudal distal femoral cortex. A small area of medullary trabecular destruction can be seen beneath the cortex just distal to the bony mass. Sclerosis was recorded in over half the tumours but was seen most commonly in the group classified as characteristic. Although the frequency of this radiological feature among the equivocal and atypical groups was significantly lower, its presence or absence did not contribute significantly to classification. Review of the breed distribution of the dogs with atypical lesions revealed that all five crossbreds and the Dachshund fell into this category, the remainder being large or giant breeds. No direct relationship could be established between the duration of clinical signs and the degree of advancement of radiological changes. However, both dogs with a short history of less than one week had pathological fractures. Of the six dogs subjected to amputation of the affected limb, two with early tumours of the proximal tibia of equivocal radiological type were treated

11 RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOGS 87 (b) ( c) FIG. 6. Osteosarcomas showing equivocal radiographic features. (A) Alsatian, 8 years; duration of signs months. There is a localized ill-defined area of medullary destruction and sclerosis on the medial aspect of the distal radial diaphysis. The cortex is intact and there is no evidence of new bone formation. In radiological terms, this is an early case. (b) Greyhound, 7 years; duration of signs 4 weeks. An indistinct, patchy area of medullary bone destruction occupies the proximal third of the diaphysis. Although the caudal humeral cortex appears to be intact. a layer of homogenous periosteal new bone lies outside it. (c) Crossbred. years; duration of signs 5f months. The caudal border of the distal scapular blade has been destroyed and the cranial border is irregular. Linear aggregations of amorphous new bone lie in a soft tissue mass cranial to it. Extension of the lesion into the shoulder joint is demonstrated by absence of the caudal third of the glenoid subchondral bone.

12 88 CHRISTINE GIBBS, H. R. DENNY AND D. F. KELLY subsequently with Bacillus Calmette Guerin (B.C.G. Glaxo Laboratories Ltd) according to the protocol described by Owen & Bostock (974); both were destroyed within six months as a result of metastasis to other parts of the skeleton. A further two cases, one with an equivocal proximal tibia lesion and one with a parosteal tumour, are lost to follow-up. One dog, in which an advanced distal radial

13 RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOGS 89 tumour was characteristic of osteosarcoma, died of an unrelated cause without signs of metastases after -6 months. The remaining dog, which developed a typical turnour at the site of a traumatic tibia fracture, is well six months after amputation. Four tumours which occurred at surgically accessible sites and were either equivocal or atypical in radiological character were excised. Three, originating respectively in the distal ulna, olecranon and proximal femur recurred within six months. The remaining dog, a small crossbred, survived for at least 4 years following removal of the fibular tarsal (Fig. 8). This case was exceptional because in addition to being atypical of breed, site and radiographic signs, the tumour had been present for over a year at the time of presentation. Pulmonary metastases were recorded from thoracic radiographs of eight of the 6 dogs submitted for necropsy, when their presence was confirmed in each case. No macroscopic evidence of pulmonary metastasis was detected post mortem among the 56 cases in which thoracic radiographs were negative. None of the ten dogs which were treated showed radiographic evidence of metastasis and no other thoracic abnormalities were recorded in any case. DISCUSSION In this series, the breed distribution is broadly similar to that reported by other workers (Brodey et al., 96; Ling et a[., 974) in that large breeds were more commonly affected. The almost equal sex incidence is slightly at variance with the slight male predisposition recorded by the above authors and Liu el al. ( 977). The age range is similar to that in the series of Brodey, Sauer & Medway (96) in which approximately I0 per cent of affected dogs were under years of age. The distribution of turnours among the five predeliction sites differed from that in FIG. 7. Radiographically atypical osteosarcomas. (a) Irish Setter, 7 years: duration of signs ; months. The distal femoral diaphysis and metaphysis are occupied by a wellcircumscribed expansile destructive lesion with a somewhat ill-defined. partly sclerotic proximal margin. The caudal cortex is thin but intact and adjacent to it the pattern of bone destruction appears to be cystic. The cranial cortex is irregular. There is no evidence of new bone formation at any site. This dog had a similar lesion in the ipsilateral ilium. (b) Labrador, 0 years; duration of signs 4 days. In the proximal humeral diaphysis there is a transverse pathological fracture which bisects a clearly defined area of medullary bone destruction with a distinct but non-sclerotic margin. The anterior cortex is thin proximally and has been destroyed distally. (c) Afghan cross, 8 years; duration of signs 5 months. The lateral radiograph shows two distinctly marginated, vertically orientated elliptical areas of medullary bone destruction in the distal radius. Adjacent to the more caudal one, the cortex has been destroyed. There are also several indistinctly marginated areas of destruction in the distal ulna. There is a large soft tissue swelling cranial to the joint; from the cortex beneath a peg-like projection of new bone extends into the mass and a similar projection can be seen arising from the radial carpal. On the antero-posterior view, in addition to the radial and ulnar changes. there are two ill-defined areas of destruction in the radial carpal.

14 90 CHRISTINE GIBBS, H. R. DENNY AND D. F. KELLY FIG. 8. Osteosarcoma from a long-term survivor. Terrier cross, 7 years; duration of signs more than a year; survival time 4 years plus. A destructive expansile lesion involves the proximal three-quarters of the fibular tarsal. The cortex of the tuber calcis is thin and irregular, but there is no new bone formation. Patches of destruction interspersed with sclerosis give the medullary bone a bubbly appearance. There is virtually no associated soft tissue swelling. former series (Brodey et al., 96; Wolke & Nielsen, 966) in that the highest incidences were recorded in the proximal humerus and distal femur, as opposed to the distal radius and tibia. This variation may be explained by the fact that lesions in the proximal limb are less easily recognized and are therefore, more likely to be encountered in a referral clinic. This suggestion is supported by the fact that a number of proximally located tumours had not been suspected prior to presentation, particularly those in which pain or soft tissue swelling was absent or could not easily be appreciated on clinical examination. The lack of association between duration of signs and degree of advancement of radiographic changes illustrates the wide range of variation in the rate of growth of osteosarcomas, which in man has been shown to be directly related to the histological grading of malignancy (Ross, 964). In both man and animals, the site of origin of osteosarcomas arising in the extremities of long bones is generally considered to be metaphyseal (Brodey et al..

15 RADIOLOGICAL FEATURES OF OSTEOSARCOMA IN DOGS 9 96; Ross, 964; Morgan, 97; Greenfield, 975; Murray & Jacobson, 977; Webbon & Clayton-Jones, 978; Kealy, 979). However, in this series, careful radiological evaluation of each lesion revealed that the sites of origin were more variable, suggesting that the true metaphysis (i.e. the region of the diaphysis immediately adjacent to the epiphyseal scar) may be a less common site of origin than hitherto recognized. This observation may have some relevance with regard to ]concepts of pathogenesis (Owen, 969). Among the proximal humeral tumours in this series, the high frequency of origin at the caudal aspect is in agreement with the findings of Morgan (97) and Ling et al. (974). The occurrence of three tumours at sites of previous major trauma is in keeping with the low but consistent incidence of fracture-associated sarcomas among the series reviewed by Stevenson el al. (98). The apparent reliability of radiography for demonstration of pulmonary metastases as suggested by the absolute correlation between their radiographic and pathological detection must be evaluated in the context of routine post-mortem procedures at this school. Lungs are sliced and examined visually and by palpation, but in the absence of evidence of gross metastatic deposits, a search for microscopic lesions is not normally made. Nevertheless, the effectiveness of radiography for the detection of macroscopically demonstrable metastases is clearly shown. However, in view of the relatively low incidence (.5 per cent of 6 cases), which is similar to that recorded by Ling et al. (974) and Liu el al. (977). radiographic demonstration of their presence or absence makes little contribution to the differential diagnosis of the primary lesion. All previous radiographic descriptions of osteosarcomas of the long bones in dogs have been concerned predominantly with their characteristic features which are now well recognized. However, the frequency with which radiographically atypical lesions encountered in this clinic have returned a histological diagnosis of osteosarcoma suggests that the spectrum of radiographic appearances of this tumour is wider than hitherto recognized. Only 6 per cent of the tumours in this series could be classified radiographically as characteristic of osteosarcoma. This figure corresponds almost exactly with that given for a group of human osteosarcomas reviewed by Lindbom et al. (96). Nevertheless, most of the equivocal lesions showed features sufficiently suggestive of the earlier radiographic signs (Ross, 964) for a clinical diagnosis to be based on the prediction of subsequent development of characteristic changes. Two equivocal and all 4 atypical lesions showed distinctly uncharacteristic features. While all these cases showed radiographic features strongly suggestive of neoplasia and usually indicative of a hopeless prognosis for the affected limb, a histological diagnosis of osteosarcoma could not have been reliably anticipated. Nine of the atypical tumours showed radiographic signs comparable with those illustrated in a recent account of purely lytic osteosarcoma in man (de Santos & Edeiken, 98); the incidence of per cent is also closely similar.

16 9 CHRISTINE GIBBS, H. R. DENNY AND D. F. KELLY It is of interest to note that the frequency of atypical lesions was relatively higher in less common sites of origin and among dogs of small and medium size. In the one long-term survivor, the lesion was slow growing and expansile, which suggests that a long history and radiographically benign appearance may be more significant with regard to the prediction of malignancy of a tumour than its histological characteristics. It should also be recorded that during the period under review, no case of osteomyelitis was erroneously diagnosed as neoplastic on the basis of radiographic signs nor was an osteosarcoma mistaken for infection. ACKNOWLEDGEMENTS We are indebted to colleagues in the Department of Pathology, University of Bristol, for some of the histological reports. The illustrations were prepared by Mr J. Conibear and the script was typed by Mrs V. Beswetherick and Mrs C. Francis. REFERENCES BANKS, W.C. (97) Parosteal osteosarcoma in a dog and a cat. J. Am. vet. med. Ass. 58, 4. BRODEY, R.S., SAUER, R.M. & MEDWAY, W. (96) Canine bone neoplasms. J. Am. vet. med. Ass. 4,47. DE SANTOS, L.A. & EDEIKEN, B. (98) Purely lytic osteosarcoma. Skeletal Radiol. 9,. DOUGLAS, S.W. & WILLIAMSON, H.D. (970) Veterinary Radiological Interpretation. Heinemann, London. GILLETTE, E.L., THRALL, D.E. & LEBEL, J.C. (977) Carlson s Veterinarji Radiology, rd edn. Lea & Febiger, Philadelphia. GREENFIELD, G.B. (975) Radiology ufbone Diseases, rd edn. Lippincott, London. KEALY, J.K. (979) Diagnostic Radiology of the Dog and Cat. W. B. Saunders, Co., London. KIRK, H. (945) Radiographical diagnosis of bone tumours with special reference to osteomyelitis. Vet. Rec. 5,45. LINDBOM, A., SODERBERG, G. & SJUT, H.J. (96) Osteosarcoma. A review of 96 cases. Acta radiol. 56,. LING, G.V., MORGAN, J.P. & POOL, R.R. (974). Primary bone tumours in the dog: a combined clinical, radiographic and histological approach to early diagnosis. J. Am. vet. med. Ass. 65, 55. LIU, S-K, DORFMAN, H.D., HURVITZ, A.I. & PATNAIK, A.K. ( 977) Primary and secondary bone tumours in the dog. J. small Anim. Pract. 8,. MORGAN, J.P. (97) Radiology in Veterinary Orthopaedics. Lea & Febiger, Philadelphia. MURRAY, R.O. & JACOBSON, H.G. (977) The Radiology of Skeletal Disorders, Vol. I, nd edn. Churchill Livingstone, London. OWEN, L.N. (969) Bone Tumours in Man and Animals. Butterworths, London. OWEN, L.N. & BOSTOCK, D.E. (974) Effects of intravenous BCG in normal dogs and in dogs with spontaneous osteosarcoma. Eur. J. Cancer, 0,775. SCHNELLE, G.B. (97) Roentgen diagnosis of neoplasms involving bone. N. Am. Vet. 8,44. STEVENSON, S., HOHN, R.B., POHLER, O.G.M., FETTER, A.W., OLMSTEAD, M.L. & WIND, A.P. (98) Fracture-associated sarcoma in the dog. J. Am. vet. med. Ass. 80, 89. SUTER, P.F. ( 96) Klinische und rontgenologische Diagnosen und Fehlddiagnosen bei Knocken tumoren von Hund und Katze. Schweizer Arch. Tierheilk. 05,469. WEBBON, P.M. & CLAYTON-JONES, D.G. (978) Radiological refresher, 0: Bone turnours. J. small Anim. Pract. 9,5. WOLKE, P.C. & NIELSEN, S.W. (966) Site incidence of canine osteosarcoma. J. small Anim. Pract. I. 489.