A definitive diagnosis of bone neoplasia cannot be made from radiographs. A fine-needle aspirate or preferably a bone biopsy is recommended in all cases. Ultrasonography can be used to guide the needle into the affected region of bone (Figure 9.29). It is also prudent to radiograph or perform a CT of the thorax to check for metastases in cases of suspected neoplasia.
Primary monostotic bone neoplasms:
Osteosarcoma: This is the most common primary mono-stotic bone neoplasm in dogs. Osteosarcomas account for 80% of malignant bone neoplasms in large-breed dogs and 50% in small-breed dogs. Osteosarcomas or osteo-genic sarcomas are malignant neoplasms primarily arising in bone; 75% of osteosarcomas occur in long bones, most arising in the medullary cavity, but some originate from the cortical surfaces, periosteum (peri o steal, parosteal or juxtacortical osteosarcoma) or in extraskeletal sites.
Osteosarcomas cause destruction of the surrounding soft tissue and readily metastasize to the lungs. The most com-mon long bone affected is the metaphyseal region of the distal radius, followed by the proximal humerus, prox i mal or distal femur, and proximal or distal tibia. Osteo-sarcomas appear to have a biphasic peak of age occur-rence. There is an increase in incidence of osteo sarcomas in dogs at 2 years of age and again at 8 years of age.
There has been a lack of standardization of termin-ology for primary bone neoplasms, other than central osteo sarcoma, within the medical literature, with the terms parosteal osteoma, parosteal osteosarcoma, periosteal osteosarcoma, juxtacortical osteosarcoma and surface osteosarcoma all used to describe neoplasms with similar histological and radiological characteristics. There has been corresponding inconsistency in terminology in the veterinary literature. Thomas et al. (1997) suggested that surface osteosarcoma could be classified histologically into three distinct groups: parosteal, periosteal and high-grade surface osteosarcoma.
• Parosteal osteosarcoma (also called juxtacortical osteosarcoma) is a slow-growing, low-grade malignant neoplasm, which arises from the surface of the bone (the periosteal connective tissue). It is less common than central osteosarcoma, which arises from within
(a)
(e) (f) (g) (h) (i) (j)
(b) (c) (d)
Examples of osteosarcoma. (a) ML view of the distal tibia of a mixed-breed dog. There is an aggressive periosteal reaction, osteolysis, mineralization of the soft tissues, a poorly defined zone of transition in the distal third of the tibial diaphysis and an extensive encircling soft tissue swelling. This is termed a ‘sunburst’ reaction. (b) ML view of the shoulder of a 10-year-old Rottweiler which had been lame for 3 months. There is complete destruction of the proximal third of the humerus with faint mineralization in the adjacent soft tissues (arrowheads). (c) CrCd view of the distal radius of a 5-year-old St Bernard with a swollen distal limb. There is a spiculated periosteal reaction (arrowed) on the distomedial aspect of the radius with localized soft tissue swelling medially. (d) ML view of the proximal tibia of an 8-year-old Rottweiler. There is focal lysis of the proximocaudal tibia with adjacent active periosteal reaction. (e) ML view of the stifle of a 9-year-old spaniel cross-breed. There is a subtle change in the trabecular pattern of the cranioproximal tibia (arrowed) with pinpoint areas of osteolysis and sclerosis. (f) ML and (g) CrCd views of a 7-year-old pointer cross-breed with a proliferative sclerotic bone reaction (arrowed) in the proximocranial and proximolateral radius. This was a chondroblastic osteosarcoma. Incidental moderate elbow arthrosis is present. (h) ML view of the radius and ulna showing an expansile septated osteolytic lesion affecting the mid-diaphysis of the ulna. This is an unusual site for an osteosarcoma. (i) Sagittal reconstruction of a CT series, displayed using a bone window, from the dog shown in (h).
The distal extension of the expansile neoplasm into the medullary cavity is clearly seen. (j) A T2W MRI study (dorsal plane) of the distal radius of a dog with an osteosarcoma. The destructive lesion (arrowhead) can be seen in the distal radius. It extends proximally into the medullary cavity. There is invasion into the adjacent soft tissue structures. The normal hyperintense medullary cavity is visible (arrowed).
(j, Courtesy of RD Pechman)
9.30
Fibrosarcoma: This is another primary bone neoplasm that can have a monostotic distribution. It can be difficult to differentiate a fibrosarcoma from an osteosarcoma, but a fibrosarcoma may be more osteolytic (Figure 9.31). They are slower growing than osteosarcomas and are more likely to invade an adjacent joint space.
Haemangiosarcoma: This rarely presents as a primary bone neoplasm and may be radiographically indistin-guishable from the other sarcomas (Figure 9.32, and see Figure 9.22a).
Primary polyostotic bone neoplasms: Multiple myeloma is the commonest polyostotic primary bone neoplasm. It results from a neoplastic proliferation of plasma cells in the bone marrow. This in turn leads to osteoclastic act-ivity, which results in bone resorption. Radiographs can reveal multiple focal lytic lesions in the vertebrae, ribs and long bones (Figure 9.33). Other neoplasms that produce multifocal bone lysis include lymphoma/lymphosarcoma of bone and polyostotic osteosarcoma.
Soft tissue neoplasms: In cases of a malignant soft tissue neoplasm invading bone, a soft tissue swelling with
osteo lysis and cortical disruption are the main radio-graphic changes. The tumours are usually sarcomas (i.e. fibrosar coma, haemangiosarcoma, liposarcoma).
Slowly developing soft tissue neoplasms may provoke a mild periosteal reaction or minimal change in the adja-cent bone contour. Benign soft tissue neoplasms do not usually invade bone (Figure 9.34).
Histiocytic sarcoma: Previously termed malignant histio-cytosis, histiocytic sarcoma can be divided into two cate-gories: localized and disseminated. Localized histiocytic sarcoma is a single primary lesion with or without local lymph node metastasis and should be considered as a differential diagnosis in middle-aged to older Golden Retrievers, Rottweilers and Bernese Mountain Dogs when radiographs demonstrate an aggressive bone lesion characterized by bone destruction or osteolysis. If there is a soft tissue mass in association with the aggressive bone lesion, this should further raise the index of suspicion.
With specific reference to long bones, there is a predilec-tion site for the proximal humerus and periarticular sites.
Disseminated histiocytic sarcoma involves multiple organ systems and may represent either multicentric disease or metastatic spread of the localized form. Bone lesions
Examples of fibrosarcoma. (a) CrCd view of the distal antebrachium of an adult Boxer that presented with a sudden history of acute lameness.
Moth-eaten osteolysis is present in the distal aspect of the radial diaphysis. A faint vertical radiolucent pathological fracture line is also present (arrowed). (b) ML and (c) CrCd views of the stifle of an adult dog. This case illustrates the more productive features that can be associated with fibrosarcoma.
New bone formation encircles the proximal tibia (arrowheads).
9.31
(a) (b)
(c)
CrCd view of the distal radius and ulna of a cross-breed dog with a haemangiosarcoma.
There is a radiolucent expansile lesion in the distal ulna which has disrupted the cortical margin (big arrow).
A smooth periosteal reaction is seen proximally (tiny arrow), and distally the cortical bone is thinned (arrowhead).
9.32
Multiple myeloma.
(a) ML view of the tibia of a 10-year-old Boxer showing multiple pinpoint radiolucent foci (arrowed) scattered throughout the proximal two-thirds of the medullary cavity. (b) VD view of the pelvis of a retriever.
Discrete, well circumscribed areas of osteolysis (arrowed) are seen in the femoral head and greater trochanter.
9.33
(a)
(b)
CT images in a bone window of a 10-year-old Collie with lameness and a swollen left thigh region. (a) Transverse (right and left pelvic limbs) and (b) sagittal
reconstruction show a large 10 x 8 cm well marginated fat-attenuating mass (arrowheads) interposed between the
semimembranosus and semitendinosus muscles and extending proximally and caudal to the ischium. The fat mass extends to but apparently does not involve the cortical bone of the femur. CT aids surgical planning. This was a lipoma.
9.34 (a)
(b)
CrCd view of the stifle of a small-breed dog.
Osteolytic ‘moth-eaten’ areas are seen in the medullary cavities of the distal femur and proximal tibia. There is a wide transition zone and an aggressive periosteal reaction on the distomedial femoral cortex. These changes were due to a primary prostatic adenocarcinoma that underwent metastasis to the stifle.
9.35
A series of images obtained over a 3-year period from a 7-year-old (at initial presentation) Labrador Retriever cross-breed which presented with a hard swelling on the mid-tibial diaphysis that underwent malignant transformation over the 3-year period. (a) ML view of the tibia and fibula showing a discrete well marginated osteolytic area (arrowhead) surrounded by a collar of endosteal and smooth periosteal new bone formation along the cortex (arrowed). (b) The ML view obtained 3 months later, following aspiration (which indicated necrosis) and subsequent curettage and antimicrobial therapy, shows that the lesion is more quiescent and organized (arrowheads). (c) ML view obtained 2 years later shows that the lesion has changed into a lobulated, well marginated mixed radiolucent granular area (arrowheads). (d) CT sagittal reconstruction in a bone window, made at the same time as the image in (c), shows that the lobulated lesion (black arrow) is well demarcated cranially (arrowhead) but extends into the medullary cavity (white arrow). A bone biopsy revealed necrotic tissue, and surgical curettage was repeated. (e) One year later and 3 years after the initial presentation, another CT was performed. This sagittal reconstruction CT image in a bone window shows that the lesion has become aggressive and expands proximally and distally. istopathology indicated that it was an aberrant fibrosarcoma.
9.36
(a) (b) (c) (d) (e)
are similar to that of localized histiocytic sarcoma and predilection sites are periarticular regions, vertebrae, the proximal humerus and ribs.
Secondary or metastatic neoplasia: Malignant neo-plasms may metastasize to bones. The long bones affected are usually the humerus and femur. Metastatic neoplasia tends to be located in the diaphysis adjacent to the nutrient foramen because of spread via the nutrient artery, and may be polyostotic. They are often only diag-nosed when a pathological fracture occurs.
The radiographic appearance of metastatic bone neo-plasia can vary from osteolytic to osteoblastic in nature (Figure 9.35). Scintigraphy can be useful in the detection of metastatic lesions that are not clinically apparent. Detection of malignancy in another organ, along with a diaphyseal polyostotic distribution in an older animal with no exposure to mycotic infections, is highly suggestive of metastatic bone neoplasia. Biopsy is required for a definitive diagnosis.
Malignant transformation: Malignant transformation (see also Chapter 10) of previous fracture sites to fracture-associated sarcomas has been reported. Possible inciting factors for malignant transformation include:
• Tissue damage resulting from the initial trauma
• Altered cellular activity associated with prolonged healing of the fracture
• Infection.
Most malignant transformations occur in the diaphysis, because this is the most common site for fracture occur-rence (Figures 9.36 and 9.37).
Radiation-induced osteosarcoma: Radiation-induced sarcomas occur following radiation therapy (either ortho-voltage or megaortho-voltage radiation treatment). Several criteria must be fulfilled in order to reach the diagnosis of a radiation-induced osteosarcoma. The patient must have had no radiographically detectable lesion in the affected area prior to irradiation, the neoplasm must have devel-oped in the irradiated field, there is usually a long latent period, and the affected area should have a low incidence of bone neoplasia. The osteosarcoma must be confirmed histopathologically.
Examples of malignant transformation. (a) ML view of the humerus of an adult dog that had a mid-diaphyseal humeral fracture repaired.
The implant was removed 6 months postoperatively. Three years later, the dog presented acutely lame. There is a pathological fracture through the mid-diaphyseal region (black arrow), with a mottled radiopacity and an aggressive periosteal reaction (white arrows). A separated fragment is visible distocranially to a large radiolucent cavity (arrowhead) resulting from a pathological fracture (double arrowheads). (b) ML and (c) CrCd radiographs of the right femur of a 10-year-old Boxer that had a surgical repair for a femoral fracture 7 years previously and presented acutely lame for 4 weeks. There is marked osteolysis within the distal medullary cavity (arrowed). This was a malignant osteoblastic osteosarcoma and may be a coincidental development but malignant transformation is likely.
9.37
(a) (b) (c)