Vet Times The website for the veterinary profession https://www.vettimes.co.uk Malignant or benign? Diagnosis from radiographic features of bone lesions Author : NATALIE WEBSTER Categories : Vets Date : March 3, 2008 NATALIE WEBSTER discusses x-ray diagnosis of non-aggressive and aggressive bone lesions in small animals DECIDING whether a bony lesion seen on a radiograph is malignant or benign is a challenge frequently faced by veterinarians. Bone responds to injury or a pathological process with osteolysis, osteoproduction or a combination of the two. In veterinary medicine, very few processes are easily categorised as black or white, and bone lesions are no exception. Non-aggressive and aggressive lesions A spectrum of bony changes may be seen, ranging from severely aggressive to intermediate, lowgrade malignancy (non-aggressive and aggressive features) to non-aggressive. Often a lesion will have both aggressive and non-aggressive characteristics representing different stages of the disease process. Aggressive bone lesions are characterised by rapid growth with minimal or absent host response to confine and wall-off the lesion. Aggressive lesions are usually malignant - for example, due to a primary bone neoplasia - but may also be due to an infectious process such as fulminant osteomyelitis. Non-aggressive lesions are slow-growing and relatively benign. They are characterised by a smooth, non-active host response with clear demarcation of the lesion as seen with a healing 1 / 39 fracture callus. The distinction between aggressive and non-aggressive lesions is important because of the potentially life-threatening consequences of their underlying causes. The prognosis and treatment vary widely depending on whether the lesion is thought to be aggressive or non- aggressive. With aggressive bony changes (as seen with osteosarcoma), timely identification and implementation of a diagnostic plan can be crucial in the treatment of the disease. However, with a benign condition, performing a full diagnostic work-up - including invasive procedures, such as surgical biopsy - may not be indicated. Evaluation The parameters for evaluating whether a bone lesion is radiographically aggressive or nonaggressive include the following. • Demarcation of lesion Non-aggressive lesions usually have well-defined margins, whereas aggressive lesions are poorly demarcated. Non-aggressive lesions are slow growing and the bone is able to wall-off the lesion. In contrast, aggressive lesions grow rapidly and the bone is not able to form a response to confine the lesion. • Zone of transition Non-aggressive lesions have a short zone of transition and it is easy to tell where the lesion ends and normal bone begins. Aggressive lesions have a long zone of transition of abnormal to normal bone as they permeate into the surrounding bone, and the bone is not able to confine the lesion. • Pattern of osteolysis Non-aggressive lesions either do not produce osteolysis or produce a localised form. - Geographic osteolysis, the least aggressive form, is seen with slower-growing lesions. Geographic osteolysis usually consists of a single radiolucent area with a sclerotic rim, with or without cortical thinning and displacement, but no disruption. Such an appearance denotes an expansile lesion. Non-aggressive lesions are slow growing and the body has time to form a response such as cortical remodelling or new bone formation surrounding the lesion. Geographic osteolysis is typically seen with bone cysts, osteoclastomas and enchondromas. Aggressive lesions show more diffuse forms of osteolysis: - Moth-eaten. Characterised by multiple, small, ill-defined areas of osteolysis that may coalesce into larger lesions. 2 / 39 - Permeative. The most aggressive form of osteolysis, which is seen with rapidly growing aggressive lesions, such as osteolytic osteosarcomas and haematogenous osteomyelitis. Permeative osteolysis is characterised by a large number of small to pinpoint areas of osteolysis with poorly defined borders. Permeative osteolysis is often best visualised superimposed over the cortex and displays a long zone of transition. - Punched-out lesions. Radiolucent, well-defined areas of osteolysis are seen and there is no evidence of a sclerotic margin demarcating the lesion. They may appear well defined and, thus, less aggressive. However, the absence of a host response indicates that the lesions are more aggressive. Punched-out lesions are typically seen with multiple myeloma or metastatic bone disease. • Cortical involvement Non-aggressive lesions may cause thickening or displacement and thinning of the cortex, but will rarely cause disruption. Aggressive lesions will usually display cortical destruction, interruption and disruption due to the rapid and aggressive growth of the lesion. • Periosteal reaction Non-aggressive lesions display continuous solid or smooth periosteal reactions: - Smooth and solid. Indicative of a slow-growing, benign process. The degree of opacity of the periosteal reaction is an indicator of chronicity; the more radioopaque the periosteal reaction, the more long-standing the process. Causes include fracture callus, panosteitis and chronic low-grade osteomyelitis. - Lamellar and lamellated. Onion skin-like periosteal reaction as the periosteum is slowly and repeatedly lifted by the disease process and new bone is laid down beneath it. Causes include metaphyseal osteopathy and repeated trauma. Lamellated periosteal reaction may also be seen with a more aggressive lesion that has settled with time and/or treatment - for example, remodelled osteomyelitis. Aggressive lesions display interrupted, irregular forms of periosteal reactions, when new bone is present: - Brush border and palisading. As a bone lesion grows and displaces or destroys the cortical bone, the periosteum is lifted rapidly over a large area of the cortex. New bone is formed in a plane perpendicular to the cortex, resembling a brush border or palisade. If the reaction is less aggressive and takes place over a relatively longer time, the spicules are thicker and known as palisading or thick brush border, as seen in hypertrophic osteopathy (Marie’s disease). 3 / 39 More aggressive lesions cause rapid and continual lifting of the periosteum and the resulting brush border is composed of finer, more slender spicules. This is termed a thin brush border and indicates a more aggressive lesion - for example, acute haematogenous osteomyelitis or neoplasia. - Sunburst. Highly aggressive lesions grow extremely rapidly, expanding outwards from the bone and causing rapid lifting of the periosteum overlying the lesion. A sunburst periosteal reaction consists of spicules of new bone radiating out from the centre of the lesion and an osteosarcoma is the most likely diagnosis. - Amorphous. The mos t aggressive periosteal reaction and highly suggestive of a primary bone neoplasia. There is unstructured, neoplastic new bone with a complete loss of recognisable structure. • Presence of Codman’s triangle Lifting of the periosteum causes production of a solid triangle of new bone at the periphery of the lesion. It is often seen at the edge of a primary malignant bone tumour. However, Codman’s triangle can occur with benign processes. • Presence of sclerosis Sclerosis can be used as a radiological term to describe increased bone radiopacity. Sclerosis is either due to increased density of the bone or superimposed periosteal or endosteal new bone, and sclerosis represents the new bone laid down by the body to confine the lesion. Non-aggressive lesions will often have surrounding sclerosis, while aggressive lesions will not have surrounding sclerosis due to their rapid rate of growth. • Rate of change Non-aggressive lesions will have a slow rate of change or appear static, whereas a feature of aggressive lesions is that they have a rapid rate of change. Many of the radiological signs of an aggressive lesion are due to the rapid rate of change where there is minimal time for the bone to adapt or remodel. Follow-up radiographs are a valuable tool in assessing lesions that display intermediate or low- grade aggressive features. Repeating the radiographic examination several weeks later will allow assessment of any radiographic changes and, thus, allow interpretation of the degree of aggression. A non-aggressive lesion will not change significantly in radiographic appearance over this time scale, whereas an aggressive lesion will. • Location of the lesion 4 / 39 The location of a lesion can help in formation of a differential diagnosis list as certain disease processes have predilection sites where lesions are common. - Osteosarcoma predilection sites are the metaphysis of the distal radius, proximal humerus, distal femur and proximal tibia (ie, away from the elbow and towards the knee). If an aggressive bone lesion is seen in one of these sites, osteosarcoma or other primary bone neoplasia should be ranked high on the differential list. - Hypertrophic osteopathy (Marie’s disease) is characterised by a palisading periosteal reaction and soft tissue swelling, initially on the medial surface of the second digit, and the lateral surface of the fifth digit, before progressing proximally. - Number of lesions. Polyostotic lesions (those that are present in more than one bone) occur with multiple myeloma, neoplasia that has metastasised to bone and, occasionally, with haematogenous infection. - Presence of joint involvement. A lesion affecting two or more adjacent bones - for example, at a joint - is likely to be a