Paediatric Bone Tumours

Abstract

Imaging characteristics of a bone lesion- a malignant tumour, a benign tumour or a tumour-like process- can be very specific, but may also be misleading at times, making characterisation and differentiation more challenging or even impossible [1, 2]. Malignant bone tumours are rare in the paediatric age group. Ewing sarcoma and osteosarcoma are the most frequent primary malignant bone tumours with a variable incidence according to age. Other malignant bone tumours such as primary skeletal lymphoma, chondrosarcoma, fibrosarcoma, haemangioendothelioma and adamantinoma are even rarer [3]. In young children bone metastases may be the presenting manifestation of neuroblastoma and leukaemia. Bone lesions in a context of Langerhans cell histiocytosis (LCH), now considered a neoplasm, occur mainly in the flat bones, spine and proximal long bones. The most common benign condition that can show aggressive features and may thus mimic a malignant bone tumour on imaging studies is osteomyelitis, particularly when the patient has no fever [2, 4]. Plain radiograph is the initial and most useful examination for differentiating benign from malignant bone processes. CT and MRI are in many cases the next diagnostic test. Paediatric bone has specific features and particularities such as growth, bone marrow conversion and variation of vascularisation of the bone with age. Furthermore, certain bone lesions are age-related [5], hence age is an important element in the differential diagnosis. A thorough analysis of the images, including the number of lesions, the location, the appearance and size of the lesion(s), and the appearance of the adjacent bone and periosteal reaction, is fundamental. In case of multiple lesions, LCH, chronic recurrent multifocal osteomyelitis and polyostotic fibrous dysplasia are possible diagnoses. The type of bone, flat, short or long, in which the lesion is located, as well as epi-, meta- or diaphysis is important in the differential diagnosis. The size of the lesion is not a very specific feature although lesions larger than 5-6 cm are more suspicious for malignancy. The morphology of the lesion is equally non-specific: most benign lesions are elliptical but some malignant tumours can have the same appearance (lymphoma, low-grade osteosarcoma). Benign lesions usually grow at a slow pace and have sharp borders. Malignant lesions commonly have poorly defined margins and show cortical destruction. A periosteal apposition occurs whenever an infection or a tumour, either malignant or benign, irritates the periosteum or as a reaction to trauma. The pattern of periosteal reaction can be benign (as seen in benign lesions or trauma) or aggressive (as seen in malignancies, infections or LCH). Benign bone tumours generally have well-defined and often sclerotic margins, show cortical expansion and may produce solid periosteal reaction. Malignant tumours usually have poorly defined margins, show cortical destruction and periosteal reaction of the spiculated, onionskin or interrupted type and are most of the time accompanied by a soft tissue mass. In conclusion, a meticulous analysis of all available imaging studies with the age of the patient in mind is required for a reliable diagnosis or differential diagnosis.