Introduction
This topic covers three malignant or plasma-cell bone lesions that an examiner expects you to tell apart on plain radiographs and then describe in terms of CT, MRI and clinical course: Ewing sarcoma, multiple myeloma and chondrosarcoma. Each is defined by its cell of origin, age group, typical site and characteristic radiological pattern, and the differential diagnosis depends on putting these features together.
Ewing sarcoma
Ewing sarcoma is a small round blue cell malignancy of primitive neuroectodermal-type origin, characterised by a recurrent translocation involving the EWSR1 gene, most often t(11;22) with the FLI1 gene. It is the second most common primary malignant bone tumour of childhood after osteosarcoma, typically presenting between about 10 and 20 years of age, with a male predominance.
- Site: diaphysis of long bones (femur, tibia, humerus, fibula), pelvis, ribs, scapula and vertebrae; the lesion is central and metaphyseal extension is possible.
- Plain film: permeative or moth-eaten lytic destruction with ill-defined margins, a large soft tissue component, and a periosteal reaction that is classically lamellated, producing the onion-skin pattern; Codman triangle may also be seen.
- Mixed lytic and sclerotic change can occur, but the lesion is generally less sclerotic than osteosarcoma.
AP and lateral views of various bones: grouped osteolytic foci, interfocal and perifocal osteosclerosis, periosteal reaction and spicules. Diagnosis: Ewing sarcoma.
- Clinical picture: pain and swelling, often mistaken for infection or trauma, with fever, raised ESR and leukocytosis that can mimic osteomyelitis.
- MRI shows marrow replacement with a soft tissue mass and is the best modality for local extent and for planning surgery or radiotherapy.