Question
Ewing’s sarcoma
Answer
Ewing sarcoma is a small round blue cell malignant bone (and soft tissue) tumour, typically affecting children and adolescents, arising in the diaphysis of long bones (femur) and flat bones (pelvis).
Genetics: Characteristic translocation t(11;22)(q24;q12), producing the EWSR1-FLI1 fusion gene (member of the Ewing sarcoma family of tumours, related to primitive neuroectodermal tumour/PNET).
Gross: Grey-white, soft, necrotic/haemorrhagic mass arising in the medullary cavity, permeating through cortex.
Radiology: Classic “onion-skin” periosteal reaction (layered periosteal new bone formation) due to repeated cycles of tumour breaking through and periosteum reacting; permeative, moth-eaten bone destruction.
Microscopy: Sheets of small, uniform, round cells with scant clear cytoplasm (rich in glycogen — PAS positive) and round nuclei with fine chromatin and inconspicuous nucleoli; occasional Homer-Wright rosettes (tumour cells arranged around a central fibrillary space) may be seen, reflecting neuroectodermal differentiation.
Immunohistochemistry: Strongly positive for CD99 (MIC2).
Clinical: Presents with pain, swelling, and sometimes fever/leukocytosis mimicking osteomyelitis. Highly aggressive but chemosensitive; combined chemotherapy, surgery, and radiotherapy improve survival significantly.