A 13-year-old boy is brought to the office because of a dull, aching pain in his right leg that has persisted for several weeks. The pain is worse at night and is not relieved by ibuprofen. Recently, he has developed a low-grade fever and noticed a lump on his right thigh. Past medical history is negative. On physical examination, there is a firm, tender mass over the mid-shaft of the right femur with associated mild erythema and warmth. Laboratory studies show a leukocyte count of 14,000/mm3 and a significantly elevated lactate dehydrogenase (LDH) level. An X-ray of the femur reveals a destructive lytic lesion in the mid-shaft of the femur with a moth-eaten appearance and onion-skin periosteal reaction.
Which of the following is the most likely diagnosis?
The correct answer is:
A) Ewing sarcoma
Ewing sarcoma is the second most common malignant bone tumor in children and adolescents after osteosarcoma. It classically affects the diaphysis (mid-shaft) of long bones, particularly the femur. Ewing sarcoma can mimic an infection, presenting with pain, swelling, fever, and leukocytosis. X-rays typically demonstrate a characteristic "onion-skin" periosteal reaction, caused by layers of new bone formation as the tumor pushes through the cortex. It may also show a "moth-eaten" appearance. Elevated LDH levels often correlate with larger tumor burden and a poorer prognosis.
Answer choice B: Osteoid osteoma, is incorrect. Osteoid osteoma is a benign bone tumor that typically presents with nocturnal pain that is greatly relieved by NSAIDs like ibuprofen. Radiographically, it appears as a small radiolucent nidus surrounded by sclerotic bone.
Answer choice C: Osteosarcoma, is incorrect. While osteosarcoma is more common than Ewing sarcoma, it typically occurs at the metaphysis (near the growth plate, like the distal femur or proximal tibia). Its classic X-ray finding is the "sunburst" pattern or Codman triangle representing elevation of the periosteum.
Answer choice D: Osteomyelitis, is incorrect. Although Ewing sarcoma can mimic bone infection with fever, pain, and leukocytosis, the radiographic "onion-skinning" and the presence of a large soft-tissue mass point more toward a malignancy. However, these two must often be differentiated via biopsy.
Answer choice E: Scurvy, is incorrect. Vitamin C deficiency can cause subperiosteal hemorrhage and bone pain in children, but it typically presents with systemic signs like gingival bleeding, petechiae, and corkscrew hairs.
Key Learning Point
Ewing sarcoma is a highly malignant small round blue cell tumor that typically involves the diaphysis of long bones. The classic radiographic finding is an onion-skin periosteal reaction. It often presents with systemic symptoms such as fever and weight loss that can mimic osteomyelitis.
