A 15-year-old boy presents to the clinic with a 2-month history of worsening pain in his left knee. The pain initially occurred only with activity but now wakes him up at night. Ibuprofen does not help with the pain. He denies any recent trauma. Past medical history is negative. On physical examination, there is a firm, non-tender mass palpable over the distal femur. There is no joint effusion, and the range of motion of the knee is slightly restricted. Laboratory studies show elevated serum alkaline phosphatase and lactate dehydrogenase (LDH). Radiography of the left femur reveals a destructive lesion in the metaphysis with a sunburst appearance and lifting of the periosteum.
Which of the following is the most likely diagnosis?
The correct answer is:
E) Osteosarcoma
Osteosarcoma is the most common primary malignant bone tumor in children and adolescents. It has a bimodal distribution, occurring most frequently during the adolescent growth spurt and again in adults over age 65 when it is often associated with Paget disease of the bone. It characteristically arises in the metaphysis of long bones, most commonly around the knee in the distal femur or proximal tibia. Radiographic findings: include a sunburst pattern with spicules of calcified bone extending into the soft tissue and the Codman triangle, with a triangular area of new subperiosteal bone formed when the tumor raises the periosteum away from the bone. Elevated alkaline phosphatase, due to high osteoblast activity, and elevated LDH, reflecting high cell turnover, are common.
Answer choice A: Ewing sarcoma, is incorrect. Ewing sarcoma typically occurs in the diaphysis (shaft) and shows an "onion-skin" periosteal reaction rather than a sunburst pattern.
Answer choice B: Osteoblastoma, is incorrect. Osteoblastoma is a rare, benign but locally aggressive bone-forming tumor. It most commonly occurs in the vertebrae and presents with chronic back pain that is not relieved by NSAIDs.
Answer choice C: Osteochondroma, is incorrect. Osteochondroma is the most common benign bone tumor. It appears as a bony outgrowth (exostosis) covered by a cartilage cap, typically pointing away from the joint. It does not cause bone destruction or the "sunburst" reaction.
Answer choice D: Osteoid osteoma, is incorrect. While osteoid osteoma also causes nocturnal pain, it is a small benign lesion (<2 cm) with a radiolucent nidus. Crucially, the pain of osteoid osteoma is exquisitely relieved by aspirin or NSAIDs, whereas the pain of osteosarcoma is not.
Key Learning Point
Osteosarcoma is a malignant bone-forming tumor that occurs in the metaphysis of long bones in adolescents. Look for the sunburst pattern and Codman triangle on X-ray, along with elevated alkaline phosphatase.
