A 14-year-old boy is brought to the clinic for a 4-month history of right thigh pain. The pain is persistent and consistently worse at night, often waking him from sleep. His mother mentions that the pain completely vanishes for several hours after he takes a single tablet of ibuprofen. Past medical history is unremarkable. On physical examination, there is no localized swelling, warmth, or palpable mass, though there is mild tenderness over the mid-femur. Plain radiography reveals a small, 1.2-cm radiolucent area in the cortex of the femoral diaphysis surrounded by a thick rim of dense, sclerotic bone.
Which of the following is the most likely diagnosis?
The correct answer is:
C) Osteoid osteoma
Osteoid osteoma is a common benign, bone-forming tumor that typically affects children and young adults and is predominant in males. It most frequently occurs in the cortex of long bones, such as the femur or tibia. The classic clinical "clues" for the USMLE are:
Nocturnal Pain: Pain that is worse at night and unrelated to activity. This is caused by high levels of prostaglandins produced by the tumor cells within the lesion.
NSAID Response: The pain is exquisitely relieved by aspirin or NSAIDs like ibuprofen, which inhibit the prostaglandin synthesis.
Radiographic nidus: On X-ray, it appears as a small (usually <2 cm), radiolucent zone (the nidus) surrounded by prominent reactive sclerosis (thickened bone).
Answer choice A: Ewing sarcoma, is incorrect. Ewing sarcoma is a malignant tumor with systemic symptoms such as fever and weight loss, an "onion-skin" periosteal reaction, and pain that does not resolve completely with NSAIDs.
Answer choice B: Osteoblastoma, is incorrect. Osteoblastoma is histologically similar to osteoid osteoma but is larger (>2 cm) and most commonly involves the vertebral column. Importantly, the pain of an osteoblastoma is usually not relieved by NSAIDs.
Answer choice D: Osteomyelitis, is incorrect. A Brodie abscess (a form of chronic osteomyelitis) can look like a radiolucent lesion with surrounding sclerosis. However, the patient would likely have history of fever, elevated inflammatory markers, and would not have the classic dramatic response to ibuprofen.
Answer choice E: Osteosarcoma, is incorrect. This is a malignant tumor in the metaphysis with a sunburst appearance and periosteal elevation (Codman triangle) on radiography. It does not present with a small, sclerotic-rimmed nidus.
Key Learning Point
Osteoid osteoma is a benign bone tumor characterized by a radiolucent nidus (<2 cm) and severe nocturnal pain that is relieved by NSAIDs. If the lesion is larger than 2 cm and involves the spine without NSAID relief, think osteoblastoma.
