A 17-year-old boy is brought to the clinic after noticing a hard, painless lump just above his right knee. He first noticed it a few months ago and says it hasn't changed much in size, though it occasionally feels uncomfortable when his gym shorts rub against it. He denies any recent trauma, weight loss, or night pain. Past medical history is unremarkable. On physical examination, there is a firm, immobile, non-tender mass on the medial aspect of the distal femur. There is no associated swelling or skin changes. An X-ray of the right femur shows a pedunculated bony outgrowth arising from the metaphysis, pointing away from the knee joint. The cortex and medullary cavity of the outgrowth are continuous with the parent bone.
Which of the following is the most likely diagnosis?
The correct answer is:
C) Osteochondroma
Osteochondroma, also known as an exostosis, is the most common benign bone tumor. It is essentially a cartilage-capped bony outgrowth that occurs near the growth plate (metaphysis) of long bones. Key diagnostic features for the boards include:
"Point Away" Rule: The growth typically points away from the nearest joint; in this case, pointing up toward the hip from the distal femur.
Cortical/Medullary Continuity: On X-ray, the most pathognomonic finding is that the cortex and the marrow cavity of the tumor are continuous with the bone from which it originates.
Symptoms: They are usually painless unless they compress a nearby nerve, interfere with a tendon, or are traumatized.
Answer choice A: Enchondroma, is incorrect. Enchondroma is a benign cartilaginous tumor that occurs inside the medullary cavity, usually of the small bones of the hands and feet. It appears as a radiolucent lesion with "popcorn" calcifications on X-ray, not an outward growth.
Answer choice B: Ewing sarcoma, is incorrect. Ewing sarcoma is a malignant tumor of the diaphysis (shaft) that presents with systemic symptoms such as fever and weight loss and an "onion-skin" appearance on X-ray.
Answer choice D: Osteosarcoma, is incorrect. Osteosarcoma is a malignant tumor of the metaphysis. It is highly destructive and presents with a "sunburst" appearance and Codman triangle, unlike the well-defined, benign-looking outgrowth of an osteochondroma.
Answer choice E: Osteoid osteoma, is incorrect. Osteoid osteoma presents with a small radiolucent nidus and severe nocturnal pain that is characteristically relieved by NSAIDs.
Key Learning Point
Osteochondroma is a benign bony outgrowth covered by a cartilage cap. The hallmark radiographic finding is a pedunculated or sessile mass with cortex and medulla continuous with the host bone, typically pointing away from the joint. While usually benign, a sudden increase in size or pain in an adult may suggest rare malignant transformation into a chondrosarcoma.
