A 25-year-old male is referred with a painless, palpable, firm mass around the knee. He reports it has been present since childhood and has not changed significantly. Anteroposterior and lateral radiographs are performed.
Describe the radiographic features seen in Figure 1 and state the most likely diagnosis.
Candidate: "The radiograph shows a bony exostosis arising from the distal femoral metaphysis. Key features are the continuity of the cortical and medullary bone of the lesion with the host bone. The lesion points away from the physis. This is diagnostic of a solitary osteochondroma."
Candidates often fail to use the phrase "medullary continuity," which is the single most important diagnostic buzzword. Some candidates suggest biopsy or MRI immediately, forgetting that the radiographic diagnosis is pathognomonic and no further imaging is required if the patient is asymptomatic.
A high-scoring answer describes the lesion systematically: "It is a sessile/pedunculated bony exostosis arising from the metaphysis. Most importantly, it demonstrates cortical and medullary continuity with the parent bone. The direction of the lesion is away from the knee joint, which is characteristic. Given these features, this is a classic solitary osteochondroma, and I would reassure the patient as no further imaging is required if there is no pain or neurovascular compromise."
The patient returns 6 months later reporting new, deep aching pain in the area and a feeling that the mass has "grown." How do you manage this?
Candidate: "New pain and growth in an adult are red flags for malignant transformation into a peripheral chondrosarcoma. I would request an MRI to measure the thickness of the cartilage cap. If it is greater than 2cm in an adult, it is highly suspicious for malignancy."
Failure to define "Adult" vs "Child" regarding the 2cm threshold. Also, omitting the importance of the biopsy for histological grading before planning definitive resection.
A structured answer: "I would categorize the assessment into Clinical (pain, growth) and Radiological (MRI cartilage cap). In an adult, a cartilage cap >2cm is the threshold for suspicion of secondary peripheral chondrosarcoma. I would also perform a core needle biopsy to grade the chondrosarcoma, as this dictates the margin required for surgical resection. I would also screen for HME (Hereditary Multiple Exostoses) given the potential for systemic disease."
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