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Findings: There is a large broad based, lobulated, predominantly sessile exostosis projecting from the left proximal humeral metadiaphysis.

Diagnosis: Osteochondroma.

Discussion: Osteochondroma (OC) or exostosis is a benign tumor-like lesion that consists of a bony projection covered by a cap of hyaline cartilage. OC may be solitary or multiple. Occasionally, they may form spontaneously following injury.

Clinically, 70% to 80% of these lesions occur in patients who are younger than 20 years of age. A painless, slowly growing mass is the most common characteristic clinical manifestation.

Frequently, these lesions are found to arise from the metaphysis of a tubular bone (femur, tibia) and ribs in 80% of the patients. Another 10% of these lesions are seen in the pelvis and scapula (both bones which are preformed in cartilage). OC are rare in the diaphysis. If they are seen in the epiphysis, they are indicative of Trevor's disease. OC are typically seen in regions corresponding to rapid bony growth.

This lesion is characterized radiographically by an osseous protuberance which arises from the external surface of the tubular bone and has a cortex which is continuous with that of the parent bone. Outgrowths may be pedunculated or sessile. The tip of an OC is covered by a cap hyaline cartilage. If the cap is small and well defined with regular calcification, the appearance is most consistent with a benign outgrowth. If it is large and poorly defined and contains irregular calcification, malignant transformation should be considered. Sarcomatous degeneration is seen in approximately 5% of patients with this entity.

References:
1. Resnick D. Bone and Joint Imaging. WB Saunders. 1985.

2. Giudici MA, et al. Cartilaginous bone tumors. Radiologic Clinics of North America. 3(2):237-259, 1993.

3. Margolis M, McLennan MK, Radiology Rounds. Osteochondroma. Canadian Family Physician.
41:216, 220-222, 1995.

4. Blickman JG. Pediatric Radiology, The Requisites. Mosby, 1994.

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