Osteochondroma

Fig. 1: Frontal radiograph of the right shoulder shows a broad-based exostosis with continuity of bone cortex and medullary marrow to the metaphysis of the humerus. 

Fig. 2: Axial T2-weighted MR image shows an exostosis with a bright cartilagenous cap (arrowheads), which is thin and smooth. 

This is a classic radiographic appearance of 'sessile osteochondroma' at the most common location (metaphysis) of bone. 

Facts:

• Most common benign bone tumor
  
• Due to aberrant epiphyseal development 
  
• Growth stops after skeletal maturation
  
• Malignant transformation in 1-2% of solitary lesion 
  

Points:

• Suspect malignant transformation if: growth after skeletal maturation, pain in the absence of fracture, bursitis or nerve compression
  
• MRI  and CT to demonstrate thickness of cartilaginous cap, if >2 cm suspect malignancy
  

Reference:

Stoller et al. Diagnostic Imaging: Orthopedics. 2004