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.
Most common benign bone tumor
Due to aberrant epiphyseal development
Growth stops after skeletal maturation
Malignant transformation in 1-2% of solitary lesion
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
Stoller et al. Diagnostic Imaging: Orthopedics. 2004
Image Courtesy of RadiologyinThai.com