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

Image Courtesy of RadiologyinThai.com

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