Secondary Chondrosarcoma from Osteochondroma in proximal humerus.

Radiograph of humerus 3 years before the present MRI showing sessile osteochondroma (arrow)

The present radiograph showing development of secondary chondrosarcoma.

The present CT scan axial section of the proximal humerus showing large exophytic mass with calcification seen in the posterior aspect. The mass is continuous with the cortex of the humerus (arrows).

T1 weighted image showing mixed signal intensity of the mass with multiple areas of calcification.

STIR coronal image showing the large mass predominantly hyperintense signals.

Axial T2 image showing the cortical continuity with the mass lesion.

Discussion:

Malignant Transformation of solitary osteochondroma <1% and for multiple exostosis is ~13%.

The cartilaginous cap deserves the most attention when differentiating a benign osteochondroma from a secondary chondrosarcoma that arose from a pre-existing osteochondroma.

In adults, the cartilaginous cap regresses and becomes thin due to enchondral ossification of the majority of the cap.

Malignant transformation is suggested by: 

• Cartilaginous cap thickness greater than 2cm 
• Cortical destruction 
• Back growth of the cartilaginous cap into the stalk or medullary canal
• Lysis of calcifications in cap

Imaging:

Radiograph:

• Chondroid Calcification in cap
• Increasing destruction or change in appearance is worrisome for malignancy

Ultrasound - Good for cap and bursae

Bone Scan - Increased uptake in the cap

MRI: Best test for evaluating thickness of cap and surrounding bursa

• Intermediate T1W Images
• High Intensity T2W Images because of fluid content

CT:

• The cap will appear as soft tissue with calcification
• Can be difficult to distinguish from muscle

Cap thickness:

• Benign < 1.5cm (0.1 - 3.0cm; Avg. 0.6 - 0.9 cm).
• Malignant > 1.5 cm (1.5 - 12cm; Ave. 6cm).