Parosteal Osteosarcoma

Clinical Scenario

An 18-year-old female presents with a slowly enlarging painless mass on the posterior aspect of her right distal thigh. She first noticed it 2 years ago when it was small, and it has gradually increased in size. There is now mild discomfort with prolonged sitting.

History:

• 2-year history of posterior thigh mass
• Slow, painless enlargement
• Now causing mild discomfort with sitting
• No trauma history
• No constitutional symptoms
• No family history of bone tumours
• Otherwise fit and well, university student

Examination Findings:

• 8 x 6 cm firm, non-tender mass posterior distal thigh
• Fixed to deep structures, limited mobility
• Skin normal, no overlying changes
• No regional lymphadenopathy
• Full knee range of motion
• Normal neurovascular examination
• Normal gait
• Contralateral limb normal

Investigations

Laboratory Results

Imaging

Plain X-ray Right Femur (AP and Lateral):

• Densely mineralised, lobulated mass arising from posterior femoral cortex
• 'String sign' - radiolucent line separating mass from cortex
• Mass appears to wrap around femur ('stuck-on' appearance)
• Dense, homogeneous ossification (mature bone)
• No periosteal reaction
• No cortical destruction
• Medullary canal appears uninvolved

MRI Right Femur with Gadolinium:

• 8.2 x 5.5 x 4.8 cm lobulated surface mass
• Predominantly low T1 and T2 signal (mature bone)
• Thin rim of high T2 signal at periphery (cartilage cap ~3mm)
• String sign confirmed - thin plane between mass and cortex
• No medullary involvement (critical finding)
• Mass wraps around 60% of femoral circumference
• Popliteal vessels displaced posteriorly but not encased
• No heterogeneous high-grade areas

CT Chest (Staging):

• No pulmonary metastases
• No mediastinal lymphadenopathy

Histopathology

Core Needle Biopsy Result:

• Well-differentiated osteosarcoma, low-grade
• Mature bone trabeculae with bland spindle cells
• Minimal atypia, low mitotic rate
• Parallel bone formation (organised pattern)
• Fibrous component with streaming spindle cells
• No high-grade/dedifferentiated areas in this sample
• FISH: MDM2 and CDK4 amplification positive
• Diagnosis: Parosteal osteosarcoma, low-grade

Questions & Model Answers

Key Teaching Points

Common Examiner Follow-up Questions

1. "What is the difference between parosteal and periosteal osteosarcoma?"

   • Parosteal: low-grade, dense ossification, posterior metaphysis
   • Periosteal: intermediate-grade, chondroblastic, diaphysis, perpendicular spicules
   • Both are surface tumours with better prognosis than conventional

2. "What if the tumour encases the popliteal vessels?"

   • Preoperative angiography/CT angiography
   • Vascular surgery involvement
   • May need vessel resection and reconstruction
   • If reconstruction not possible, amputation may be required

3. "How do you confirm the diagnosis if imaging is equivocal?"

   • CT-guided core biopsy
   • FISH for MDM2/CDK4 amplification
   • Serial imaging if myositis ossificans suspected
   • Myositis ossificans matures over 6-8 weeks

4. "What chemotherapy regimen is used for dedifferentiated parosteal OS?"

   • Same as conventional high-grade OS
   • MAP protocol: Methotrexate, Adriamycin (doxorubicin), cisPlatin
   • Neoadjuvant + adjuvant
   • Assess histological necrosis post-neoadjuvant

Related Topics

• Conventional Osteosarcoma
• Periosteal Osteosarcoma
• Myositis Ossificans
• Osteochondroma
• Surface Bone Lesions