Synonyms

• Juxtacortical osteosarcoma
• parosteal OS

Category

Bone

Behaviour

Malignant

Grade

Low

Gender

Female

Tissue of Origin

Bone

Epidemiology

• Most common surface osteosarcoma (65% of surface OS)
• Peak incidence in 3rd–4th decades
• Slight Female predominance
• Better prognosis than conventional OS

Clinical Features

• Slow-growing painless posterior distal femoral mass
• Restricted knee flexion
• Hard, fixed bony mass on examination
• Long history (months to years) before diagnosis

Location

• Posterior distal femur (>70%)
• Proximal humerus
• Proximal tibia
• Posterior surface of Bone - characteristic

Imaging

• Dense, heavily mineralised lobulated surface mass wrapping around posterior distal femur
• String sign: radiolucent cleavage plane between tumour and cortex (early lesions)
• CT: confirms surface location and mineralisation
• MRI: Low signal sclerotic mass with Variable soft tissue component

Pathology

• Low-grade spindle cell stroma with well-formed osteoid and Bone
• Bland Fibrous stroma (no nuclear pleomorphism in Low-grade)
• Dedifferentiated parosteal OS: High-grade sarcomatous component (worse prognosis)
• MDM2, CDK4 amplification in most cases

Genetics

• MDM2 and CDK4 amplification (12q13-15) in >90%
• Ring chromosome 12 (supernumerary ring containing 12q)
• Dedifferentiated component: additional genomic complexity

Treatment

• Wide local excision with negative margins - curative in most
• No chemotherapy needed for Low-grade lesions
• Dedifferentiated parosteal OS: treated as High-grade OS with chemotherapy
• Limb salvage feasible in majority

Prognosis

• Excellent for Low-grade: 5-year survival >90%
• Local recurrence 10% after wide excision
• Dedifferentiated parosteal OS: prognosis similar to conventional High-grade OS
• Negative margins are the key prognostic factor

Key Points

• MDM2 and CDK4 amplification are diagnostically useful - also present in DDLPS (different anatomical context)
• Posterior distal femur is the classic location
• Low-grade lesion does NOT require chemotherapy
• Dedifferentiated component (High-grade) must be identified - changes management completely

Workup - Blood Tests

• FBC, U&E, LFTs, Bone profile - baseline
• Alkaline phosphatase - often normal (unlike High-grade OS)

Workup - Local Imaging

• Plain radiograph - dense surface lesion arising from metaphyseal Bone; 'caulifLower' appearance
• MRI primary site with gadolinium - local staging; assess cortical breakthrough and medullary involvement
• CT chest - staging

Workup - Biopsy

• Core needle biopsy - confirm Low-grade osteosarcoma
• Histology: Low-grade spindle cell sarcoma with abundant osteoid formation
• MDM2/CDK4 amplification present (seen in surface/Low-grade OS)
• No significant atypia or mitotic activity

Workup - Staging

• CT chest - pulmonary staging (Low metastatic risk)
• Medullary involvement assessment critical - prognostic

Follow-up Summary

• Post-op visit at 6 weeks
• Years 1–2: 6-monthly clinical review; Xray and CXR 6-monthly
• Years 3–5: Annual clinical review + CXR
• Dedifferentiated parosteal OS: escalate to High-grade OS protocol (MAP chemo + intensive Follow-up)
• Discharge at 10 years for confirmed Low-grade; document self-monitoring advice