Parosteal Osteosarcoma - Pathology

Introduction

A low grade osteosarcoma
Epidemiology
- demographics
  - more common in females, age 30-40
- location
  - occurs on surface of metaphysis of long bones
  - most common sites include posterior distal femur, proximal tibia, and proximal humerus
    - 80% cases occur in the femur
  - marrow invasion occurs in 25% of cases
Prognosis
- 95% long term survival when local control has been achieved
  - dedifferentiation of parosteal osteosarcoma is a poor prognostic factor
  - invasion into the medullary cavity does not affect long-term survival

Symptoms

Presentation
- often presents as a painless mass
- limited joint motion can also be a presenting complaint for characteristic large posterior distal femoral lesions

Imaging

Radiographs
- heavily ossified, lobulated mass arising from cortex
- appears stuck onto cortex
CT chest
- mandatory staging study
- used to evaluate for pulmonary metastasis
MRI
- mandatory and must include entire involved bone
- helps determine soft tissue/marrow involvement and skip lesions
Bone scan
- mandatory imaging study
- always hot

Histology

Characteristic histology
- low grade lesion
- regularly arranged normal osseous trabeculae
- most prominent histologic feature are the atypical spindle cells seen between regularly arranged osseous trabeculae
- some cellular atypia should be seen to make diagnosis
Histology can be mistaken for fibrous dysplasia
high rate of MDM2 amplification and ring chromosome

Treatment

Groups & Differentials

	Bone surface lesion (cortex)	Similar Histology	Treated with surgery alone (2)
Parosteal osteosarcoma	•		•
Periosteal osteosarcoma	•
Periosteal chondroma	•		•
Osteochonroma / MHE	•		•
Developmental defect	•
Myositis ossifican	•
Chondrosarcoma			•
Adamantinoma			•
Chordoma			• (controversial)
Fibrous Dysplasia		•
ASSUMPTIONS: (1) Older patient is > 40 yrs; (2) assuming no impending fracture

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