Parosteal Osteosarcoma

Introduction

A low grade osteosarcoma Age & location more common in females, age 30-40 previous exam question ages: 75, 74 yrs occurs on surface of metaphysis of long bones most common sites include posterior distal femur, proximal tibia, and proximal humerus marrow invasion in 25%

Symptoms

Presentation often presents as a painless mass can limit joint motion (knee)

Imaging

Radiographs heavily ossified, lobulated mass arising from cortex (appears as if sticking to cortex) Bone scan mandatory and always hot CT chest mandatory to rule out pulmonary mets MRI mandatory to determine soft tissue involvement and skip lesions

Histology

Characteristic histology regularly arranged normal osseous trabeculae  slightly atypical spindle cells within trabeculae cartilage is often present and may take the form of a cartilage cap Pathologist ocassionally mistakes for fibrous dysplasia

Treatment

Operative wide local surgical excision often curative chemotherapy not indicated unless there is a high grade component

Prognosis

95% long term survival when local control has been achieved dedifferentiation is a poor prognostic factor

Groups & Differentials

Fibrous dysplasia (similar on histology, but xrays are different) Myositis Ossificans Traumatica (juxtaposed to bone) Osteochondral exostosis (shares cortex with bone) Developmental defect at insertion of adductor magnus