summary Parosteal Osteosarcoma is a malignant, low-grade, osteosarcoma that occurs on the surface of the metaphysis of long bones. Patients typically present between ages 30 and 40 with a painfless mass. Diagnosis is made radiographically with a heavily ossified, lobulated mass arising from the cortex with biopsy showing cellular atypia seen between regularly arranged osseous trabeculae. Treatment is generally wide surgical resection. Epidemiology Demographics more common in females, age 30-40 Anatomic 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 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 Differential Fibrous dysplasia similar on histology, but xrays are different Myositis Ossificans Traumatica Osteochondral exostosis (shares cortex with bone) Developmental defect at insertion of adductor magnus Differential of Parosteal Osteosarcoma Bone surface lesion (cortex) Similar Histology Treated with surgery alone Parosteal osteosarcoma o o Periosteal osteosarcoma o Periosteal chondroma o o Osteochondroma / MHE o o Developmental defect o Myositis ossificans o Chondrosarcoma o Adamantinoma o Chordoma o (controversial) Fibrous Dysplasia o Treatment Operative wide local surgical excision indications standard of care in most patients technique many consider geometric osteotomy of involved bone to decrease long term morbidity and retain native joint chemotherapy chemotherapy not indicated unless there is a high grade component outcomes often curative 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