Parosteal Osteosarcoma:
- See:
- Bone Tumor Menu:
- Classic Osteosarcoma
- Discussion:
- parosteal osteosarcoma is a low-grade malignant bone tumor that usually occurs on the surface of
the metaphysis of long bones;
- arises between cortex and muscle as a low grade stage I-A surface tumor (ie does not initially
invade underlying medullary canal);
- most common in adolescents and yound adults;
- parosteal osteosarcoma is distinguished from classic osteosarcoma
by its much slower, less aggressive clinical course.
- there is a low propensity to metastasize;
- about 10% of parosteal tumors exhibit areas of dedifferentiation into
high grade sarcoma & are thus considered stage IIb lesions.
- tumor remains separated from normal bone, especially in the early stage;
- extension into the underlying bone is associated with a higher incidence of dedifferentiation and pulmonary metastasis;
- as growth continues, radiolucent zone between tumor & underlying bone may be obliterated as cortex becomes involved;
- late in dz, tumor extends through the underlying cortex to invade medullary canal as well, converting to a stage Ib tumor;
- invasion into the overlying displaced soft tissues is rare;
- differential dx:
- Osteochondroma;
- myositis ossificans,
- Periosteal chondroma,
- Location: often presents as a fixed, painless mass on:
- proximal humerus is the second most common location;
- posterior aspect of the distal femur (50% of cases);
- in this location, its slow growth may result in late invasion of the underlying cortex as well as circumferential
growth around the anterior aspect of the femur;
- when the tumor invades the canal, its stage changes from Ia to Ib;
- Radiographs:
- dense, heavily ossified, broad based fusiform mass that appears to encircle the metaphysis.
- tumor is separated from cortex by thin, uninvolved, radiolucent zone;
- medullary canal should not show tumor involvement;
- Histology:
- irregular "matured" bony trabeculae;
- osteoid trabeculae lie parallel to one another in a hypocellular stroma;
- regularly arranged trabeculae have intervening spaces w/ atypical cells;
- trabeculae have pattern of cement lines similar to those of Pagets Dz;
- trabeculae may contain varying degrees of cartilage;
- diff dx:
- Fibrous Dysplasia;
- surface location of parosteal osteosarcoma excludes dx of fibrous dysplasia;
- parosteal osteosarcoma may be underdiagnosed as benign due to benign appearing histologic features;
- dedifferentiation will have a worse prognosis;
- Treatment and Prognosis:
- wide excision w/ limb salvage is treatment of choice;
- prognosis is good & chemotherapy is not indicated;
- w/ indication of inadequate margins or with tumor invasion into soft tissues will require postop XRT;
- in the report by VO Lewis et al (J Bone Joint Surg [Am] 82-A: 1083-8, 2000), the authors report on a new
technique for operative management of parosteal osteosarcoma located in the popliteal fossa;
- method involves resection of the mass through separate medial and lateral incisions, which allows
for wide margins yet limits the amount of dissection of the soft tissues and the NV bundle.
- 6 patients with parosteal osteosarcoma located on the posterior aspect of the distal part of the femur
underwent resection of the lesion and reconstruction with a posterior hemicortical allograft through
dual medial and lateral incisions;
- average time until the last follow-up assessment was 4.3 years.
- no metastases developed, and there were no local recurrences.
- technique advantages:
- two incisions allow the surgeon to clearly visualize the sites of the osteotomies and to accurately define
the margins of the resection;
- NV bundle is easily dissected from the posterior aspect of the tumor;
- most of the distal femoral articular cartilage is salvaged and the joint remains stable;
