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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.
    - recognition of dedifferentiated areas with angiography can localize the area that should be biopsied in order to obtain the diagnosis;
    - 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 (heavy mineralization), 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;
                  - ref: The prognostic and therapeutic factors which influence the oncological outcome of parosteal osteosarcoma
          - w/ indication of inadequate margins or with tumor invasion into soft tissues will require postop XRT;
    - in the report by Lewis VO, et al (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
                   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 osteotomies and to accurately define margins of 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
          - Parosteal osteosarcoma of posterior distal femur. Oncological and functional results following a new resection technique.

The meaning of radiolucencies in parosteal osteosarcoma.

The surgical treatment of parosteal osteosarcoma in long bones.

Osteosarcomas arising on the surfaces of long bones.

Dedifferentiated parosteal osteosarcoma.

Parosteal Osteosarcoma. A Clinicopathological Study.