Extraskeletal Osteosarcoma
Extraskeletal osteosarcoma is a rare entity accounting for less than 5% of all osteosarcomas, and is most commonly found in the soft tissues of the thigh and upper extremity [1]. Approximately 10% of extraosseous osteosarcomas occur in sites that have received prior radiotherapy [2].
Routine radiographs and CT of osteosarcoma usually show a large soft tissue mass dominated by an osteoid matrix which appears most dense centrally (
Fig. 1a, b, c). The non-mineralized portions of these tumors have a similar appearance to muscle, and are low to intermediate in signal intensity on T1 weighted images. The signal intensity increases on T2 weighted pulse sequences [3].
Differentiating between extraskeletal osteosarcoma and myositis ossificans can be difficult, however, since both extraskeletal osteosarcoma and myositis ossificans tend to occur in older patients (50 to 60 years of age), and although myositis ossificans classically shows peripheral density, both can appear dense centrally. Extraskeletal osteosarcoma will enlarge even when no history of trauma is present, while myositis ossificans tends to occur in locations that are susceptible to trauma, such as superficially around large joints or near the buttocks and tends to decrease in size with time.
Recently, a patient presented for mammography in which a newly palpated mass was present in the right breast without abnormal superficial skin changes (
Fig. 2a, b). Prior mammograms from one year earlier showed no evidence of soft tissue mass or calcification where the abnormality was noted on physical examination. The current examination demonstrated a well—circumscribed oval—appearing mass with internal calcifications. There were some sharp spiculated macrocalcifications as well as lobulated rounded calcifications present. At this time, the preliminary diagnosis was that the lesion represented a fibroadenoma, though the rather acute onset of both the mass and its internal calcifications was unusual. Surgical excision was performed. During the pathology examination, no fibroadenoma was seen. Immunoperoxidase stains for cytokeratin, an aid in diagnosing metaplastic carcinoma, were negative.
This mass proved to be a primary osteosarcoma of the breast. Although these lesions in the breast are associated with prior radiation therapy, our patient had none. Some reports indicate that these lesions can arise in a pre—existing fibroadenoma or Phylloides tumor [4].
Osteosarcoma of the Jaw
Gnathic osteosarcoma is an intramedullary form of osteosarcoma that affects the maxilla and mandible and demonstrates significant differences from the previously described osteosarcomas.
Histologically, gnathic osteosarcoma shows less anaplasia, is better differentiated, and tends to metastasize later than conventional osteosarcoma, allowing for a better prognosis [5]. Clinically, patients are older than the long bone osteosarcoma patients, commonly presenting in the fourth decade with jaw swelling or palpable mass. These lesions can be firm, painful or pain free, and can be associated with loosening of teeth or parasthesias. Pre—existing conditions including radiation therapy, Paget's disease and fibrous dysplasia have been reported [5]. Radiographically, these tumors are typically sclerotic due to the predominance of chondroblastic matrix. However, there are often associated areas of lytic bone destruction (
Figure 3). An aggressive periosteal reaction with long thin spicules of bone extending into a soft tissue mass is seen in almost all of these lesions [1].
Both cross sectional imaging with CT, which can identify the mineralized matrix, and MR imaging, which shows both intramedullary involvement and soft tissue mass, are advocated for staging workup [3]. This form of osteosarcoma tends to have low to intermediate signal intensity on T1 weighted MR images and high signal intensity with T2 weighting.
Conclusion
Our conclusion from this four—part series of articles is that osteosarcoma is the most common primary malignant bone neoplasm in young patients. This tumor can originate within the intramedullary space, cortex or bone surface as well as within the soft tissues. Knowledge of the various radiographic appearances and clinical presentations as well as possible predisposing entities is essential for accurate and confident diagnosis and treatment planning.
We feel that standard radiographs are a must in the initial workup of all types of osteosarcoma. MR imaging is indicated for intramedullary lesions, and MR imaging with CT scanning is most beneficial in lesions arising in bones that are difficult to radiograph in two orthogonal planes. CT scanning, with or without MR imaging, is useful for surface lesions. Our initial cross sectional imaging for suspected secondary osteosarcoma as well as extraskeletal osteosarcoma begins with MR imaging. Radionuclide total body bone imaging and routine chest radiography is used for initial evaluation in those lesions with a high incidence of metastasis. Total body bone MR imaging using Turbo STIR, T1 Turbo SE and single shot EP imaging to detect metastatic disease and may have promise for future studies.
