Musculoskeletal case of the day. Extraskeletal osteosarcoma of the thigh.

Extraskeletal osteosarcoma is a malignant mesenchymal neoplasm that produces neoplastic osteoid, bone, and occasionally chondroid material [1-4]. It is located within the soft tissues without attachment to the underlying bone and, therefore, is appropriately considered extraskeletal [2, 4, 5]. Compared with osteosancoma of bone, extraskeletal osteosarcoma is rare, as reflected in a relativeratioof approximately 25:1 [2,4, 6, 7]. Patients with osteosarcoma of bone typically present between the ages of 10 and 25 years, whereas those with extraskeletal osteosarcoma are older [1 , 3, 4, 7-9]. Among othens, Enzinger et al. [1 , 2] maintain that the mean age at presentation is 54 years. Scattered reports of extraskeletal osteosancomas arising in diverse locations such as breast, lung, thyroid gland, kidney, prostate, and urinary bladder have been published [2, 3, 6, 10, ii]. The most frequent site of involvement is the lower extremity, especially the thigh [1 , 3, 4, 7-9, 12, 13]. The typical clinical finding is a slowly growing, soft-tissue mass that may be (but is not invariably) painful [1-3, 7-9]. Most lesions are deeply seated and fixed to the soft tissues [1 , 2]. The most common sites of metastases are lung and liven. Labonatory findings are usually within normal limits except for alkaline phosphatase activity, which can be elevated, especially with metastases [2, 4, 9]. It is speculative what role, if any, antecedent trauma plays in the development of extraskeletal osteosarcoma, which has also been reported as a complication of prior radiation therapy [1-4, 6, 7, 9, ii , 1417]. Nevertheless, the patient described here had neither prior trauma non radiation exposure. The prognosis of this tumor is poor, and most patients die of metastases within 2-3 years from the time of initial diagnosis [1 , 3, 4, 6, 9]. Early combination therapy consisting of radical or limb-sparing resection (if possible), radiation thenapy, and adjuvant chemotherapy constitutes the current regimen that is most likely to achieve prolonged survival [1-4, 6, 7,9]. The imaging studies of this patient showed some particulanly interesting features (Fig. 1). On the chest radiograph (Fig. 1 B), the pulmonary parenchymal metastases were disproportionately “clustered” around the heart-findings confirmed by CT and scintignaphy. In fact, the CT scans and scintigrams (Figs. 1C and 1 D) suggested possible penicardial metastases. When the scintigrams were interpreted mitially, metastases to ribs could not be excluded, although when correlated with the findings on the chest radiograph, this possibility was deemed unlikely. Both CT scans and MR images (Figs. 1 D-1 F) suggested that the extraskeletal osteosarcoma was well marginated. These tumors are typically delineated by a “pseudocapsule,” however, beyond which additional malignant material is frequently encountened on careful histologic examination [1]. The MR image (Fig. 1 F) also revealed lobulation within the tumor, but the significance of this finding is unclear [1]. Skeletal metastases from extraskeletal osteosarcoma have been reported infrequently, and we were unable to find any radiologic illustrations of this phenomenon [4, 8]. Because of the largeness of the soft-tissue mass in this case, we believe that the malignant tumor arose in the thigh. On the radiographs, no lesion was discernible in the proximal diaphysis of the right femur. In this latter location, however, abnormalities were noted on the scintigrams, CT scans, and MR images (Figs. iC-iF), strongly suggestive of a skeletal metastasis, particularly considening the extensive metastases in the chest. The scintigram also suggested several metastases in the pelvis, although this area was not assessed by CT or MR imaging. The radiographic differential diagnosis of extraskeletal osteosarcoma includes synovial sarcoma, chondrosarcoma of soft tissue, liposancoma of soft tissue, and myositis ossificans [3]. Because of the extensive mineralization within the lesion (Fig. iA), only myositis ossificans was a reasonable alternative diagnosis to extraskeletal osteosarcoma [3, 7]. Naturally, the extensive metastatic disease excluded the diagnosis of myositis ossificans. In addition, on CT scans (Fig. 1D), the mineralization within the mass was greatest in