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Extraskeletal osteosarcomas are rare malignant mesenchymal neoplasms characterised by the direct production of osteoid or bone by tumour cells. By definition, they are located in the soft tissues without primary bone or periosteal involvement. The most common location of these tumours is the lower extremity, especially the thigh, followed by the upper extremity and the retroperitoneum.1–4
We report the radiological presentation of a retroperitoneal extraskeletal osteosarcoma, which may be helpful in the consideration of its differential diagnosis.
A 68 year old man presented with a painless palpable mass in the right side of the abdomen. A previous trauma was denied. The patient was not on anticoagulant medication. Except for a cholecystectomy in 1986, his medical history was unremarkable. Physical examination revealed a firm mass measuring approximately 10 × 10 cm. Laboratory findings were within normal limits, with the exception of a slightly raised alkaline phosphatase concentration of 140 U/litre (normal range, 40–120).
Plain radiography of the abdomen demonstrated a large ill defined dense lesion projecting over the right side of the pelvis. Contrast enhanced helical computed tomography (CT) of the abdomen identified a large, non-homogeneous soft tissue mass in the right side of the retroperitoneum (fig 1). The tumour measured 9 × 12 × 14 cm. The medial part of the mass was predominantly mineralised; the lateral side showed a large soft tissue mass with low density in the centre suggestive of necrosis or haemorrhage. The radiological features suggested an osseous, rather than chondroid, nature because of the poorly defined and homogeneous aspect of the mineralisation. The upper border of the mass was in close anatomical proximity to, but clearly separate from, the adjacent right kidney on three dimensional reformatting. The tumour definitely did not arise from adjacent osseous structures and the psoas muscle was compressed by the tumour.
Magnetic resonance imaging (MRI) demonstrated a mass surrounded by a pseudocapsule near but not originating from the lower pole of the right kidney. In addition to the ossified zone, the mass contained areas of necrosis, old haemorrhage, or secondary lacunae formation filled with protein substance indicated by intermediate signal intensity on T1 weighted sequences and very high signal intensity on T2 weighted images. Based on the clinical history and radiographic findings, the diagnosis of an extraskeletal osteosarcoma was suggested.
Macroscopic examination of the resected specimen revealed a 19 × 12 × 9 cm tumour including the resected margins, partly bony, partly firm, partly weak of consistency, with a white pink colour. There was a large cystic area, measuring 7 × 6 cm filled with serous fluid.
Microscopically the tumour was composed of storiform oriented bundles of spindle shaped tumour cells, admixed with areas of polygonal shaped tumour cells with abundant deposition of primitive osteoid matrix in between (fig 2). The osteoid matrix showed a trabecular arrangement and was focally admixed with chondroid forming areas. In these fields, the tumour cells showed lacunae. In all areas the tumour cells showed moderate pleomorphism and mitotic activity of up to eight mitosis/mm2. Scattered areas of necrosis were seen. No relation with a pre-existing nerve could be documented. Immunohistochemically, the tumour showed diffuse reactivity with antibodies against vimentin and focal reactivity with antibodies against the S-100 protein in the chondroid containing fields and the spindle cells. Antibodies directed against neurofilaments and p53 showed no reactivity. The differential diagnosis included high grade extraskeletal osteogenic sarcoma, malignant peripheral nerve sheath tumour with heterologous elements, and dedifferentiated liposarcoma. A combined liposarcomatous part was not identified. Because of the lack of an identifiable nerve, the morphology of the spindle cells, focal reactivity only with S-100, and neurofilaments being negative, the diagnosis was extraskeletal osteogenic sarcoma.
Extraskeletal osteosarcoma is a rare tumour, constituting approximately 1% of all soft tissue sarcomas and approximately 4% of all osteosarcomas.1–3 Although primary osteosarcomas of bone occur predominantly in the first decades of life, extraskeletal osteosarcomas are rarely encountered under 40 years of age.4
The pathogenesis of the tumour is unclear; the tumour may occur and be induced at sites that have received previous radiotherapy. In addition, a history of trauma has been reported in 12–30% of patients. There are cases described in which extraskeletal osteosarcoma is presumed to be preceded by myositis ossificans lesions.1–3
Few reports of extraskeletal osteosarcoma have detailed the radiological findings of this rare neoplasm.5–9 The imaging techniques showed a large soft tissue tumour, for a large part demonstrating ossification, located in the retroperitoneum. Another primary osteosarcoma of bone was not found elsewhere in the body. On T1 weighted sequences the tumour was hypointense and isointense compared with muscle, and exhibited high signal intensity on T2 weighted imaging in the lateral part of the tumour, suggesting necrosis, haemorrhage, or secondary lacunae formation filled with protein substance. This latter correlated with the histological findings. Compression but no involvement of the psoas muscle, as visualised by CT, was confirmed.
The radiological differential diagnosis of extraskeletal osteosarcoma includes benign and malignant lesions that show mineralisation. The most important benign lesions are calcified haematoma and myositis ossificans. Several mesenchymal tumours can show reactive or metaplastic bone formation—for example, synovial sarcoma, epithelioid sarcoma, liposarcoma, and malignant peripheral nerve sheath tumour.4 Both possible benign lesions could be ruled out. The first because the patient definitely denied previous trauma. Furthermore, the patient did not use anticoagulant medication and the aorta was normal on all studies. Myositis ossificans was unlikely because there was no previous trauma and because of the large size of the lesion. Most myositis ossificans lesions measure 3–6 cm in diameter.4 Moreover, this lesion demonstrated ossification throughout a large part of the tumour and not at the periphery as is seen in myositis ossificans. Furthermore, the adjacent muscles were normal. Differentiating our patient's tumour from other malignant retroperitoneal sarcomas that can show bone formation is more difficult.
In conclusion, this case demonstrates that radiological imaging can help in the diagnosis of extraskeletal osteosarcoma. However, a biopsy is mandatory for a definitive diagnosis.