case_images: PMC2833305_01
case_images: Abdominal CT revealed a large mass with diffuse calcification in the right retroperitoneal space.
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case_images: fig1
case_images: Abdominal CT showed a large mass with diffuse calcification of size about 130 x 110 mm in the right retroperitoneal space between the right kidney and the inferior vena cava, with slight hydronephrosis (Figure 1).
case_images: The cut surface of the retroperitoneal tumor, showing a solid, bony, and yellowish-white mass.
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case_images: PMC2833305_01_CRM2010-327591.002.jpg
case_images: fig2
case_images: Gross examination revealed a rounded, bony, and hard tumor of size about 140 x 110 x 80 mm and weight 1100 g that was yellowish-white in color (Figure 2).
case_images: (a) Low-power photomicrograph of the primary tumor, showing irregularly arranged woven bone trabeculae and atypical cartilage islands along with intervening fibrous tissue. (b) High-power photomicrograph, showing irregular osteoid seams with atypical osteoblasts and atypical chondrocytes within the cartilage matrix. These findings are consistent with low-grade osteosarcoma.
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case_images: fig3
case_images: Operative findings showed no continuity with skeletal bones, leading to a final diagnosis of well-differentiated (low-grade) extraskeletal osteosarcoma of the retroperitoneum (Figures 3(a) and 3(b)). 
case_images: High-power photomicrograph of the recurrent tumor, showing cellular proliferation of anaplastic tumor cells with many mitotic figures.
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case_images: fig4
case_images: These findings led to a diagnosis of high-grade malignant anaplastic spindle cell sarcoma, consistent with the recurrence of extraskeletal osteosarcoma (Figure 4).