PMC2866243_01
(a) Axial MRI images of the iliac mass reveal its enlarged cartilage cap of 5-6 cm thickness and the nodular or ring and arc enhancement (large arrow). (b) Coronal MRI images reveal the enlarged cartilage cap, nodular or ring and arc enhancement (large arrow), and invasion of the inner table cortex by tumor (small arrow).
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fig1
Much of the lesion consisted of a 5-6 cm thick cartilaginous cap, metabolically active as seen by uptake of gadolinium on T2 images in a nodular  or ring and arc enhancement pattern, associated with a central iliac bony erosion (Figure 1).
(a)  Low-power micrograph (10x) of the low-grade chondrosarcoma: H&E stained normal cartilage (left) is separated by a fibrous band from an expansile nodule of malignant cartilage (right). (b) 40x photomicrograph reveals a region of typical benign cartilage of an osteochondroma, with cells of similar sizes organized in rows. (c) 40x photomicrograph of malignant cartilage demonstrating both the crowded conditions and the irregular configuration of the large nuclei in the malignant component.
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fig2
The areas considered to have progressed cytologically deviated from areas of the lesion that had features of an osteochondroma (Figure 2).
(a) Gross tumor specimen revealing cartilage overlying cortex of the ilium. (b) Gross tumor specimen revealing cortical invasion by cartilage.
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fig3
Invasion of the cortex of the ilium was evident on gross sections (Figure 3).
PMC2866243_02
(a) Anteroposterior radiograph reveals the large osteochondroma of the left distal femur in a 13-year-old with MO. (b) Lateral view further depicts the size of the large osteochondroma of the left distal femur. (c) Six-month follow-up anteroposterior radiograph, illustrating the obvious growth of the posterolateral lesion of the distal femur compared to the posteromedial osteochondroma, including erosion into the posteromedial osteochondroma by the tumor (arrow). (d) Six-month follow-up lateral radiograph reveals irregular calcifications within the cartilage cap, cortical erosions, and indistinct osseous margins, all suggesting malignant progression of the posterolateral osteochondroma of the distal femur.
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fig4
The preoperative radiographic appearance of this mass demonstrated a 10 x 10 x 7 cm lesion with a sclerotic margin at the distal femoral diaphysis displaying obvious growth, with an apparent 50% increase in volume over the preceding six months, including invasion of the cortex of the stalk of a nearby osteochondroma (Figure 4).
(a) CT scan includes both distal femurs prior to excision of osteochondromas. Contrast the benign appearance of the osteochondroma on the patient's right femur to the thickened cartilage cap with stippling on the left (arrow). (b)  MRI includes the left distal femur after resection of the benign posteromedial osteochondroma and biopsy of the posterior chondrosarcoma, with the thickened cartilage cap and invasion of femoral cortex (arrow) evident.
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fig5
Additional imaging studies including MRI and CT demonstrated stippling of the thickened cartilage cap and apparent invasion of the cortex of the shaft of the femur, with periosteal new bone being evident, suggesting malignancy (Figure 5).
(a) Low-power (10x) photomicrograph of the low-grade chondrosarcoma includes cells that are more crowded (small arrow) and disorganized (large arrow) than most osteochondromas. (b) High-power (40x) photomicrograph of chondrocytes of a low-grade chondrosarcoma is often enlarged (small arrow), and binucleation is common (large arrow).
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fig6
Histopathologic examination revealed a well-differentiated chondroid neoplasm composed of abundant hypercellular nodules of hyaline cartilage consistent with a low-grade chondrosarcoma arising from an osteochondroma (Figure 6), similar to findings noted on biopsy.