Teaching Case

A 38-year-old female is referred for imaging of a mass around the right iliac crest. She noticed the lump 5 years ago, progressive growing since 3 months. The mass is painless, but there are esthetical and psychological implications.

Radiography (Figure 1) shows an ill-defined lesion containing calcifications, projected over the right iliac bone. The exact relationship to the bone is difficult to evaluate.

CT scan (Figure 2) shows a pedunculated lesion, arising from the medial cortex of the spina iliaca anterior superior. There is continuity between the medullary and cortical bone of the lesion and the iliac wing. The lesion is surrounded by a cartilage cap containing ring-and-arc calcifications. There is contact between the cartilage cap and the lamina interna of the ala ossis ilii. Cortical disruption is absent.

MRI (Figure 3) shows a thick lobulated cartilage cap, compressing the overlying abdominal musculature and the psoas muscle. The lesion is predominantly hyperintense on T2-WI (WI) and iso-intense to muscle on T1-WI, except for the intralesional calcifications which are of low signal. Dynamic contrast MRI reveals lesion enhancement exceeding 1,5 x that of muscle.

Chondrosarcomas are malignant cartilaginous tumours accounting for about 25% of all primary malignant bone tumours. They can be divided into three grades based primarily on cellularity. Chondrosarcomas are either primary or secondary arising from a pre-existent cartilaginous lesion such as osteochondroma (aka cartilaginous exostosis) or enchondroma [1], [2].

Patients, typically in the 4^th - 5^th decade, may present with pain, pathological fracture, palpable lump or local mass effect. Chondrosarcomas are mostly seen in the long bones (45%) such as the femur (20-35%), tibia (5%) and proximal humerus (10-20%), as well as in the iliac bone (25%). Unfortunately, chondrosarcomas are usually large at the time of diagnosis (> 4 cm). Early detection hence, is mandatory to avoid mutilating surgery [1], [3].

Imaging is important to differentiate benign osteochondromas from chondrosarcomas and to identify hereditary multiple exostoses syndrome, who is more likely to become malignant. Malignant degeneration occurs in 1% of solitary osteochondromas and in 5-25% with hereditary multiple exostoses [4].

On plain radiography and CT, the cartilage cap may be thin and difficult to identify, or thick with rings-and-arcs calcifications and irregular subchondral bone. New cortical irregularity or continued growth after skeletal maturity, as well as frankly aggressive features (e.g. bony destruction, large soft tissue component, metastases) are all worrying for malignant transformation to chondrosarcoma. 

MRI is the best imaging modality to assess malignant degeneration by evaluating the cartilage thickness. In adults, a cartilage cap of over 1.5 cm in thickness is suspicious for malignant degeneration, while the cartilage cap may be seen up to 3 cm in thickness in young patients [5]. 

In general, a chondrosarcoma arising in an osteochondroma is multilobulated with central high water content (chondroid matrix) accounting for the high T2 – low T1 signal. The peripheral enchondral ossifications result in the typical ring-and-arcs or popcorn calcifications. Gradient echo shows blooming of mineralized/calcified components. Enhancement of benign lesions is normally seen in the tissue that covers the cartilaginous cap which is fibrovascular in nature. The cartilaginous cap itself should not enhance. 

If malignant transformation of an osteochondroma occurs, then the chondrosarcoma is usually of low grade (67-85% of cases), and surgery is usually curative (70-90%). Limb-sparing wide local excision usually suffices [2], [6].

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