Applied Radiology

Diagnosis

Chondrosarcoma

Findings

Contrast-enhanced computed tomography (CT) images demonstrate a large lobulated mass involving the posterior aspect of the sacrum and left iliac wing with involvement of the left S2 neural foramen. The mass extends into the left obturator internus, piriformis, paraspinous, and gluteal muscles. There are numerous calcifications throughout the mass with a “rings and arcs” morphology, consistent with chondroid matrix. Further evaluation of the remainder of the examination reveals numerous bony excrescences along the left lesser trochanter, the left ischium, the right iliac wing, and multiple right sided ribs. The excresences demonstrate cortical and medullary continuity with the underlying bone, consistent with osteochondromas.

Discussion

Osteochondromas are the most common benign bone tumor, representing approximately 40% of all benign bone tumors. They are cartilage-capped excrescences, which demonstrate cortical and medullary continuity with the underlying bone. Approximately 90% of lesions are solitary with the remaining 10% occurring in patients with the autosomal dominant syndrome, hereditary multiple exostoses (HME). Osteochondromas are often asymptomatic when small and solitary, and are found incidentally on imaging exams. Symptomatic lesions often present with physical deformity, pain due to fracture, impingement of nerves or tendons, vascular compromise, development of an overlying bursitis, or malignant transformation. The incidence of malignant degeneration to chondrosarcoma occurs in <1% of solitary osteochondromas, but the incidence of malignant transformation is approximately 3% to 5% in patients with HME. The lesions almost always degenerate to chondrosarcomas, although they may also degenerate to osteosarcoma and malignant fibrous histiocytoma.

Osteochondromas occur most commonly in the metaphyses of the extremities (95%). Approximately 40% occur in the femur, 20% in the tibia, and 10% to 20% in the humerus. The remainder usually occur within the small bones of the hands and feet, scapula, pelvis, and spine. They may develop in a sessile or pedunculated form. The sessile form is more common and may have the appearance of an undertubulated bone. The pedunculated form is more easily detected due to their size with the masses extending away from the joint. The pedunculated form is also more likely to demonstrate enchondral calcifications. Osteochondromas tend to stop growing after skeletal maturity, an important characteristic for evaluating follow-up examination.

Osteochondromas are diagnosed with plain radiographs, but the previously mentioned complications are best imaged using magnetic resonance imaging (MRI). Patients with known HME are often followed sporadically with plain radiographs although routine follow-up imaging is not recommended. Plain radiographs are also performed to assess for complications when symptoms arise. Imaging findings suggestive of degeneration to chondrosarcoma include destruction of the adjacent bone, increase in size following skeletal maturity (often suggested by increased mass effect), focal lucency within the lesion, an irregular lesion surface, and change in the calcifications. MRI findings of osteochondromas include normal bone marrow extending into the exostoses, continuity of the cortex with the underlying bone, a high-signal cartilage cap on T2-weighted images, possible low-signal calcifications within the cap, and no associated soft-tissue mass. The thickness of a normal cartilage cap varies with age, often reaching up to 3 cm in growing adolescents, while it is often only several millimeters thick in a skeletally mature adult. Although there is no consensus cartilage cap thickness threshold for suspicion of chondrosarcoma, a reasonable cutoff is 1.5 cm. Unfortunately, bone scintigraphy is not useful for differentiating between metabolically active osteochondromas and chondrosarcomas.

Treatment of osteochondromas consists of watchful waiting and patient education regarding potential complications. Complicated lesions may be resected at their base with a recurrence rate of less than 2%. Patients with HME are much more likely to develop symptomatic lesions and average 2 to 3 procedures per patient over their lifetime. Malignant transformation to chondrosarcoma is treated with wide surgical resection and limb salvage. Radiation therapy and chemotherapy are reserved for cases of dedifferentiated tumors.

1. Murphey MD, et al. Imaging of osteochondroma: variants and complications with radiologic-pathologic correlation. Radiographics. 2000;20:1407-1434.
2. Malghem J, Vande Berg B, Noel H, Maldague B. Benign osteochondromas and exostotic chondrosarcomas: evaluation of cartilage cap thickness by ultrasound. Skeletal Radiol 1992;21:33-37.
3. Hudson TM, Springfield DS, Spanier SS, Enneking WF, Hamlin DJ. Benign exostoses and exostotic chondrosarcomas: evaluation of cartilage thickness by CT. Radiology. 1984;152:595-599.