Summary: A 62-year-old male presents with a slowly enlarging gluteal mass.
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.
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
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.
- Murphey MD, et al. Imaging of osteochondroma: variants and
complications with radiologic-pathologic correlation. Radiographics. 2000;20:1407-1434.
- 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.
- Hudson TM, Springfield DS, Spanier SS, Enneking WF, Hamlin DJ. Benign exostoses and exostotic
chondrosarcomas: evaluation of cartilage thickness by CT. Radiology.