Teaching Case

A 32-year-old male was referred to our Sarcoma Reference Centre regarding a growth in the right groin. The patient was otherwise asymptomatic. Physical exam showed vascular and nerve compression phenomena, with signs of venous stasis in the right lower limb and paraesthesia on the lateral aspect of the right thigh.

Plain anteroposterior and oblique radiographs of the right femur (Figure 1) showed a 10 cm-wide exophytic mass emerging from the anterior wall of the right acetabulum, with femoral neck widening.

A skeletal survey was obtained (Figure 2), showing multiple smaller bony lesions located in the ribs, forearms and legs. There was also metaphyseal widening in long bones (femora, tibiae), also known as Erlenmeyer flask deformity, and shortening of both ulnae with deformity of the radius diaphysis (pseudo-Madelung deformity). Hence, the diagnosis of hereditary multiple exostoses was made.

A CT scan (Figure 3) showed cortical and marrow continuity between the mass and the parent bone. In MRI (Figure 4), there was a layer of tissue of up to 4 cm covering the bony lesion, with intermediate signal in T1wi and high signal in T2wi, consistent with a cartilaginous cap. It was suggestive of osteochondroma with malignant transformation, which pathology confirmed.

Background

Hereditary multiple exostoses (HME) is an autosomal dominant condition characterised by the presence of multiple osteochondromas (OC) and exostoses [1].

Malignant transformation is the most important complication of OCs, with an incidence of 1% in solitary OCs and an incidence of 3-20% in HME. It also occurs at a younger age in patients with HME, the average age being 25-30 years versus 50-55 years in solitary OC [1, 2].

Clinical perspective

In HME, masses are usually asymptomatic, with occasional mild pain or mass-effect related symptoms. Lesions that grow or cause pain after skeletal maturity should be suspected of malignant transformation, as in this particular case [1].

Imaging perspective

Plain radiograph remains the mainstay of radiological diagnosis in HME. The findings are single or multiple exophytic masses near the joints of long bones. The most common locations include the distal femur, proximal tibia, wrist and hand, humerus, ankle, pelvis and ribs. The key radiologic feature is cortical and marrow continuity between the lesion and parent bone, often with a cartilage cap. Widening of the distal femora (Erlenmeyer flask deformity) and a more unusual shortening of the ulnae (pseudo-Madelung deformity) might be present, as in our case [2, 3].

Some features of malignancy of an OC have been described, such as growth after closure of growth plates, irregular margins and central location (pelvis, hips, shoulders); but the most reliable indicator of malignancy is the thickness of the cartilage cap. Studies have proven that a cut-off set at 2 cm can have a sensitivity and specificity of 100% and 98%, respectively, in adults. In children, the limit is set at 3 cm [4].

MRI is the best method for imaging the morphology of tumours, including cartilaginous ones. The medullary and cortical continuity with the underlying bone is confirmed. The cartilage cap has a low-intermediate signal on T1-weighted images and high signal on T2-weighted images due to its high-water content. Septal enhancement after gadolinium administration suggests malignant degeneration. MRI also provides information about possible complications (vascular compromise, neurologic sequelae, etc.) [2, 4].

Outcome

OCs with malignant transformation often require large surgical resection. Preoperative MRI is essential for procedure planning [1].

Secondary chondrosarcomas are mostly low-grade with good prognosis: 70-90% long-term survival, 0-15% recurrence rate after complete resection and distal metastases in only 3-7% of patients. Moreover, a recent systematic review suggested that annual MRI screening in young patients with HME may be effective [5].

Written informed patient consent for publication has been obtained.