Osteochondroma (osteocartilaginous exostosis) is one of the most common neoplasm occuring in the skeleton(although strictly speaking it is not a neoplasm). The cartilage capped subperiosteal bone projection accounts for 20-50% of benign bone tumours. They occur most frequently in the first two decades of life with a male to female ratio of 1.5:1.They occur most often in long bones, especially the distal femur and proximal tibia, with 40% of the tumours occurring around the knee.
Osteochondromas of the ribs are rare and nearly always arise at or near the anterior end of the rib. They cause expansion or deformity of the rib with calcification of the cartilaginous cap. They can be mistaken for an area of pneumonia or a solitary pulmonary mass. CT allows a more definitive evaulation, especially of the cap.
Osteochondromas are most likely caused either by a congenital defect or by trauma of the perichondrium which results in the herniation of a fragment of the epiphyseal growth plate through the periosteal bone. They can either be sessile or pedunculated (stalk-like), with a slender pedicle directed away from the joint, and appear in the juxta-epiphyseal location. They occur only in bones that develop from cartilage (endochondral ossification). They have been reported to occur as a result of radiation therapy in children. There is normally no further growth of the exostosis after epiphyseal fusion.
Clinically, they present with pain due to mechanical irritation or a painless mass. A fracture can occur through the stalk of the lesion. Occasionally they are an incidental finding.
Hereditary multiple osteochondromatosis (diaphyseal aclasis) is an autosomal dominant condition with lesions (both sessile and pedunculated) occurring on different bones or on the same bone. Nearly 50% of these patients have a rib lesion.
Plain films are normally sufficient to diagnose the condition. Sessile lesions cover a wide area and can cause metaphyseal widening. The cartilaginous cap displays irregular areas of calcification. CT is helpful in determining whether the marrow and cortices of the lesion are continuous with the bone. The thickness of the cartilage cap is best delineated on MRI. The cartilage cap ranges from 1mm to 6mm in thickness; over 2cm of cartilage or renewed growth is a sign of possible malignant transformation.
The differential diagnosis includes hereditary multiple osteochondromatosis (diaphyseal aclasis). The differential diagnosis of rib lesions includes enchondroma, osteoblastoma, osteoid osteoma, chondroblastoma and haemoangioma.
Complications may occur; osteochondromas in areas other than the knee are more likely to undergo malignant transformation. Secondary chondrosarcoma occurs in 0.5-1% of patients with a solitary osteochondroma. Chondrosarcoma transformation is more common in the hereditary form.Dynamic Gd-enhanced MR can be used to differentiate benign from low grade malignant cartilaginous tumor.Both early and exponential enhancement being predictors of malignancy.Gd-enhanced MR can further help in tumor mapping in case a biopsy is indicated. Spontaneous haemothorax or pneumothorax (as in this case) have been reported in rib lesions, as have fractures.
Pathologically, on gross examination, an osteochondroma is an irregular bony mass with a bluish grey cap of cartilage. Opaque yellow cartilage has calcification within the matrix. The base of the lesion has a rim of cortical bone and central cancellous bone. Microscopically, the lesion shows endochondral ossification on the basal surface of hyaline cartilage. The cartilage is more disorganised than normal, the thickness of the cap is usually 1-3mm, and greater thickness may imply malignant transformation.
No treatment is necessary for asymptomatic osteochondromas. If the lesion is causing pain or neurological symptoms due to compression it should be excised at the base. The prognosis of a solitary exostosis is excellent. In this case, the patient had surgery as she developed spontaneous pneumothorax and the diagnosis was confirmed at pathology.