Joint Program in Nuclear Medicine

Osteochondroma

C. Dong Park, MD

May 15, 1984

Presentation

A 28 year-old male had a 10 year history of a "lump" on his left buttock. This mass was radiographed and was told by his physician not to worry about it several year ago. Recently, the patient laims that the mass has been enlarging. Otherwise he is asymptomatic.

Imaging Findings

The plain x-ray of pelvis with spot view of ileum showed a large, irregular, calcified mass projecting out laterally and posteriorly from the left ileum. The bone scintigraphy showed markedly increased uptake in the mass.

Discussion

Osteochondroma is by far the most common of the benign tumors of the bone, comprising 9.3%% of all bone tumors in Dahlin's series. These are benign bone tumors characterized by cartilage capped bony growths that project from the surface of the affected bone. When the lesion is seen only in a single bone, it is called a solitary osteocartilaginous exostosis or osteochondroma. When the tumors are distributed over the skeleton and there is a familial history, they are then designated as multiple hereditary exostoses or osteochondroma. The individual lesions of solitary and multiple osteochondromas are basically identical radiographically and pathologically. The structure of the tumor consists of central core of bone marrow, which is identical to and continuous with the marrow of the underlying bone. This is enveloped by a cortex and periosteal covering, which are continuous with that of the affected bone. The tumor is capped by a layer of hyaline cartilage, which frequently extends well over the pedicle and sometimes approaches the normal cortex. The exostosis is produced by a progressive enchondral ossification of this growing cartilaginous cap. The cartilaginous portion of the osteochondroma acts as an enchondral plate for this abnormal growth and persists as long as there is growth activity. The growth usually stops when the nearest epiphysis has closed. The great majority of these tumors then becomes inactive during the adult life. Subsequently the cartilaginous cap tends to involve, but remnants usually persists.

A solitary osteochondroma usually arises in the areas where cartilage is ordinarily found. Although the lesions are not infrequently seen in the pelvis, most are seen in the metaphyseal regions of the long bones. About 1/3 are located in the distal femur and another 1/3 equally divided between proximal humerus and proximal tibia. About 7%% are seen in the ileum. In multiple hereditary osteochondroma, the lesions number from a few to a thousand with an average of 10 per patient. They are most frequently located in the metaphyseal portion of the long bones, particularly in the knee, ankle, shoulder and wrist.

The tumor usually comes to attention in childhood or adolescence, the greater majority in the second decade of life. The patient usually notices firm swelling not associated with any symptoms. If the tumor becomes large and bulky or located near nerves and blood vessels, it may cause discomfort by compression of the surrounding structures. Uncommonly, a fracture through the stalk can cause severe pain and hematomas. More uncommonly there have been reported cases of cauda equina syndrome from bulky osteochondroma of the lumbar spine and cases of traumatic aneurysm of popliteal arteries from sharply pointed osteochondroma.

The difficulty in the patient management is due to the fact that the remnants of the cartilage cap may occasionally, after a latent interval of many years, show renewed growth and undergo malignant transformation to chondrosarcoma. Secondary chondrosarcoma occurs in 0.5-1%% of the patients with solitary osteochondromas; a routine prophylactic surgical removal of the tumor is not advocated. The rate of occurrence of chondrosarcoma in multiple hereditary exostosis is much greater; the rate ranges from 5-25%%. The development of secondary chondrosarcoma is usually slow and it may take months to years for its evolution. Such tumors may reach significant size before being clinically recognizable. Therefore, it is strongly emphasized that any osteochondroma in adults, and occasionally even in young patients, which take on a spurt of growth, should be regarded as chondrosarcoma. Such tumors are widely excised surgically. This is the basis for Epstein degeneration as such, but it will accurately distinguish those osteochondromas which are growing from those which are quiescent earlier than the plain radiographs. They suggest that the bone scan is the better choice for periodic routine surveillance of adult patients with osteochondroma.

Hudson, in his series where histologic correlation was made with radionuclide uptake, showed that the increased uptake in benign osteochondroma occurred in areas of enchondral ossification, and uptake in chondrosarcomas occurred in areas where ossification, osteoblastic activity, and hyperemia were found. The uptake was not related to the amorphous cartilage calcification. Radionuclide uptake correlated with areas of ossification visible radiographically, and large masses of nonossifying cartilage were not detected. They also noted that there was large overlap of intensity of uptake between the benign and malignant exostosis. In their series, intense uptake raised the suspicion of malignancy, but a normal study did not exclude the possibility of malignancy.

Conclusion:

Bone scan cannot distinguish between benign and malignant exostosis, and is not an adequate modality for anatomic staging. Thorough anatomic staging of chondrosarcomas rests on careful CT and sometimes angiography. There may be some potential for early detection of spurt of growth that may signal malignant transformation, but this needs further study.

References

1) Dahlin DC. Bone Tumors. Springfield: Thomas, 1978.

2) Wilner D. Radiology of bone tumors and allied disorders. Philadelphia: Saunders Company, 1982.

3) Hudson TM, et al. Scintigraphy of benign exostoses and exostotic Chondrosarcomas. ÊAJR 1983; 140:581-586.

4) Epstein DA, Levin EJ. Bone scintigraphy in hereditary multiple exostoses. AJR 1978; 130:331-333.

5) Simon MA, Kirchner PT. Scintigraphic evaluation of primary bone tumors. J of Bone and Joint Surgery. 1980; 62A:758-764.

6) Kenney PJ, et al. The use of computed tomography to distinguish osteochondroma and chondrosarcoma. Radiology 1981; 139:129-137.

7) Garrison RC, et al. Chondrosarcoma arising in osteochondroma. Cancer 1982; 49:1890-1897.

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J. Anthony Parker, MD PhD, Tony_Parker@bidmc.harvard.edu