We present a case of osteosarcoma arising in an osteochondroma of the right fibula in a 30-year-old woman. The available radiographic studies of the lesion were not suggestive of malignant transformation. The lesion and underlying bone were excised. Histologic examination showed a conventional high-grade osteoblastic osteosarcoma that focally eroded the fibrocartilaginous cap. The patient received postoperative chemotherapy and shows no evidence of disease 27 months following operation. The occurrence of osteosarcoma in an osteochondroma is an extremely rare event, and only a few cases are on record in the literature.

Osteochondroma or osteocartilaginous exostosis is a common benign bone lesion whose prevalence is unknown because many cases are asymptomatic.1 The occurence of malignancy in a solitary osteochondroma is exceptional.1 We present a case of such an osteosarcoma that arose in a slowly growing solitary osteochondroma.

A 30-year-old woman was referred to our institution from another hospital because of a firm, painless swelling on the lateral aspect of the proximal part of her right calf. She first noticed this swelling 10 years previously; it had slowly increased in size during the ensuing years. Radiographs taken 4 years previously at another hospital showed an osteochondroma of the fibula.

On physical examination, there was a firm, bony mass of 3 cm in its largest dimension on the lateral aspect of the right fibula just under its neck. It was not painful on palpation, and no neurologic or circulatory deficits were evident. An anteroposterior radiograph of the right leg showed a broad-based osteochondroma arising from the lateral and posterior cortex of the proximal diaphysis of the right fibula (Figure 1, A). On the lateral view, the cortex of the lesion merged continuously with the cortex of the fibula (Figure 1, B). In the periphery of the osteochondroma, scattered calcifications were present. No similar lesions were found in the rest of the skeleton by routine radiographic studies.

On operation, a 7.0-cm-long segment of cortical and underlying cancellous bone with the exostosis was resected. It was covered by some connective adipose and muscle tissue. Following operation, the patient received postoperative chemotherapy of doxorubicin hydrochloride and cisplatin. This treatment was interrupted after she had received 4 courses because of severe toxic complications. Twenty-seven months after operation, there is no evidence of local recurrence or distant metastases.

A gross, irregularly shaped 3.0 × 2.5 × 1.5-cm bony excrescence that was partially covered by a bluish cartilaginous cap was found. On cut surface, most of the lesion was represented by irregular and dense bone of cancellous appearance. The cartilaginous cap was of uneven thickness, being only 1.0 mm in maximum thickness; in lateral parts of the upper surface of exostosis, the cap appeared fibrous. The base of the resected specimen was represented by regular-appearing spongy bone.

Microscopically, the lateral surfaces of the exostosis were represented by cortical bone, and the upper surface was covered by a cartilaginous and fibrous cap. The spongy substance of the core of the lesion was filled by high-grade osteoblastic osteosarcoma of conventional appearance that eroded the inner surface of the cartilaginous cap (Figure 2, A). The cap was focally interrupted and replaced by fibrous tissue that was partly infiltrated but not overgrown by tumor tissue. Osteosarcoma cells produced a fair amount of osteoid but no cartilaginous matrix; many osteoclast-type giant cells were also present (Figure 2, B). The cartilage cap was paucicellular, with no atypical chondrocytes. Cortical bone on the lateral surface of the exostosis was also infiltrated by tumor in several foci but was still intact on its outer surface. Toward the base of the lesion, tongues of osteosarcoma tissue grew into cancellous substance of the parent bone. The resection margins of bone and soft tissues were free of tumor.

Malignant transformation of a solitary osteochondroma is a rare event, occurring in about 1% to 2% of cases,1 and the tumor is almost exclusively chondrosarcoma.2 In multiple osteocartilaginous exostoses, chondrosarcomatous change occurs more often, although the reported frequency varies considerably, from 5% to 25% of cases,3 with higher estimates most probably related to the selection bias.4 

In solitary osteochondroma, the occurrence of a sarcoma other than chondrosarcoma is exceedingly rare and only sporadically reported; it is usually in the form of osteosarcoma.4–10 We perused the English-language literature and performed a MEDLINE search from 1966 to April 1998, which produced 10 cases of osteosarcoma arising in osteochondroma. In the Mayo Clinic series of 1649 osteosarcomas, there were 3 cases that arose in osteochondroma; in 2 of those cases, the patients had a solitary osteochondroma, and the third case occurred in a patient with multiple exostoses.4 The rest of the published cases were single case reports; osteosarcoma in these cases arose either in a solitary osteochondroma5–9 or in association with multiple exostoses, in which one of the osteochondromas underwent malignant transformation.10–11 Although the number of the reported cases is evidently small, it appears that there is a higher proportion of those patients who are beyond the age when osteosarcoma appears most frequently.4–11 Rare cases of osteosarcoma arising in enchondroma are also on record,4,12,13 sometimes associated with enchondromatosis (Ollier disease).14 In addition to the cases discussed herein, there are also cases of so-called dedifferentiated chondrosarcoma that arise in osteochondromas and may have an osteosarcomatous component. Malignant transformation in such cases is stepwise: osteochondroma giving rise to peripheral low-grade chondrosarcoma that in turn dedifferentiate into a high-grade sarcoma that may appear as fibrosarcoma, malignant fibrous histiocytoma, and osteosarcoma.15,16 The latter cases are essentially chondrosarcomas arising in osteochondromas.

In our case, we were not able to obtain radiologic studies of the lesion that were done 4 years before the present admission. However, the plain radiographs of the current lesion were quite typical for osteochondroma and not suggestive of malignant transformation. In some previously reported cases of osteosarcoma arising in osteochondroma, some features in the radiographs of the lesion, such as heavily ossified foci admixed with lucent areas, suggested malignant transformation of osteochondroma.6,8–10 In other studies, as in our case, no such changes in radiographs were seen.5 The idea of a benign lesion was also reinforced by the history of long unaccelerated growth of the tumor; if any acceleration in growth of exostosis is reported, it should always raise a suspicion of malignant transformation.5–10 Moreover, the lesion was never painful, which is often the case in those osteochondromas that harbor malignant tumor.5–10 

Our case shows that malignant transformation in osteochondroma may not, at least in its initial stage, be heralded by any telltale radiologic appearances. The continuous, although slow, growth of the osteochondroma in adults should, however, alert the clinician to the possibility of malignant transformation, because osteochondromas cease their growth at the time of skeletal maturity.4 In such cases, the use of other imaging techniques is mandatory.

In regard to prognosis, the follow-up in most reported series was relatively short, and no firm conclusion could be given. There were, however, 2 long-term survivors among the 3 Mayo Clinic patients.17 

We wish to thank to K. K. Unni, MB, BS, Mayo Clinic, Rochester, Minn, who reviewed the case and concurred with the diagnosis.

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