Diagnosis

Giant cell tumor (GCT)

CASE FOLLOW-UP

An open bone biopsy was performed and the tumor was interpreted as a benign GCT (Figure 3). Because of the interruption of the subchondral bone with possible pathologic fracture and osteoarthritis, it was thought that the patient would be better served by resection of the tumor and reconstruction, rather than curettage and reconstruction. At surgery, the subchondral end of the tibia was fractured transversely, but there was no gross tumor at the joint. The tumor was very vascular. A radical resection of the tibia with reconstruction by an allograft and prosthetic device was performed. Thorough sampling of the resected tumor showed that part of the tumor was malignant with fibrosarcomatous differentiation. Vascular invasion was also identified (Figures 4 and 5).

The patient had an uneventful recovery. She has been regularly followed-up for 3 years. She walks with a cane and is doing well, with no evidence of either local recurrence or metastases.

Findings

Radiographs of the knee showed a large, sharply defined, lytic lesion that involved the posterolateral aspect of the proximal end of the right tibia. There were no matrix calcification, sclerotic changes, or perosteal bone formation. There was an interruption of the articular margin, an invasion of the lateral and posterior cortex with soft tissue extension, and advanced osteoarthritic changes (Figure 1). The MRI study showed the tumor mass to have a homogeneous low signal intensity on T1-weighted imaging (T1WI) (Figure 2A and 2B). The signal was heterogeneously intermediate on T2-weighted imaging (T2WI) (Figure 2C).

Discussion

Primary malignant GCT of the bone is extremely rare, and only a handful of well-documented cases have been reported in the English liter-ature.^1-3 It constitutes only 1% of all GCTs. Benign GCT has a slight female predominance.^1-4 Although the peak incidence of benign GCT is the third decade, the sixth decade of life is the most common for primary malignant GCT.^1,2 As in benign GCT, the most common sites of occurrence are around the knee, in the distal end of the femur, or the proximal end of the tibia.¹ Secondary malignant GCT constitutes roughly 4% of all cases of GCT and has the same sex and site distribution as primary malignant GCT, with peak incidence in the fifth decade.⁵ Radiographically, primary malignant GCT may be easily confused with aggressive benign GCT; in most in-stances, there is no clear-cut differentiation.¹

The preoperative diagnosis in the case reported here was aggressive benign GCT. Most GCTs exhibit low-to-intermediate signal intensity on T1WI, and intermediate-to-high signal intensity on T2WI. Tissue inhomogeneity has been noted on T2WI.⁶ The tumor in this case exhibited low signal intensity on T1WI, and heterogeneous intermediate signal intensity on T2WI.

Hutter et al⁷ and Dahlin⁸ have clarified the definition of malignant GCT of the bone, stating that it is a sarcoma of the bone arising either in a pre-existing GCT or in conjunction with and juxtaposed to a GCT. The concept of juxtaposition is critical to the definition, implying that there is no gradual transition between the 2 components, but rather an abrupt change that results in the characteristic bimorphic histologic appearance. Histopathologically, fibrosarcoma occurred 3 times as frequently as osteosarcoma in secondary malignant GCTs.⁵ In primary malignant GCTs, there are 2 patterns described: fibrosarcoma or malignant fibrous histiocytoma, or, in some cases, a combination of the 2 patterns.¹

Malignant GCTs can be divided into primary and secondary types; the primary lesions are those with a sarcomatous component that occurs de novo, and secondary lesions are those that occur after definitive therapy.^1-9 The vast majority of secondary malignant GCTs of the bone have occurred several years after radiation therapy.^1-8 Only a minority of cases are sarcomas that have arisen after multiple surgical resections for recurrent lesions or after a long latency period. McGrath⁴ has called these lesions evolutionally malignant GCT. A literature search revealed 2 cases of GCT of the proximal tibia, 1 reported by Sundaram et al⁹ and the other by Hefti et al.¹⁰ Both patients were young-20 and 31 years old, respectively. They were treated by curettage and filling of the defect by bone chips. In the first case, because of persistent pain 2.5 years following surgery, an open biopsy was performed, which revealed a poorly differentiated osteo-sarcoma. In the second case, also because of pain 6 years after an operation, an open biopsy was performed, which showed grade III osteosarcoma. These 2 cases may fit the description of evolutional malignant GCTs, especially because they were not treated by radiation therapy.

The distinction between primary versus secondary malignant GCTs is important. Based on the study of 8 cases of primary malignant GCT reported by Nascimento et al,^l these patients have a better prognosis than those with secondary lesions. This may be related, in part, to the lethal nature of the radiation-induced sarcomas that constitute the majority of secondary malignant GCTs. The follow-up and median 4-year survival (range 0.5 to 15 years) in the series by Nascimento and colleagues far exceeded the usual 1-year survival that is typical of radiation-induced sarcoma.^2-4

CONCLUSION

We have described a case of primary malignant GCT that was diagnosed only after resection of the whole tumor and after histologic examination of the whole specimen. This result indicates that such malignant GCT can be easily confused with an aggressive benign GCT and, in most instances, there is no clear-cut differentiation.

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