Aggressive aneurysmal bone cysts of the upper tibia mimicking giant cell tumour

1 Description

One 18-year-old male presented with a 2-year history of painful, progressive left knee swelling and deformity. He was diagnosed as having an expansile lesion of the left upper tibia which had undergone two open biopsies in Africa - giving varied histological diagnoses of an aneurysmal bone cyst (ABC) and giant cell tumour (GCT). Local examination demonstrated a healed scar, firm and tender bony swelling on the anteromedial aspect of the proximal leg, associated with a fixed flexion knee deformity of 30°. The plain radiographs (Fig. 1a and b) revealed a well-defined osteolytic lesion, eccentrically located in the epiphyseo-metaphyseal region of the proximal tibia, but not extending into the knee joint. It had a trabeculated matrix and minimal surrounding sclerosis. The Magnetic Resonance Imaging (MRI) revealed a 7x7x6.5 ​cm solid-cystic, expansile lesion, which was hypointense in T1W and heterogeneously hyperintense on T2W images. There was no breach of the articular congruity of the knee joint (Fig. 2a,b,c,d). A core biopsy of the lesion demonstrated some inflammatory cells with a predominant bloody aspirate. An extended curettage (with 1% phenol ablation) was done and the bone cavity was filled with allografts and bone cement (Fig. 1c, and 1d), via the ‘‘sandwich technique’’.¹ Histopathological examination (Fig. 3) illustrated large blood-filled cystic spaces with fibrous septa containing fibroblasts, occasional giant cells, and reactive woven bone rimmed by osteoclasts, suggestive of an ABC. He had an uneventful post-operative recovery period and was put on bisphosphonates.

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Fig. 1 a, b- Pre-operative Antero-Posterior and Lateral radiographs of the knee showing and expansile osteolytic lesion of the proximal tibia. c, d - Post-operative radiographs showing filling of the tumour cavity with bone cement and bone grafts.

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Fig. 2 a, b- T1 weighted coronal and sagittal cuts of MRI of the knee joint, showing an expansile osteolytic lesion involving the proximal tibia, with fluid. c, d- T2 weighted coronal and sagittal cuts of MRI of the knee joint, showing an expansile osteolytic lesion involving the proximal tibia, with fluid.

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Fig. 3 Histopathology (x100 magnification) showing a predominant blood mixed lesion with multiple fibrous septae and giant cells.

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Primary osteolytic bone lesions can either be benign or malignant and commonly occur around the knee. These can be difficult to differentiate using plain radiographs and MRI alone. Histological analysis is paramount to accurately identify the nature of the lesion. ABC accounts for 1% of primary bone tumors with a characteristic well-defined margin, lytic appearance with ballooning of the cortex on plain radiograph and fluid-fluid interface on T2 weighted MRI images.² They occur commonly in children and adolescents. GCT also similarly appears as a lytic lesion with a well-defined but non-sclerotic margin, eccentric location, extending into the subchondral bone. They often occur in patients with closed physes. GCT may exhibit aggressive features such as cortical expansion with soft-tissue invasion. Fluid-fluid levels, consistent with the secondary formation of ABC are seen in 14% of cases.³ GCT of bone are composed of mononuclear stromal cells and multinucleated giant cells. Secondary changes such as fibrohistiocytic or aneurysmal bone cyst-like components and coagulation necrosis have been noted in conventional GCT of bone and thus tend they to be an important differential diagnosis of each other.⁴ Other differentials include fibrous dysplasia, osteosarcoma, and chronic osteomyelitis. The use of bone cement with extended curettage, adjuvant therapy like phenol irrigation and the use of Denosumab or bisphosphonates helps in decreasing the recurrence of aggressive benign bone tumors.⁵

2 Take home messages

•Osteolytic bone lesions e.g. ABC, and GCT commonly occur around the knee and have a varied range of differentials that have similar clinical and radiological characteristics and thus require accurate histological analysis.•Secondary changes within GCT show ABC like components thus necessitating additional sampling to definitively confirm the diagnosis.

Consent

An informed consent of the patient was obtained.

Funding

This case report did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.