Clinical History
A 21-year-old female patient presented with swelling near the right knee associated mild pain for 2 months. She had been treated in another hospital one year before for trauma in the same region. No previous images were available for comparison.
Imaging Findings
AP & lateral radiographs of the knee showed a solitary, well-defined, expansile, osteolytic lesion with a narrow zone of transition, which was eccentrically placed and involved the metaphyseal region of the right tibia. The lesion showed cortical thinning with a pathological fracture. All imaging parameters on radiography pointed to a biologically non-active (benign) tumour. Further evaluation by MRI showed the lesion to be hypointense on T1 with multiple fluid-fluid levels on T2, STIR and PD images. Post contrast the lesion showed enhancement of the septae. Biopsy evaluation of the lesion showed proliferating spindle cells with giant cell lining blood-filled cystic structures.
Discussion
Aneurysmal Bone Cyst (ABC) is a benign bone lesion representing less than 3% of all primary bone tumours. It is of unknown origin and was first described in 1942 by Jaffe and Lichtenstein. Although it can affect any age, it is more commonly seen in the first two decades of life, with a slight female predominance. On histological analysis, ABC shows cystic spaces filled with blood, separated by a spindle cell stroma with giant cells showing osteoid production. ABC may also present as a solid variant which is rare and comprises 3-8% of all conventional ABCs. Three hypotheses have been postulated regarding the aetiopathogenesis, which are (1) the lesion is a sequela to an improper healing process following a traumatic subperiosteal haemorrhage; (2) a bone vascular disturbance; (3) bleeding into a pre-existing lesion. In nearly 30-35% of ABCs a pre-existing lesion may be identified, the most common being a Giant Cell Tumour (GCT). Other common pre-existing conditions include: osteoblastoma, angioma, chondroblastoma, fibrous dysplasia, fibrous histiocytoma, langerhans cell histiocytosis, osteosarcoma, and even metastatic carcinoma. [1, 2] The most common sites affected by ABC are femur, tibia, humerus, spine, and pelvis, and the small bones of the hands and feet. They are usually metaphyseal or diaphyseal in location and eccentrically placed, which helps in differentiating from a unicameral bone cyst. [2, 3] On radiographs ABCs present as an expansile osteolytic eccentrically placed lesion involving predominantly the metaphyseal region of long bones or the intramedullary region if involving the epiphysis. Occasionally ABCs may have an aggressive appearance with cortical thinning with or without erosion and periostitis. CT may show fluid- fluid levels, and are useful for identifying a fracture of the cortex when it is not apparent on radiographs. MRI shows macrolobulated, well-circumscribed cystic lesions containing multiple fluid-fluid levels which correlate with blood-filled spaces with non-endothelial lining. Fluid-fluid levels, while non-specific, can be seen in other conditions such as fibrous dysplasia, simple bone cyst, and chondroblastomas, and are a strong predictor for ABC. Various treatment modalities that have been used in the past for ABCs include surgical resection, radiation therapy, cryotherapy and embolization. At present curettage and filling with bone graft or polymethylmethacrylate are the preferred techniques. Radiotherapy is avoided as there is a risk for malignant transformation. Regardless of treatment method, a recurrence rate of 5-40% has been reported. [2, 4, 5]
Differential Diagnosis List
Aneurysmal bone cyst of the right tibia
Giant cell tumour
Unicameral bone cyst
Langerhans cell histiocytosis
Telangiectatic osteosarcoma
Final Diagnosis
Aneurysmal bone cyst of the right tibia
