Osteoblastomas are rare and benign primary bone tumours. They may be locally aggressive and tend to affect the axial skeleton more often than their histologic relative, osteoid osteoma.
They account for 1-3% of all primary bone tumours 2-3. Patients typically present around the second to third decades of life. There is a recognised male predilection with a male to female ratio of approximately 2.5:1.
With spinal lesions a painful scoliosis is a common presenting symptom. Otherwise, it presents with an insidious onset of dull pain, worse at night, with minimal response to salicylates in only 7% of patients (unlike osteoid osteoma). The area will characteristically be swollen and tender with a decreased range of motion.
Osteoblastoma is histologically similar to an osteoid osteoma except that it is much larger. The tumour is bone and osteoid forming and is comprised of osteoblasts. There is high associated vascularity.
- spinal column: ~40% (range 32-46% 2); often involves the posterior column
- cervical spine: 9-39% of all spinal lesions 2
- sacrum: 17% of all spinal lesions 3
- usually located in the metaphysis and distal diaphysis of the long bones
- aggressive (malignant) osteoblastoma: has a high of number epithelioid osteoblasts with nuclear atypia
Osteoblastomas can have a wide range of radiographic patterns. Lesions are typically larger than 2 cm in size 4-5.
- lesions are predominantly lytic, with a rim of reactive sclerosis
- tend to be expansile
- internal calcification may sometimes be present
- an associated soft tissue mass may also be present
- demonstrate a rapid increase in size with associated cortical expansion in the vast majority of patients, sometimes with cortical destruction
- there may be surrounding sclerosis or periostitis in up to 50%
- there may be a secondary aneurysmal bone cyst in 20%
- similar to the radiograph, lesions are often demonstrated as predominantly lytic
- internal matrix mineralisation is better appreciated on CT
MRI features tend to be non specific and often overestimates the lesion 9:
- T1: typically hypo to isointense on T1 with areas of decreased intensity that correspond to foci of calcification
T2: typically isointense to hypointense on T2 with foci of decreased intensity corresponding to the foci of calcification
- high signal may be seen in surrounding bone marrow and soft tissues due to oedema "flare phenomenon" 11
- C+ (Gd): this is a highly vascular tumor and therefore typically avidly enhances, with associated enhancement of the surrounding soft tissues 9
- Tc99m MDP or HMDL: often shows intense uptake although this is nonspecific and is typical in all lesions exhibiting increased bone turnover
Treatment and prognosis
Radical surgical excision is often the treatment of choice. Pre-operative embolisation is commonly carried out to reduce bleeding risk although surgery needs to be performed at the very short time interval in order to avoid reconstitution of collateral blood supply. Percutaneous ablation is an emerging modality for treatment of these lesions (as well as osteoid osteoma) 10.
Lesions are prone to extensive intra-operative bleeding due to intrinsic vascularity.
The differential diagnosis for bone tumours is dependent on the age of the patient, with a very different set of differentials for the paediatric patient.
- bone-forming tumours
- cartilage-forming tumours
- chondromyxoid fibroma
- fibrous bone lesions
- bone marrow tumours
- other bone tumours or tumour-like lesions
- aneurysmal bone cyst
- benign fibrous histiocytoma
- giant cell tumour of bone
- Gorham massive osteolysis
- haemophilic pseudotumour
- intradiploic epidermoid cyst
- intraosseous lipoma
- musculoskeletal angiosarcoma
- musculoskeletal haemangiopericytoma
- primary intraosseous haemangioma
- simple bone cyst
- impending fracture risk
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