Osteoblastoma

Osteoblastomas are rare bone-forming tumours that may be locally aggressive. They are larger (>1.5-2;2 cm) and tend to affect the axial skeleton more often than their histologic relative, osteoid osteoma¹².

Epidemiology

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.

Clinical presentation

With spinal lesions, 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 (only 7% of patients respond, unlike osteoid osteoma). The area will characteristically be swollen and tender with a decreased range of motion.

Pathology

Osteoblastoma is histologically similar to an osteoid osteoma but they are larger. They are bone- and osteoid-forming and is comprised of osteoblasts. There is high associated vascularity.

Location

• spinal column: ~40% (range 32-46% ²); often involves the posterior column
  • cervical spine: 9-39% of all spinal osteoblastomas ² 
  • sacrum: 17% of all spinal osteoblastomas ³
• usually located in the metaphysis and distal diaphysis of the long bones

Variants

• aggressive (malignant) osteoblastoma: has a high of number epithelioid osteoblasts with nuclear atypia

Radiographic features

Osteoblastomas can have a wide range of radiographic patterns. Lesions are typically larger than 1.5-2 cm in size although smaller lesions may occur ^4,5. 

Plain radiograph

• lesions are predominantly lytic, with a rim of reactive sclerosis
• tend to be expansive
• may have a bubbly appearance ¹²
• 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%

CT 

• similar to the radiograph, lesions are often demonstrated as predominantly lytic 
• internal matrix mineralisation is better appreciated on CT

MRI

MRI features tend to be non-specific and often overestimate 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
  • a 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 tumour and therefore typically avidly enhances, with associated enhancement of the surrounding soft tissues ⁹

Nuclear medicine

• Tc-99m MDP or HMDL: often shows intense uptake although this is non-specific 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 a 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.

Complications

Lesions are prone to extensive intraoperative bleeding due to intrinsic vascularity.

Differential diagnosis

• osteoid osteoma: <1.5-2; 2 cm