Osteoblastoma

Osteoblastomas are rare and benign primary bone tumors. They may be locally aggressive and tend to affect the axial skeleton more often than their histologic relative, osteoid osteoma.

Epidemiology

They account for 1-3% of all primary bone tumors ^2,3. Patients typically present around the second to third decades of life. There is a recognized 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 lesions ² 
  • sacrum: 17% of all spinal lesions 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 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
• 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 edema "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

Nuclear medicine

• Tc⁹⁹m 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 embolization 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.