Osteoblastoma

Osteoblastomas are rare bone-forming tumours that may be locally aggressive. They are larger (>2 cm) and tend to affect the axial skeleton more often than their histologic relative, osteoid osteoma ⁴¹.

Terminology

‘Epithelioid osteoblastoma’ is considered an acceptable alternative term ¹.

The terms 'pseudomalignant osteoblastoma' or 'aggressive osteoblastoma' are no longer recommended ¹.

Epidemiology

TheyOsteoblastomas account for 1-3≤1% of all primary bone tumours ^2,31-4. Patients typically present around the second to third decades of life¹. There is a recognised male predilection with men approximately two times more commonly affected than women ¹.

Diagnosis

The diagnosis of osteoblastoma is based on a malecombination of typical radiological and pathological features.

Diagnostic criteria

Diagnostic criteria according to female ratiothe WHO classification of approximately 2.5:1.soft tissue and bone tumours (5^thedition)¹:

Clinical presentation

With spinal lesions, painful scoliosis is a common presenting symptom. Otherwise, it presentsOsteoblastomas commonly present with an insidious onset of dull pain, worse at night, with minimal response to salicylates (only 7% of patients respond, unlike osteoid osteoma)¹. The area will characteristically be swollen and tender with a decreased range of motion.

Spinal lesions might present with painful scoliosis or neurological symptoms ¹. 

Pathology

Osteoblastomas are histologically similar to osteoid osteomas, are bone- and osteoid-forming with a rim of osteoblasts and are associated richly vascularity ¹²¹.

Location

• spinal column: ~40% (range 32-46% ²); often involves the posterior column
  • cervical spine: 9-39% of all spinal osteoblastomas ²⁵ 
  • sacrum: 17% of all spinal osteoblastomas ³⁶
• 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
  • controversial diagnosis, not recommended by the WHO ¹²¹
  • epithelioid osteoblastoma is the preferred term ¹²¹

Genetics

• FOS gene rearrangement is present in ~90% of cases (similar to osteoid osteoma) ¹²¹

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 ^4,57,8. 

Plain radiograph

• lesions are predominantly lytic, with a rim of reactive sclerosis
• tend to be expansive
• may have a bubbly appearance ⁴⁷
• 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-like changes 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" ¹¹¹⁰
• 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

Radiology report

The radiological report should include a description of the following ¹¹:

• location and size
• tumour margins and transition zone
• aneurysmal bone cyst-like changes
• concerning features
  • pathologic fracture
  • cortical destruction
  • aggressive periosteal reaction
  • surrounding bone marrow oedema
  • solid mass like enhancement
  • soft tissue extension

If features are typical the lesion should be categorised as Bone-RADS 4 on CT or MRI ¹¹.

Treatment and prognosis

En bloc surgical excision is often the treatment of choice ^ref with curettage another surgical option ¹²¹. 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 ^ref. Percutaneous ablation is an emerging modality for the treatment of these lesions (as well as osteoid osteoma) ¹⁰¹¹. Recurrence rates may be as high as ~25~23% ¹²¹.

Complications

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

Differential diagnosis

• osteoid osteoma: <2 cm
• aneurysmal bone cyst
• giant cell tumour of bone
• osteoma with osteoblastoma-like features
• osteosarcoma