Osteoblastoma

Osteoblastomas are rare bone-forming tumors 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 ¹.

Terminology

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

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

Epidemiology

Osteoblastomas account for ≤1% of all primary bone tumors ^1-4. Patients typically present around the second to third decades of life ¹. There is a recognized male predilection with men approximately two times more commonly affected than women ¹.

Diagnosis

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

Diagnostic criteria

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5^thedition) ¹:

Essential features include ¹:

• lytic bone tumor >2 cm in size on imaging
• well-defined tumor borders
• no evidence of permeation of the host bone
• histological evidence of a bone-forming tumor consisting of trabeculae of remodeled woven bone framed by plump osteoblasts in a vascularized background

Clinical presentation

Osteoblastomas 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 have rich vascularity ¹.

Location

• spinal column: ~40% (range 32-46% ²); often involves the posterior column
  • cervical spine: 9-39% of all spinal osteoblastomas ⁵ 
  • sacrum: 17% of all spinal osteoblastomas ⁶
• usually located in the metaphysis and distal diaphysis of the long bones

Macroscopic appearance

Macroscopically osteoblastomas are usually well-defined tumors. They might show osseous expansion with thinning of the cortex rimmed by sclerotic host bone ^1,4. Due to their rich vascularity, osteoblastomas display a red-tannish appearance and might show blood-filled cystic spaces ^1,4.

Microscopic appearance

Microscopically osteoblastomas are similar to osteoid osteoma and are characterized by the following ^1,4:

• interconnecting trabeculae of woven bone rimmed by a single layer of osteoblasts
• trabeculae with different degrees of mineralization (from osteoid to pagetoid appearance) connecting to the bony edge in the periphery
• richly vascularized loose stroma
• possible central sclerotic nidus
• scattered osteoclastic giant cells
• well-defined borders without destructive bone permeation and no soft tissue extension
• no atypical mitotic figures
• possibly aneurysmal bone cyst-like changes

Variants

• aggressive (malignant) osteoblastoma
  • ​has a high of number epithelioid osteoblasts with nuclear atypia
  • controversial diagnosis, not recommended by the WHO ¹
  • epithelioid osteoblastoma is the preferred term ¹

Genetics

• FOS gene rearrangement is present in ~90% of cases (similar to osteoid osteoma) ¹

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 ^7,8. 

Plain radiograph

• lesions are predominantly lytic, with a rim of reactive sclerosis
• tend to be expansive
• may have a bubbly appearance ⁷
• 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 mineralization 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 the edema "flare phenomenon" ¹⁰
• T1 C+ (Gd): this is a highly vascular tumor 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
• tumor margins and transition zone
• aneurysmal bone cyst-like changes
• concerning features
  • pathologic fracture
  • cortical destruction
  • aggressive periosteal reaction
  • surrounding bone marrow edema
  • solid mass-like enhancement
  • soft tissue extension

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

Treatment and prognosis

Management is surgical with the selection of the surgical procedure depending on the location and aggressiveness ¹². Intralesional curettage can be performed in most cases but should be extended to the normal bone and can be combined with cryotherapy, chemical cauterization with phenol as adjuncts and bone grafting ¹². En bloc surgical excision is associated with fewer recurrence rates and can be done in locally aggressive and/or large tumors or in recurrent lesions ¹². Pre-operative embolization can be carried out to reduce bleeding risk. However, 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) ¹². Recurrence rates may be as high as ~23% ¹.

Complications

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

History and etymology

Henry Lewis Jaffe and Leo Mayer first described osteoblastoma as an ‘osteoblastic osteoid tissue forming tumor' in 1932 ^12,13.  It was later described as ‘osteogenic fibroma of bone’  by the American bone pathologist Louis Lichtenstein in 1952 ^14,15 and as ‘giant osteoid osteoma’ by D C Dahlin and E W Johnson, Jr ¹⁶  before the name osteoblastoma was suggested in 1956 again by Louis Lichtenstein ¹⁷.

Differential diagnosis

Differential diagnoses of osteoblastoma include ⁴:

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