Osteoblastoma

Osteoblastomas are rare bone-forming tumours that may be locally aggressive. Compared to their histological relative, the osteoid osteoma, they are larger (>2 cm) and more frequently affect the axial skeleton ¹.

Osteoblastoma accounts for the 'O' in the popular mnemonic for lucent bone lesions FEGNOMASHIC.

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 tumours ^1-4. Patients typically present around the second to third decades of life ¹. 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 combination of typical radiological and pathological features.

Diagnostic criteria

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

Essential features include ¹:

• lytic bone tumour >2 cm in size on imaging

• well-defined tumour borders

• no evidence of permeation of the host bone

• histological evidence of a bone-forming tumour consisting of trabeculae of remodelled woven bone framed by plump osteoblasts in a vascularised 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 locally aggressive lesions that 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 elements

  • 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 tumours. 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 characterised by the following ^1,4:

• interconnecting trabeculae of woven bone rimmed by a single layer of osteoblasts

• trabeculae with different degrees of mineralisation (from osteoid to pagetoid appearance) connecting to the bony edge in the periphery

• richly vascularised 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 expansile

• 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 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 the oedema "flare phenomenon" ¹⁰

• T1 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 can be categorised 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 tumours or in recurrent lesions ¹². Pre-operative embolisation 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 tumour' 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 tumour of bone

• osteoma with osteoblastoma-like features

• osteosarcoma