Osteoblastoma

Tumor biology and incidence

• Benign aggressive osteoblastic lesion of bone
• Sometimes called "giant osteoid osteoma" -- histologically identical to osteoid osteoma, but larger
• Not known to metastasize if properly diagnosed
• 1% of all primary bone tumors; <5% of all benign bone tumors

Age

Generally affects persons 10-35 years of age

Gender

M:F = 2:1

Presentation

• Often a long history of pain not as readily relieved by salicylates as in osteoid osteoma
• Pain may be present at rest, but does not usually have nocturnal worsening commonly associated with osteoid osteoma
• Patient may be asymptomatic, but pain is generally predominant feature
• Spinal lesions usually present with pain, rarely with acute neurological deficit

Physical findings

• Spinal involvement may present with myelopathic or radicular symptoms, progressive painful scoliosis, or torticollis if lesion is in cervical spine
• Tender mass or soft tissue atrophy may be present
• Direct palpation often localizes a lesion, even when it cannot be seen on a plain radiograph
• Neurologic deficit rare, but can occur if there is compression of dura by pathologic fracture or epidural extension of tumor

Plain films

Site

• Most common locations are spine (posterior elements), femur, tibia, and skull
• In long bones, usually metaphyseal or diaphyseal
• Epiphyseal involvement alone is rare
• Less common locations are humerus and small bones of hands and feet

Size

Greater than 2 cm

Tumor effect on bone

• Lesions may be sclerotic, mixed or entirely lucent
• Usually lytic, especially if lesions are diaphyseal
• Lytic areas may have appearance of aneurysmal bone cyst with "neo-corticalization" around destructive lesions
• Large, aggressive lesions may show extensive lytic destruction of bone

Bone response to tumor

• Lesions generally geographic
• Usually present with sclerotic rim
• Less reactive sclerosis than osteoid osteoma
• Aggressive lesions may have "neo-corticalization" with periosteal new bone formation outlining soft tissue mass

Matrix

• Osteoblastic matrix may be present
• Bone formation may be quite dense in less aggressive lesions

Cortex

• Cortical breakthrough may occur in aggressive osteoblastoma
• Rim of "neo-cortex" usually evident

Soft tssue mass

Rarely occurs

Bone scan

Lesions are hot on bone scan

CT scan

Useful in demonstrating extent of bone expansion/destruction

MRI

• Helpful to rule out extension into epidural space in vertebral lesions
• Fluid-fluid levels may be present in cases with aneurysmal bone cyst degeneration

Fig. 1. Typical location in the posterior elements of the spine. Radiodense, expansile, bone producing neoplasm.

Fig. 2. Radiodense bone producing tumor. DDx Osteosarcoma (classic, parosteal), osteoblastoma, osteoma.

Differential diagnosis

• Osteoid osteoma
  • Sclerosis around the nidus
  • Nidus <2 cm
• Aneurysmal bone cyst
  • Fine, incomplete, septae forming lobulations
  • Balloon-like expansion of bone
  • In posterior spine, may be indistinguishable from osteoblastoma on conventional radiographs

• Giant cell tumor
• Brodie's abscess
• Osteosarcoma
  • More poorly defined
  • Soft tissue mass not limited by neo-corticalization
• Chondrosarcoma
  • Heavily and irregularly calcified
  • Indistinct borders
• Chondroblastoma
  • Epiphyseal location

Pathology

• Gross: Curettings may be ossified and surrounding bone is sclerotic
• Microscopic: Interconnected trabeculae of bone in a fibrovascular stroma
• Osteoblasts and osteoclasts present on trabecular surfaces
• Lace-like osteoid, usually considered pathognomic of osteosarcoma, may be present
• Cells forming bone are cytologically benign
• Lesion merges with adjacent cortical bone and does not infiltrate through pre-existing bone trabeculae
  • Helps differentiate this lesion from osteosarcoma on biopsy.
• May co-exist with giant cell tumor or aneurysmal bone cyst

Fig. 3. Typical gross appearance, dense cancellous cortical bone.

Treatment

• Should be excised surgically; otherwise, continues to enlarge and damage the bone and adjacent structures
  • Lytic lesions are generally treated with curettage and bone grafting or cementing
    • As much of surrounding bone should be removed as possible
    • Most osteoblastomas controlled by extended curettage, but recurrence is not uncommon (approximately 10-20%)
  • En bloc excision may be preferred for lesions in expandable bones (eg, rib, fibula), aggressive-appearing lesions, and some vertebral lesions
• Reconstruction of defect formed by excision depends on the site, but most defects require bone grafting
• Spinal lesions may require irradiation if the soft tissue mass and epidural disease is extensive