Osteoblastoma

G. Petur Nielsen, MD

Andrew E. Rosenberg, MD

Key Facts

Clinical Issues

Benign bone-forming neoplasm > 2 cm in dimension
Diagnosed in young adults with male predominance of 2:1
Commonly arises in tubular bones and posterior elements of spinal column
Presents with pain and swelling, with neurologic symptoms in some spinal lesions
Treated by curettage or en bloc excision
Excellent prognosis with local recurrence rate of 20%

Image Findings

Expansile well-defined mixed lytic and blastic mass with sclerotic margins

Macroscopic Features

Well-circumscribed, gritty, tan-white, dark red mass
Usually 2-5 cm in size

Microscopic Pathology

Sharply demarcated from adjacent preexisting and reactive bone
Composed of haphazardly interconnecting trabeculae or sheet-like aggregates of woven bone rimmed prominently by metabolically active osteoblasts
Intertrabecular tissue consists of loose, richly vascular connective tissue
Variants contain cartilage, epithelioid osteoblasts, degenerative nuclear changes, and aneurysmal bone cyst-like changes

Radiograph of humerus shows a lytic osteoblastoma of the mid shaft surrounded by solid periosteal new bone. It extends into periosseous soft tissues where cloud-like neoplastic ossification is noted.

A long bone with an osteoblastoma involves the cortex and surface of bone. The well-circumscribed tumor is oval and red with speckled tan-white foci, which correspond to areas rich in neoplastic bone.

TERMINOLOGY

Abbreviations

Osteoblastoma (OB)

Synonyms

Giant osteoid osteoma

Definitions

Benign bone-forming neoplasm composed of woven bone trabeculae lined by osteoblasts
- Tumor > 2 cm in dimension

ETIOLOGY/PATHOGENESIS

Etiology

Cause unknown; not associated with any syndrome; no specific cytogenetic abnormality

CLINICAL ISSUES

Epidemiology

Incidence
- Uncommon; accounts for 1% of primary bone tumors
  - Responsible for 3% of primary benign bone tumors
Age
- Diagnosed in adolescents and young adults (2nd-4th decade)
  - 75% younger than 25 years old at diagnosis
  - Mean age: 20 years
Gender
- Males affected more frequently than females (2:1)

Site

Most commonly arise in tubular bones
- Approximately 60% develop in appendicular skeleton
  - Frequently involve metadiaphyseal region of tubular bones
  - Centered in cortex and surface in 65% and located in medullary cavity in 35% of cases
  - 12% occur in femur; 10% originate in tibia; 9% involve bones of foot and ankle
Axial skeleton frequently affected
- 30% arise in spinal column
  - Originate in posterior elements, especially lamina and pedicles; vertebral body involvement occurs secondarily
  - Cervical > lumbar > thoracic > sacral
Craniofacial bones involved in 10% of cases
- Mandible most common location

Presentation

Pain, swelling, decreased range of motion
Tumors located in spine can cause neurological symptoms
- Numbness, tingling, paraparesis, paraplegia
Small percentage associated with systemic “toxic” symptoms

Treatment

Curettage or selective en bloc resection
Radiation when located in inaccessible site

Prognosis

Curettage associated with local recurrence in approximately 20% of cases
Aggressive variant behaves similar to conventional type
Malignant transformation in osteoblastoma is exceptionally rare

IMAGE FINDINGS

Radiographic Findings

Expansile, well-defined, oval mixed lytic and blastic mass
- Peripheral lucent halo surrounds central area of mineralization
- Poorly defined margins in small minority
Periosteal reactive bone common; usually solid but may be laminated or spiculated

MR Findings

Well-delineated mass that is low to intermediate signal intensity on T1 and intermediate to high signal intensity on T2
- Mineralized areas manifest as signal void (dark)
- Tumor and edema enhance with contrast

CT Findings

Expansile lytic and sclerotic mass with circumscribed margins and surrounding reactive bone

Bone Scan

Intense uptake on scintography

MACROSCOPIC FEATURES

General Features

Solitary, well-circumscribed, tan-white, dark red, gritty
- Rarely multifocal/multicentric
Cystic changes (aneurysmal bone cyst-like changes) prominent in 10% of cases
Delineated from soft tissues by periosteal shell of reactive bone

Size

Tumors range from 2-20 cm in dimension
- Most tumors are 3-5 cm

MICROSCOPIC PATHOLOGY

Microscopic Features

Sharply demarcated
Neoplastic woven bone trabeculae deposited in haphazard, interconnecting, or sheet-like patterns
- Bone rimmed by osteoblasts and scattered osteoclasts
  - Osteoblasts are ovoid or round, with moderate amounts of eosinophilic or purple cytoplasm, and eccentric nuclei with fine chromatin
Vascular connective tissue fills intertrabecular space
Scattered mitoses with no atypical forms
Necrosis usually absent or focal
Occasional cystic change mimicking aneurysmal bone cyst
Tumors composed of 75% epithelioid osteoblasts known as aggressive osteoblastoma
- Epithelioid osteoblasts are large and polyhedral with abundant eosinophilic cytoplasm, vesicular nucleus, and prominent nucleolus
Cartilaginous variant (5%) contains well-formed hyaline cartilage
Pseudomalignant type contains osteoblasts with large, hyperchromatic, vacuolated nuclei that represent degenerative changes

DIFFERENTIAL DIAGNOSIS

Osteoid Osteoma

< 2 cm; characteristic clinical symptoms

Aneurysmal Bone Cyst

No haphazardly joining trabeculae of woven bone

Osteoblastoma-like Osteosarcoma

Grows with infiltrative pattern

DIAGNOSTIC CHECKLIST

Pathologic Interpretation Pearls

Tumor is well demarcated and neoplastic bone is rimmed by osteoblasts

SELECTED REFERENCES

1. Berry M et al: Osteoblastoma: a 30-year study of 99 cases. J Surg Oncol. 98(3):179-83, 2008

Image Gallery

Imaging Features

(Left) AP bone scan shows a small oval focus of intense isotope uptake on the left of T2. (Right) CT scan through osteoblastoma of pedicle and lamina of the vertebra demonstrates a well-circumscribed, expansile, oval mass, which contains substantial internal fine ossification. The tumor impinges upon the spinal canal. Thick periosteal reaction is present along the surface of the lamina and the base of the spinous process