osteoblastoma

Digital cases

 HPC:79 : Osteoblastoma
 HPC:147 : Osteoblastoma

Definition: An osteoblastoma is a rare solitary, benign, osteoid- and bone-forming neoplasm that contains many well-differentiated osteoblasts and osteoclasts, and usually has a vascular stroma.

Clinical synopsis

Osteoblastomas predominantly affect young adults and are often painful.

Swelling and tenderness are usually the symptoms that prompt the patient to seek medical attention.

"Toxic" cases - A few patients with either solitary or multiple osteoblastomas have presented with associated weight loss, anemia, and low- grade fever. In these toxic cases, the true diagnosis is apt to be delayed because the patient is usually diagnosed as having osteomyelitis. Curettage of the lesion resolves the toxic symptoms

Location

In most cases, the long bones or vertebrae are the sites of involvement, with approximately 40% of osteoblastomas originating in the spine. In long bones, the lesion may arise in either the metaphysis or the diaphysis.

In the spine, osteoblastomas arise with about equal frequency in the cervical and lumbar regions, with the thoracic region being less frequently involved.

Occasionally, they may involve the pelvis or sacrum.

Spine - Mainly they affect the vertebral arch, involving the spinous and transverse processes as well as the laminae and pedicles. In only a few cases does the lesion appear to originate within the vertebral bodies. The clinical presentation of osteoblastoma in the spine may include myelopathic or radicular symptoms and often suggests a herniated disc. Progressive scoliosis may also appear and if the cervical spine is affected, reversal of the lordotic curve may occur and torticollis be prominent.

Radiology

On imaging studies, the lesion characteristically appears as a lucent defect with various de grees of central density.

They are usually well circumscribed, without extensive surrounding bone sclerosis.

They are similar in appearance to osteoid osteoma, with the difference being that the diameter generally exceeds 1.5 cm, and unlike osteoid osteomas, which are self-limiting lesions, osteoblastomas may continue to enlarge.

Computed tomography (CT) scanning and isotope scanning show increased uptake, and both methods are helpful in delineation.

Microscopy

On microscopic examination, the lesion consists of a vascular spindle cell stroma with abundant irregular spicules of mineralized bone and osteoid.

Osteoblasts and multinucleated osteoclasts are readily evident on the bone surfaces.

Generally no cartilage can be seen in the lesion; however, in rare cases, foci of cartilage may be found and this in association with focal atypical cells may lead to a misdiagnosis of osteosarcoma.

On rare occasions, osteoblastomas have been noted to act aggressively, with significant bone destruction and extension into adjacent soft tissues.

In these cases, microscopic examination has revealed large, plump osteoblasts that have epithelioid features and may form sheets of cells in the intertrabecular spaces.

The nucleoli of these cells may be prominent, and mitoses may be present.

In some cases, it may be very difficult to differentiate an aggressive osteoblastoma from a low-grade osteoblastoma-like osteosarcoma.

Extremely rare cases of multiple osteoblastomas have been reported.

Changes characteristic of a secondary aneurysmal bone cyst or pathologic fracture may be present within some osteoblastomas, further adding to the problems of differential diagnosis.

Synopsis

 mixed lytic and sclerotic areas
 fibrous and osseous tumor
 usual pattern of disorganized trabeculae of immature bone set in a cellular vascular stroma
 marked osteoblastic and osteoclastic activity at the bone surfaces
 atypical osteoblastoma with crowded, large epithelioid stromal cells
 small irregular foci of woven bone

Variants

 osteoblastoma with cartilaginous matrix (8447510)
 epithelioid multinodular osteoblastoma (17667552)
 atypical multiple benign osteoblastomas accompanied by simple bone cysts
 mutiple osteoblastomas

• multiple well-marginated lytic lesions

Differential diagnosis

 osteoid osteoma

• It is often difficult to differentiate an osteoblastoma from an osteoid osteoma.
• However, the tissue pattern usually appears less regular in an osteoblastoma than in an osteoid osteoma.
• Nevertheless, the lesions are similar enough that osteoblastomas have in the past been referred to in the literature as "giant osteoid osteomas".

 aneurysmal bone cyst

• Changes characteristic of a secondary aneurysmal bone cyst or pathologic fracture may be present within some osteoblastomas, further adding to the problems of differential diagnosis.

 osteosarcoma

• It is important for the pathologist to recognize that an osteoblastoma may be cellular, and to distinguish this pattern from osteosarcoma, a histologic differentiation that can at times be very difficult.
• An atypical osteoblastoma can contain crowded, large epithelioid stromal cells; small irregular foci of woven bone are present.

Cytogenetics

 unbalanced translocation resulting in the loss of the chromosome 17 short arm (8374902)

Management

Treatment consists of curettage or en bloc excision.

At surgery, an osteoblastoma is found to be composed of hemorrhagic, granular, friable, and calcified tissue

See also

 bone-forming tumors

References

 Zon Filippi R, Swee RG, Krishnan Unni K. Epithelioid multinodular osteoblastoma: a clinicopathologic analysis of 26 cases. Am J Surg Pathol. 2007 Aug;31(8):1265-8. PMID: 17667552