Osteoblastoma

Osteoblastomas are benign bone-forming neoplasms that produce woven bone spicules bordered by osteoblasts, histologic features that are similar to those of osteoid osteomas. Osteoblastomas have an extremely rich vascular supply and an aneurysmal bone cyst component is seen in up of 15% of cases.

Different forms of osteoblastoma have been described. Epithelioid osteoblastomas, also known as aggressive or malignant osteoblastomas, have plump osteoblasts with prominent nuceloli and can be difficult to distinguish from osteosarcoma. They have been described as borderline lesions between benign osteoblastoma and osteosarcoma, and have a locally destructive growth pattern and a high recurrence rate after curettage.

Toxic osteoblastomas are rare and associated with systemic symptoms (anorexia, weight loss, cachexia, and fever).

The term conventional osteoblastoma is therefore used to differentiate the traditional osteoblastoma from these varieties.

Conventional osteoblastomas typically affect adolescents and young adults, males more frequently than females. Patients present with dull localized pain that is less likely to respond to non-steroidal anti-inflammatory medications than osteoid osteomas. The pain is also less likely to get worse at night.

Conventional osteoblastomas affect the spine in 1/3 of cases, where they more commonly affect the posterior processes and may extend into the vertebral body (isolated vertebral body involvement is rare). Compared to osteoid osteomas, osteoblastomas affecting the spine are more likely to present with neurological symptoms. Scoliosis, on the other hand, is less likely to occur with osteoblastomas compared to osteoid osteomas.

Osteoblastomas can also affect the diaphyses and metaphyses of long bones, where they are more often proximal.

Radiographs and CT may reveal one of three appearances:

• Lucent lesion with surrounding sclerosis. The appearance is similar to that of the osteoid osteoma, but the lesions are larger (> 1.5 cm). This is the most common appearance in the long bones, where solid continuous periosteal reaction can be seen, sometimes distant from the lesion.
• Expansile lucency with a sclerotic rim and internal calcifications that may simulate a chondroid matrix. This is the most common form in the spine.
• Aggressive expansile lesion with bone destruction, soft tissue extension, and matrix calcifications that may simulate a chondroid matrix.

The diagnosis of osteoblastoma in the long bones is difficult, being correctly made in less than 1/3 of cases (compared to 2/3 in spinal lesions). Various degrees of mineralization may be present, and an agrressive lamellated or spiculated periosteal reaction may be present.

As may be expected from their highly vascular nature and periosteal reaction, bone scans demonstrate marked uptake.

Findings on conventional angiography are nonspecific and variable. Some osteoblastomas show no abnormality at all on conventional angiography. Those that do may reveal a contrast blush in the capillary and/or venous phase, regular or irregular vascularity, and/or large draining veins.

As with other bone tumors, the MRI appearance is nonspecific and may be misleadingly aggressive. Lesions have intermediate signal intensity on T1-weighted images and variable signal intensity on T2-weighted images. Extensive peri-tumoral T2 hyperintensity can be seen. An aneurysmal bone cyst component is seen in up to 15% of cases.

In the spine, the main differential consideration is osteoid osteoma. Osteoblastomas tend to have multifocal mineralization compared to the central mineralization that may be seen in osteoid osteomas. In addition, osteoblastomas tend to be expansile and may have a soft tissue component. Finally, serial imaging reveals slow growth, which is distinguished from the stability of osteoid osteomas.

Osteoblastomas of the spine are usually curetted with allograft or autograft packing. Aggressive and larger lesions are typically resected en block. Conventional osteoblastomas may recur in up to 25% of cases, while aggressive osteoblastomas have close to a 50% recurrence rate.

References

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