by Ashley Flaman MD and Bibianna Purgina MD FRCPC
March 4, 2022

What is an osteoblastoma?

An osteoblastoma is a rare non-cancerous bone type of bone tumour. These tumours usually range in size from about 2 to 5 cm but they can be much larger, up to 20 cm. They are most frequently found in the bones of the legs, feet, and spine, but other locations are possible. A tumour that looks very similar to osteoblastoma but measures less than 2 cm is called an osteoid osteoma.

What are common symptoms of osteoblastoma?

Most osteoblastomas occur in children and young adults but can occasionally occur in older adults. The most common symptom is pain, but other symptoms include swelling, decreased range of motion (for example, not being able to move your leg, arm or back as much as normal), and numbness and tingling depending on the location of the tumour.

How do pathologists make the diagnosis of osteoblastoma?

This diagnosis can be made after a small tissue sample is removed in a procedure called a biopsy or when the entire tumour is removed in a procedure called a resection or curettage. The tissue is then sent to a pathologist for examination under the microscope.

What does an osteoblastoma look like under the microscope?

Under the microscope, an osteoblastoma is made up of disorganized, immature new bone surrounded by osteoblasts. Immature bone is made up of a material called osteoid that looks bright pink when examined under the microscope.


Pathologists use the phrase “osteoblastic rimming” to describe osteoid surrounded by osteoblasts. This feature is important because some types of bone cancer can look like osteoblastoma under the microscope, but bone cancer does not show osteoblastic rimming.

In addition, osteoblastoma usually shows a clear border between the tumour tissue and the normal bone, a feature that can be seen by both X-ray or other imaging and under the microscope. For this reason, your pathologist may also look at your X-ray results before making the diagnosis of osteoblastoma.

Other helpful resources

Children’s hospital of Philadelphia

Dana Farber Institute

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