What is an osteoblastoma?

An osteoblastoma is a relatively 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.

This type of tumour generally occurs 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.


Bones are a special type of hard tissue that provide our bodies with support and protection. In total there are 206 bones in each of our bodies. Some bones (like the bones in our fingers) are very small while other bones (like the bones in our upper legs) are very big.

Normal bones develop when specialized cells called osteoblasts produce a substance called osteoid. Osteoid is initially soft but becomes hard over time as minerals (such as calcium) are added to it. Osteoid that is still soft is called ‘immature’ bone while mineralized, hard osteoid is called ‘mature’ bone. The immature bone starts out with a woven, disorganized appearance under the microscope before undergoing mineralization and remodelling, which makes it hard and strong.

How do pathologists make this diagnosis?

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.

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

by Ashley Flaman MD and Bibianna Purgina MD FRCPC, updated December 31, 2020
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