Osteoblastoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
October 6, 2026

An osteoblastoma is a rare noncancerous bone tumor. It is made of immature bone, called osteoid, and the cells that make it, called osteoblasts. It makes up about 1% of all tumors that start in bone.

Osteoblastoma does not spread to other parts of the body and does not usually turn into cancer. However, it can grow and erode the surrounding bone, and it can come back after surgery. For this reason, it is sometimes described as locally aggressive.

Osteoblastoma usually affects children, teenagers, and young adults, most often between 10 and 30 years old. It is about twice as common in males.

This article explains how an osteoblastoma is diagnosed and what the findings in your pathology report mean.

How is an osteoblastoma different from an osteoid osteoma?

An osteoblastoma looks the same under the microscope as an osteoid osteoma, a smaller noncancerous bone tumor. The main difference is size. A tumor larger than 2 cm is called an osteoblastoma, and a smaller one is called an osteoid osteoma. Osteoblastomas are usually 2 to 5 cm across but can grow larger.

The two also behave differently:

Location. Osteoblastomas more often develop in the spine.
Pain. Osteoblastomas cause a dull, aching pain that is less likely to worsen at night or improve with anti-inflammatory medicines such as ibuprofen.
Growth. Osteoblastomas keep growing and can wear away the surrounding bone, while osteoid osteomas stay small.
Treatment. Osteoblastomas usually need surgery, while osteoid osteomas are often treated with heat through a needle.

Where do osteoblastomas develop?

About 3 to 4 in 10 osteoblastomas develop in the spine, usually in the back parts of the vertebrae, the bones that make up the spine. Other common locations include the long bones of the legs, the bones of the feet, and the jaw.

What are the symptoms of an osteoblastoma?

The most common symptom is a dull, aching pain that builds over months. Other symptoms depend on where the tumor is:

Spine. Back or neck pain and stiffness. The spine may curve to one side. A tumor that presses on a nerve or the spinal cord can cause numbness, tingling, or weakness in the arms or legs.
Arms and legs. Swelling over the bone and difficulty moving the nearby joint.
Jaw. Swelling of the jaw or face, and sometimes loose teeth.

What causes an osteoblastoma?

Many osteoblastomas have a change involving a gene called FOS or a closely related gene called FOSB. These genes help control how bone-forming cells grow. The same changes are found in osteoid osteomas, which is why the two tumors look alike. The change develops in the tumor during a person’s lifetime. It is not inherited and cannot be passed on to children.

How is the diagnosis made?

X-rays, CT, and MRI show a tumor inside the bone, often with a clear border and areas of new bone formation. A CT scan is particularly useful for tumors in the spine.

A pathologist confirms the diagnosis by examining tumor tissue under a microscope. The tissue may come from a biopsy or from surgery that scrapes out the tumor in a procedure called curettage or removes it in one piece in a resection.

Under the microscope, an osteoblastoma is made of strands of immature bone lined by orderly osteoblasts. Pathologists describe this lining as osteoblastic rimming. The bone strands sit in a background rich in small blood vessels, and the tumor has a clear border with the surrounding normal bone.

How is an osteoblastoma different from osteosarcoma?

Osteosarcoma is a bone cancer that also makes immature bone. Distinguishing it from an osteoblastoma is the pathologist’s most important decision because the two are treated very differently.

An osteoblastoma has orderly bone-forming cells and a clear border with the surrounding bone. An osteosarcoma has very abnormal cells and grows into and around the existing bone. Your pathologist will usually look at your imaging as well as the microscope to make this decision.

When the distinction is difficult, a stain for the FOS protein can help. It is positive in most osteoblastomas and negative in osteosarcoma.

What is an epithelioid osteoblastoma?

Some osteoblastomas contain bone-forming cells that are larger and rounder than usual. Your report may call this an epithelioid osteoblastoma.

These tumors were once called aggressive osteoblastoma, because they were thought to come back more often. That name is no longer used, because studies have shown that the larger cells do not reliably predict how the tumor will behave. Epithelioid osteoblastoma is still noncancerous, although its appearance can make it harder to separate from osteosarcoma.

What is the prognosis?

The outlook is very good. Most people are cured by surgery, and an osteoblastoma does not spread to other parts of the body.

The main concern is the tumor coming back in the same place. In published series, this happens in roughly 10% to 25% of people. It is more common after curettage than after removal of the tumor in one piece, and more common in the spine, where complete removal is harder. A tumor that comes back can usually be treated with further surgery.

Very rarely, a tumor diagnosed as osteoblastoma later behaves like a cancer. A tumor that keeps coming back is usually examined closely to make sure it was not an osteosarcoma from the start.

What happens after the diagnosis?

Curettage. Scraping out the tumor and filling the space with bone graft or bone cement is the most common treatment, especially in the arms and legs.
Resection. Removing the tumor in one piece lowers the chance of it coming back. It is often recommended in the spine, although it can be technically demanding there.
Spine surgery. Spine surgery may also involve stabilizing the vertebrae with rods or screws.
Radiation therapy. Radiation is rarely used. It may be considered for a tumor that keeps coming back and cannot be removed again.
Follow-up. Regular imaging continues for several years, because most recurrences appear within the first few years after surgery.

Questions to ask your doctor

How large was the tumor?
Does my report describe an epithelioid osteoblastoma?
How sure are you that this is an osteoblastoma and not an osteosarcoma?
Was the whole tumor removed?
Will I need curettage or resection, and why?
What is the chance the tumor will come back?
How often will I need follow-up imaging?