Periosteal Osteosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
October 7, 2026

Periosteal osteosarcoma is a rare bone cancer that grows on the outer surface of a bone. It is a type of osteosarcoma, a group of cancers in which the tumor cells make new bone. Periosteal osteosarcoma accounts for only about 1 to 2% of all osteosarcomas.

The tumor starts beneath the periosteum, the thin layer of tissue that covers the outside of a bone. Most of the tumor is made of cartilage, with smaller areas where the tumor cells make bone.

Periosteal osteosarcoma is an intermediate-grade cancer. This means it behaves more aggressively than a low-grade tumor but less aggressively than most osteosarcomas, which are high-grade. As a result, the outlook is better than for the more common types of osteosarcoma.

This article explains how periosteal osteosarcoma is diagnosed and what your pathology report findings mean.

How is it different from other osteosarcomas on the bone surface?

Most osteosarcomas start inside the bone. Three types start on the surface, and the names are easy to confuse:

Parosteal osteosarcoma. This is a low-grade tumor that grows slowly, most often on the back of the thigh bone just above the knee. Our article on parosteal osteosarcoma describes it in detail.
Periosteal osteosarcoma. This is an intermediate-grade tumor made mostly of cartilage. It usually grows on the shaft of the thigh bone or shin bone.
High-grade surface osteosarcoma. This is a high-grade tumor that behaves like the common type of osteosarcoma that starts inside the bone.

The type matters because each one is treated differently and has a different outlook.

Who gets periosteal osteosarcoma?

Periosteal osteosarcoma most often affects teenagers and young adults. About half of people are under the age of 18 when it is diagnosed, although it can develop at any age. It is slightly more common in women than in men.

The tumor usually develops on the shaft, the long middle part, of the thigh bone (femur) or shin bone (tibia). Less often, it develops on the upper arm bone (humerus) or another bone.

What causes periosteal osteosarcoma?

The cause of periosteal osteosarcoma is not known. It is not known to be inherited, and no specific genetic change has been found that defines it. Nothing you did or did not do caused it.

What are the symptoms?

The most common symptoms are a firm lump on the arm or leg, pain, and swelling. Symptoms often develop over weeks to months. Because the tumor grows on the surface of the bone, you may feel the lump before it causes much pain.

How is the diagnosis made?

Doctors make the diagnosis by combining imaging findings with examination of tumor tissue under a microscope.

An X-ray usually shows a mass sitting on the surface of the bone. Thin lines of new bone often spread outward from the surface in a pattern that radiologists describe as a sunburst. The outer layer of the bone beneath the tumor is often thickened and may have a shallow, saucer-shaped dip.

An MRI shows the size of the tumor and whether it has grown into the hollow center of the bone, called the medullary cavity. In most cases, it has not.

A biopsy removes a small sample of the tumor for examination. It should be planned at the hospital where the tumor will be treated. The path of the biopsy needle is removed along with the tumor during surgery, so its position must match the surgical plan.

Under the microscope, most of the tumor is made of abnormal cartilage arranged in rounded groups called lobules. The cartilage cells look moderately abnormal. Toward the edges of the lobules, the cells become longer and thinner.

The key finding is osteoid, the new, unhardened bone made directly by the tumor cells. Finding even a small amount of osteoid confirms that the tumor is an osteosarcoma and not a cartilage tumor.

Because osteoid areas can be small, a biopsy may sample only cartilage. For this reason, pathologists sometimes make the final diagnosis only after the whole tumor has been removed and examined.

What else can look like periosteal osteosarcoma?

Because the tumor is mostly cartilage, pathologists must separate it from cartilage tumors that also grow on the bone surface. These include periosteal chondroma, which is noncancerous, and periosteal chondrosarcoma. Neither one makes osteoid.

Your report may mention tests used to help with this distinction:

IDH1 and IDH2. Changes in the IDH1 and IDH2 genes are found in many cartilage tumors. They are not found in periosteal osteosarcoma.
MDM2. Extra copies of the MDM2 gene are found in most parosteal osteosarcomas. They are not found in periosteal osteosarcoma.

A negative result for these tests is expected and supports the diagnosis.

Grade

Grade describes how abnormal the tumor cells look and predicts how the tumor is likely to behave. Periosteal osteosarcoma is an intermediate grade tumor by definition. Reports often record this as grade 2 or as “intermediate grade.”

If the pathologist sees areas where the cells look high grade, the report will say so. Your team takes this into account because high-grade areas may change the treatment plan.

Treatment effect

If you received chemotherapy before surgery, your report may describe how much of the tumor is no longer alive. This is called treatment effect or tumor necrosis, and it is reported as a percentage.

For the common high-grade type of osteosarcoma, this percentage helps predict the outlook. For periosteal osteosarcoma, it does not appear to predict the outlook in the same way. Cartilage-rich tumors often show little change after chemotherapy, so a low percentage is common.

Tumor extension

Your report will describe how far the tumor has grown. Most periosteal osteosarcomas stay on the bone surface and grow outward into the surrounding soft tissue.

Growth into the medullary cavity is uncommon. When it is present, the report will say so, and some studies suggest it may be linked to a less favorable outlook.

Margins

A margin is the edge of tissue the surgeon cuts when removing the tumor. The pathologist examines bone and soft-tissue margins to see whether any tumor cells are present at the cut edge.

Negative margin. No tumor cells are seen at the cut edge. This suggests the tumor was completely removed. The report may also give the distance from the tumor to the closest margin.
Positive margin. Tumor cells are seen at the cut edge. This suggests some tumor may remain, and more surgery may be recommended.

Margins are especially important for this tumor. The tumor coming back in the same place is the factor most strongly linked to a worse outlook, and negative margins lower that risk.

Pathologic stage

The pathologic stage describes the size of the tumor and whether it has spread. It is assigned after the tumor is removed. For tumors in the arms, legs, trunk, skull, and face, the tumor stage (pT) is:

pT1. The tumor is 8 cm or smaller.
pT2. The tumor is larger than 8 cm.
pT3. There is more than one separate tumor in the same bone.

Tumors in the spine and pelvis are staged using different rules.

Osteosarcoma rarely spreads to lymph nodes, so surgeons usually do not remove them. Your report may say that the nodal stage was not assigned. When periosteal osteosarcoma does spread, it most often goes to the lungs.

What is the prognosis?

The outlook for periosteal osteosarcoma is better than for high-grade osteosarcoma. In the largest group of patients studied, about 89% were alive five years after diagnosis, and about 83% were alive at 10 years.

Features that affect the outlook include:

Margins. Complete removal with negative margins lowers the chance that the tumor will come back.
Local recurrence. A tumor that comes back in the same place is linked to a higher chance of spread.
Spread to other parts of the body. Spread, most often to the lungs, occurs in a minority of people and makes the cancer harder to cure.
High-grade areas or growth into the medullary cavity. These features may be linked to a less favorable outlook.

Most tumors that come back or spread do so within about three years of treatment.

What happens after the diagnosis?

A team at a center that specializes in bone tumors treats periosteal osteosarcoma.

Surgery. Surgery is the main treatment. The tumor is removed in one piece with a rim of normal tissue around it, a procedure called a wide resection. In most cases, doctors can save the arm or leg.
Chemotherapy. Chemotherapy is often offered, but its benefit for this tumor is uncertain. Large studies have not shown that it improves survival. Your team will discuss whether it makes sense for you.
Radiation. Radiation is not standard treatment.
Follow-up. Follow-up includes regular examinations and imaging of the surgical site and the chest. Visits are most frequent during the first three years.

Questions to ask your doctor

Was the diagnosis confirmed by biopsy, or only after the whole tumor was removed?
Did the pathologist see any high-grade areas?
Did the tumor grow into the medullary cavity?
Were the margins negative, and how close was the nearest one?
What is the stage of my tumor?
Is chemotherapy recommended for me, and why or why not?
Has a pathologist who specializes in bone tumors reviewed my case?
What is the chance that the tumor will come back?
How often will I need follow-up visits and imaging?
What symptoms should I report between visits?