Parosteal Osteosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
October 6, 2026

Parosteal osteosarcoma is a rare, slow-growing type of bone cancer. It is one form of osteosarcoma, a cancer made of cells that produce bone.

Most osteosarcomas start inside a bone. Parosteal osteosarcoma is different because it grows on the outer surface of a bone. The word parosteal means beside the bone. The tumor starts in the periosteum, the thin layer of tissue that covers the outside of every bone.

Parosteal osteosarcoma is a low-grade cancer. It grows slowly and rarely spreads to other parts of the body, and most people are cured with surgery alone. It is the most common type of osteosarcoma that grows on the bone surface.

This article explains how doctors diagnose parosteal osteosarcoma and what your pathology report findings mean.

Who gets parosteal osteosarcoma?

Parosteal osteosarcoma usually affects adults in their 20s and 30s, which is somewhat older than the usual age for osteosarcoma. It makes up about 4% of all osteosarcomas and is slightly more common in women.

Where does parosteal osteosarcoma develop?

About 7 in 10 parosteal osteosarcomas develop on the back of the lower thigh bone, just above the knee. Other common locations include the top of the shin bone and the upper arm bone. Less often, the tumor develops in the jaw, the ribs, or the small bones of the hands and feet.

What are the symptoms of parosteal osteosarcoma?

The most common symptom is a firm, painless lump near a joint, most often behind the knee. Because the tumor grows slowly and usually does not hurt, many people lump for a year or more before it is diagnosed.

As the tumor grows, it can cause pain or make it harder to bend or straighten the nearby joint.

What causes parosteal osteosarcoma?

Parosteal osteosarcoma is caused by extra copies of a region of chromosome 12 that contains two genes called MDM2 and CDK4. Having extra copies of a gene is called amplification. These genes help control cell growth, and too many copies allow cells to keep growing.

This change develops in the tumor cells during a person’s lifetime. It is not inherited and cannot be passed on to children. Doctors do not know why it happens.

How is the diagnosis made?

On an X-ray, parosteal osteosarcoma appears as a dense, bony mass stuck to the outside of the bone, often wrapping around it. CT and MRI show how far the tumor extends and whether it has grown into the inner part of the bone or the surrounding muscle.

A biopsy confirms the diagnosis. The biopsy should be done at the center that will perform the final surgery, because the track left by the needle has to be removed along with the tumor later.

Under the microscope, the tumor is made of long strands of well-formed bone separated by spindle cells, which are long, thin cells. The spindle cells show only mild atypia, meaning they look only slightly abnormal. Some tumors also contain a cap of cartilage on the surface.

Why can parosteal osteosarcoma be hard to recognize?

Because the cells look so close to normal, parosteal osteosarcoma can be mistaken for a noncancerous bone growth. The most common look-alikes are an osteochondroma, a common bony outgrowth with a cartilage cap, and a reactive bone growth that forms after an injury.

Telling them apart matters. A parosteal osteosarcoma treated like a noncancerous growth may be removed incompletely, and a tumor that comes back is more likely to change into a high-grade cancer.

Two tests help separate them:

Immunohistochemistry. Stains for the MDM2 and CDK4 proteins are positive in most parosteal osteosarcomas and negative in the noncancerous look-alikes.
FISH. This test counts MDM2 gene copies. Extra copies confirm the diagnosis of parosteal osteosarcoma.

Grade and dedifferentiation

All parosteal osteosarcomas are low grade by definition. The cells grow slowly, look only slightly abnormal, and have a low risk of spreading.

In some tumors, part of the cancer changes into a high-grade osteosarcoma, which grows faster and is more likely to spread. This change is called dedifferentiation. Published series report it in roughly 15% to 40% of cases, and it is more common in tumors that have come back after incomplete removal.

Your report will state whether any high-grade area was found. This is the single most important finding in a pathology report for this tumor, because it changes both the outlook and the treatment.

Tumor extension

Your report may describe whether the tumor has grown through the outer shell of the bone into the inner part, called the medullary cavity. It may also note growth into the surrounding muscle.

Some studies have linked growth into the inner part of the bone with a higher chance of the tumor coming back, although this finding is not consistent across studies.

Margins

A margin is the edge of tissue cut by the surgeon. For parosteal osteosarcoma, margins are the most important part of the surgery, because a tumor left behind can come back and may return as a high-grade cancer.

Negative margin. No cancer cells are seen at the cut edge. The report may give the distance between the tumor and the closest margin.
Positive margin. Cancer cells reach the cut edge, and some tumor may remain. Further surgery is usually recommended.

Your report will usually list several margins, including the cut ends of the bone and the surrounding soft tissue.

Stage

Bone cancers are staged using the TNM system from the American Joint Committee on Cancer (AJCC), 8th edition. Some experts note that this system was designed for tumors that start inside bone and fits surface tumors such as parosteal osteosarcoma less well.

Arms, legs, trunk, skull, and face. The tumor stage depends mainly on size. A tumor 8 cm or smaller is pT1, and a tumor larger than 8 cm is pT2. Separate tumor deposits within the same bone are pT3.
Spine and pelvis. These bones have their own staging rules, based on how many bone segments are involved and whether the tumor extends beyond the bone.
Lymph nodes. Parosteal osteosarcoma rarely spreads to lymph nodes, so surgeons usually do not remove them. When no nodes are examined, the report may say the nodal stage was not assigned.

What is the prognosis?

The outlook for a low-grade parosteal osteosarcoma is excellent. When the whole tumor is removed with negative margins, about 9 in 10 people are alive five years later, and most are cured with surgery alone.

Incomplete removal. The tumor can come back, and a tumor that comes back is more likely to have changed into a high-grade cancer.
Dedifferentiation. A tumor with a high-grade area behaves more like conventional osteosarcoma and can spread, most often to the lungs. The outlook is less favorable.

Because this tumor can come back many years after surgery, long-term follow-up is recommended. These figures describe groups of people and cannot predict what will happen to one person.

What happens after the diagnosis?

A team at a center that regularly treats bone sarcomas manages parosteal osteosarcoma.

Surgery. The main treatment is removing the tumor with a rim of normal tissue. When the tumor wraps around a bone near the knee, part of the bone may need to be replaced with an implant or graft. Scraping out the tumor is not recommended, because it leaves cancer behind.
Chemotherapy. Chemotherapy is not used for a purely low-grade tumor. It may be recommended when a high-grade area is present, although its benefit in that situation is still debated.
Radiation therapy. Radiation is not used because osteosarcoma does not respond well to it.
Follow-up. Regular imaging of the original site and of the chest continues for many years.

Questions to ask your doctor

Was MDM2 testing done to confirm the diagnosis?
Did the tumor contain any high-grade or dedifferentiated areas?
Was the whole tumor removed, and were all the margins negative?
Did the tumor grow into the inner part of the bone?
Will I need chemotherapy or further surgery?
What is the chance the tumor will come back?
How often will I need follow-up imaging, and for how many years?