Low-Grade Central Osteosarcoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
October 6, 2026

Low-grade central osteosarcoma is a rare, slow-growing type of bone cancer. It is a form of osteosarcoma, a cancer of the cells that produce bone. Your report may use the shorter name LGCOS, or an older name, well-differentiated intramedullary osteosarcoma.

The word central means the tumor starts inside the bone, in the hollow center called the medullary cavity, where the bone marrow is. This sets it apart from parosteal osteosarcoma, a similar low-grade cancer that grows on the outside of the bone.

Low-grade central osteosarcoma is a low-grade cancer. It grows slowly and rarely spreads, and most people are cured with surgery alone. It can grow into surrounding bone and soft tissue, and it can come back if it is not completely removed.

This article explains how low-grade central osteosarcoma is diagnosed and what the findings in your pathology report mean.

Who gets low-grade central osteosarcoma?

Low-grade central osteosarcoma is very rare, making up only about 1% to 2% of all osteosarcomas. It usually affects adults in their 20s and 30s, which is older than the usual age for osteosarcoma. It is slightly more common in women.

Where does low-grade central osteosarcoma develop?

Most develop near the ends of the long bones of the leg, particularly the thigh bone and the shin bone, close to the knee. Rarely, the tumor develops in the jaw, the pelvis, or the small bones of the hands and feet.

What are the symptoms of low-grade central osteosarcoma?

The most common symptom is pain in the affected bone, sometimes with swelling or a lump near a joint. Because the tumor grows slowly, symptoms often build up gradually, and some people have them for months or years before the diagnosis is made.

What causes low-grade central osteosarcoma?

Low-grade central osteosarcoma is caused by extra copies of a region of chromosome 12 that contains two genes called MDM2 and CDK4. Extra copies of a gene are 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. It is the same change found in parosteal osteosarcoma.

How is the diagnosis made?

X-rays, CT, and MRI usually show a large area inside the bone that may contain both dense and thinned areas. The tumor often has a less sharp border than a noncancerous bone growth, and it may break through the outer shell of the bone.

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 spindle cells, which are long, thin cells, along with strands of bone made by the tumor. The spindle cells show only mild nuclear atypia, meaning their nuclei look only slightly abnormal. The tumor grows between the existing bone and may extend through the outer shell of the bone into the nearby soft tissue.

Why can low-grade central osteosarcoma be hard to recognize?

Because the cells look so close to normal, low-grade central osteosarcoma can be mistaken for a noncancerous bone condition. The most common look-alike is fibrous dysplasia, a condition in which fibrous tissue replaces normal bone. Another is desmoplastic fibroma, a rare noncancerous bone tumor.

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

Two kinds of tests help separate them:

Immunohistochemistry. Stains for the MDM2 and CDK4 proteins are positive in most low-grade central osteosarcomas and negative in the noncancerous look-alikes.
Molecular testing. A test such as FISH counts the copies of the MDM2 gene, and extra copies confirm the diagnosis. Fibrous dysplasia does not have this change. Instead, it usually has a change in a different gene called GNAS, which is not found in low-grade central osteosarcoma.

Grade and dedifferentiation

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

In a minority of 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. 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 for this tumor, because it changes both the outlook and the treatment.

Tumor extension

Your report may describe whether the tumor has broken through the outer shell of the bone into the surrounding muscle or other soft tissue. This is called extraosseous extension, and it is linked to a higher chance of the tumor coming back.

The report may also note whether the tumor has grown into an adjacent bone. In the spine and pelvis, which are made of several connected bones, this raises the stage.

Margins

A margin is the edge of tissue cut by the surgeon. For low-grade central osteosarcoma, margins are the most important part of the surgery, because tumor left behind usually comes 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.

Lymphovascular and perineural invasion

Your report may state whether cancer cells were seen inside small blood vessels or around nerves. Both findings are rare in low-grade central osteosarcoma, and the report will usually say neither was identified.

Stage

Bone cancers are staged using the TNM system from the American Joint Committee on Cancer (AJCC), 8th edition:

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 classified as 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. This tumor rarely spreads to lymph nodes, so doctors usually don’t remove them. When no nodes are examined, the report may say the nodal stage was not assigned.

What is the prognosis?

The outlook is very good when the whole tumor is removed with negative margins. About 9 in 10 people are alive five years after diagnosis, and more than 8 in 10 at 10 years. Fewer than 5 in 100 develop spread to other parts of the body.

Incomplete removal. A tumor that is only partly removed, or scraped out rather than removed whole, usually comes back.
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.

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 low-grade central osteosarcoma.

Surgery. The main treatment is removing the tumor with a rim of normal tissue. Part of the bone may need to be replaced with an implant or graft. Scraping out the tumor, a procedure called curettage, 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.
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 outside 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?